Curative therapies in sickle cell disease including hematopoietic stem cell transplantation and gene therapy

INTRODUCTION — Allogeneic hematopoietic stem cell transplantation with a matched sibling donor (MSD) has become an established treatment of sickle cell disease (SCD), while alternative donors (haploidentical, matched unrelated) are becoming viable treatment options. The use of gene therapy in SCD is evolving.

This topic discusses these potentially curative therapies for patients with SCD, including indications, clinical outcomes, and transplant strategies specific to this population.

Other therapies for SCD are discussed separately.

●Hydroxyurea – (See "Hydroxyurea use in sickle cell disease".)

●Other medical therapies – (See "Disease-modifying therapies to prevent pain and other complications of sickle cell disease".)

●Transfusions – (See "Red blood cell transfusion in sickle cell disease: Indications and transfusion techniques".)

●Investigational non-curative approaches – (See "Investigational pharmacologic therapies for sickle cell disease".)

●Routine pediatric care – (See "Sickle cell disease in infancy and childhood: Routine health care maintenance and anticipatory guidance".)

●Comprehensive specialist care – (See "Overview of the management and prognosis of sickle cell disease".)

TERMINOLOGY

●SCD – Sickle cell disease (SCD) includes homozygosity for the sickle cell variant in the beta globin gene or compound heterozygous inheritance of the sickle cell variant and another variant in the beta globin gene such as hemoglobin C, a thalassemia variant (beta⁰-thalassemia or beta⁺-thalassemia), or others. (See "Overview of compound sickle cell syndromes", section on 'Specific compound sickle cell syndromes'.)

Patients with SCD exhibit a clinical phenotype mainly characterized by hemolytic anemia, microvascular occlusion, and laboratory evidence of sickling. (See "Overview of the clinical manifestations of sickle cell disease" and "Overview of compound sickle cell syndromes".)

●Transplantation – Hematopoietic stem cell (HSC) transplantation (HCT, also called HSCT) refers to transplantation of hematopoietic stem and progenitor cells (HSPCs) from any source (bone marrow, peripheral blood, or umbilical cord blood).

•Allogeneic – Allogeneic transplantation uses HSCs from a donor. The donor may be related (eg, sibling) or unrelated.

-HLA matched or haploidentical – Related donors can be human leukocyte antigen (HLA)-matched (typically, matched at 10 out of 10 HLA loci) or haploidentical (matched at 5 out of 10 or 50 percent of their HLA loci with the donor having one haplotype in common with the recipient).

-Matched unrelated – Matched unrelated HSCs can come from an unrelated donor or cord blood unit.

•Autologous – Autologous transplantation uses the patient's own HSCs. This is not useful for individuals with SCD unless the autologous cells have been modified (eg, by gene therapy). (See 'Gene therapy and gene editing' below.)

•Conditioning – Conditioning regimens (also called preparative regimens) can be myeloablative, nonmyeloablative, or reduced-intensity. These are defined as follows, with additional details presented separately (see "Preparative regimens for hematopoietic cell transplantation"):

-Myeloablative – These regimens destroy the recipient's bone marrow, causing aplasia and pancytopenia that is usually long-lasting and irreversible. HSCs are required to reconstitute hematopoiesis.

-Nonmyeloablative (NMA) – These regimens do not destroy the recipient's bone marrow or cause persistent cytopenias; however, engrafting donor T cells usually eliminate host hematopoietic cells following transplantation. These may also be referred to as immunoablative regimens, and they may be used to reconstitute hematopoiesis using a second transplant after graft failure.

-Reduced-intensity conditioning (RIC) – This category consists of a range of regimens that are usually intermediate between myeloablative and nonmyeloablative and use lower doses of cytotoxic and/or noncytotoxic agents to ablate or partially ablate the bone marrow. These regimens cause cytopenias and suppress the host's immune system to allow engraftment.

●Gene therapy and gene editing – Gene therapy involves the delivery of genetically modified autologous HSCs, mostly after a busulfan-based myeloablative conditioning regimen. Genetic modification of HSCs can be achieved by gene addition (eg, using a lentiviral vector) or gene editing (manipulation of the endogenous beta globin or regulatory gene). (See "Overview of gene therapy, gene editing, and gene silencing", section on 'Gene therapy' and "Overview of gene therapy, gene editing, and gene silencing", section on 'Gene editing'.)

DECISION TO CONSIDER CURATIVE THERAPY — There is a wide range of professional opinions whether children and adults with SCD should receive curative therapy.

Various considerations contribute to the lack of consensus:

●Continued evolution of overall improvement in care for children with SCD.

●Increased number of curative therapy options, particularly in the early 2020s.

●Short lifespan in adults (median, 48 years).

●Lack of randomized controlled trials to identify short-term and long-term benefits and risks of curative therapy.

For children, opinions may vary widely and may include only offering curative therapy for children with strokes or progressive organ disease despite maximum medical management, to offering curative therapy to children as young as two years of age with no evidence of irreversible organ disease.

The nuances of these viewpoints are discussed in a pair of point and counterpoint articles from 2017 [1,2].

Opportunity for discussion and shared decision-making — The decision to consider (and/or to pursue) transplantation is challenging and highly individualized. The curative therapy evaluation does not necessarily imply that transplantation is indicated.

Patients with SCD should have the opportunity to discuss risks and benefits of hematopoietic stem cell transplantation (HCT) with their primary hematologist as well as experts at a center that treats patients with SCD (including transplantation/curative therapies as well as pharmacologic therapies).

The primary hematologist should inform patients with SCD about the possibility of HCT as a curative treatment option early in their care. The hematologist should discuss general risks and benefits of curative therapy. Perhaps the best way to ensure that all individuals interested in pursuing these therapies can reasonably weigh their options, after discussion with the primary hematologist and decision to move to the next step, is to refer to a center where experts in both SCD and in transplantation are present. Ideally a psychologist with expertise in SCD can also provide consultation and guidance.

In our practice, the hematologists and the transplant physicians work together and review candidates before or immediately after the family asks about the possibility of curative therapy.

●We discuss the severity of the disease, the different curative therapy approaches, and which clinical trials are open.

●We have the patient and family/caregivers visit with the hematologists first, followed by the transplant physician, often with the hematologist present.

●Ultimately, we provide a joint recommendation.

Indications and eligibility — We are most likely to consider curative therapy in children and adults with complications of SCD associated with early mortality or severe morbidity.

The decision to pursue transplantation must assess the benefits and risks related to standard care, which is challenging as both transplant techniques and standard care are rapidly and continuously evolving.  

The following complications are often considered appropriate reasons to prompt evaluation of transplantation when disease manifestations are not responding well to standard therapies:

●Recurrent vaso-occlusive pain episodes (>2 per year, recognizing that some pain episodes may be managed at home without hospitalization)

●Life-threatening or recurrent acute chest syndrome

●Strokes [3]

●Need for chronic blood transfusions

Indications for transplantation might be different for children versus adults [4].

●In children, pain, acute chest syndrome, and (the risk of) stroke or silent cerebral infarcts are the most common reasons.

●In adults, next to recurrent pain episodes, progressive organ dysfunction might be the reason for considering transplantation.

Additional complications that may less commonly be indications for pursuing transplantation apply primarily to adults, in whom these comorbidities are associated with a shortened lifespan. These include:

●In adults, cardiac dysfunction due to pulmonary hypertension confirmed by right heart catheterization, as expressed by elevated tricuspid regurgitant jet velocity, diastolic dysfunction, or both [5,6].

●In adults, progressive lung disease [7].

●Progressive kidney disease [8-11].

●Recurrent major priapism despite medical therapy.

●Progressive avascular osteonecrosis (AVN), especially at a relatively young age, as this tends to predict future episodes of AVN affecting other joints; however, there is also a possibility that transplantation might worsen AVN, especially if glucocorticoids are used for graft versus host disease (GvHD).

●Progressive hepatopathy, with the indications for transplantation considered in a multidisciplinary team, since advanced chronic liver injury can significantly increase transplantation-related toxicity, especially when using busulfan-containing myeloablative conditioning regimens. Mild SCD-related liver fibrosis can be an indication for transplantation [12].

●Alloimmunization of a severity that precludes transfusion when necessary. Patients with severe alloimmunization need special preparation in collaboration with transfusion specialists, and units with compatible blood type need to be reserved for the patient.

Patients or their parents and caregivers may also seek consultation regarding transplantation due to diminished quality of life, recent complications, an imminent major medical decision, or anxiety about future severe complications [13].

Conversely, some chronic complications, including kidney and liver dysfunction and alloimmunization that interferes with transfusions, may adversely affect transplant outcomes [14].

Indication for transplantations in patients with less severe disease has been a matter of debate, but transplantation is generally not pursued.

Unique considerations for children — For children in high-income countries, the proposed transplant approach must have the potential to improve survival, which is especially challenging since the likelihood of survival to adulthood is very high, yet this metric (survival to adulthood) fails to capture many nuances of quality of life [1]. Further, it may not be possible to predict which children are most likely to develop life-threatening or severe SCD complications, due to clinical variability of the disease and the absence of validated biomarkers for disease severity or mortality. The reduced life expectancy and diminished quality-adjusted life expectancy of SCD patients are also important considerations [15,16]. (See "Overview of the management and prognosis of sickle cell disease", section on 'Survival and prognosis'.)

Given the 2 to 5 percent risk of transplantation-related mortality and the relatively low (<2 percent) risk of mortality in children receiving maximum medical therapy, we recommend consideration of HCT only in patients with severe recurrent painful events and/or an organ complication such as listed above. (See 'Indications and eligibility' above.)

Unique considerations for adults — There is more discussion about risk benefit tradeoffs in older children and adults, due to the higher prevalence of organ complications and the higher risk of transplantation-related toxicities.

Life-expectancy is shortened in adults with SCD; the lifespan of approximately 48 years has not improved in recent decades [17]. Given this, we believe that adults wanting a cure for SCD should receive a formal consult with a hematologist and a transplant physician familiar with allogeneic transplantation in patients with SCD.

We would consider transplantation from a matched sibling donor or haploidentical donor. At least 90 percent of older children and adults have a related donor when haploidentical donors are included [4].

In contrast, unrelated donor transplant carries a higher risk of GvHD and would be considered only as part of a clinical trial [18].

Considerations for people without a matched related donor — The vast majority of patients with SCD do not have an HLA-matched sibling donor.

For individuals who lack an HLA-matched related donor, options include an alternative donor such as haploidentical donor (including a parent, child, or sibling), matched or 9/10 mismatched unrelated donor, matched related umbilical cord blood, or gene therapy/gene editing approaches. (See 'Terminology' above.)

●Haploidentical donor – Studies show encouraging outcomes for haploidentical transplantation using nonmyeloablative conditioning, with outcomes in adults comparable to those of myeloablative HLA-matched sibling donor transplantation in children with SCD [4,19,20]. (See 'Haploidentical related donor' below.)

●Unrelated donor – The safety and efficacy of matched unrelated donors (including cord blood donors) is still a subject of active investigation. Major and life-threatening toxicities have been reported with matched unrelated donors and unrelated umbilical cord blood in patients with SCD, including graft rejection and acute and chronic GvHD [18,21]. (See 'Matched unrelated donor (bone marrow or umbilical cord blood)' below.)

●Gene therapy or gene editing – Gene therapy or gene editing using myeloablative conditioning and autologous transplantation is another option. These therapies have only been available for a short period of time, and data on safety and efficacy are limited. (See 'Gene therapy and gene editing' below.)

PLANNING AND PREPARATION

Decisions regarding age, donor, stem cell source, and conditioning regimen — The timing of transplantation (patient age), donor selection (when an HLA-identical sibling donor is unavailable), conditioning regimen, and post-transplant immunosuppression remains unknown and continues to be studied.

In a 2019 retrospective cohort of 910 allogeneic transplant recipients, the best outcomes occurred in patients <13 years who underwent HLA-matched sibling donor transplantation [22]. While myeloablative conditioning was associated with a higher rate of disease-free survival, mortality was higher with myeloablative and reduced intensity conditioning regimens as compared with nonmyeloablative regimens, suggesting increased toxicity of more intensive conditioning regimens, especially in patients >12 years old. (See 'Survival' below.)

●Patient age – Transplantation should be considered in young children with severe disease who have an HLA-identical sibling; it is most likely to be successful in children, when using a myeloablative conditioning regimen, but it may not be possible to predict which children are most likely to benefit. (See 'Opportunity for discussion and shared decision-making' above.)

●Donor – An HLA-matched sibling donor provides the greatest chance of successful cure in children with SCD but is not typically an option due to lack of an available HLA-matched sibling. (See 'Survival' below.)

Siblings or other relatives with sickle cell trait can donate; the recipient will have sickle cell trait, a benign carrier condition. (See "Sickle cell trait".)

For individuals who lack a matched sibling donor, another option is to use a haploidentical donor (including a parent, child, or sibling) [4,20]. (See 'Haploidentical related donor' below.)

Matched unrelated donor (MUD) transplant is associated with an unacceptably high risk of GvHD, and unrelated umbilical cord donors are associated with an unacceptably high risk of graft failure; these donors should only be pursued in the context of a clinical trial [23]. (See 'Matched unrelated donor (bone marrow or umbilical cord blood)' below.)

●Stem cell source – There are no direct comparisons between bone marrow and peripheral blood stem cells. Our approach is to endorse the source of hematopoietic stem cells used in multicenter peer reviewed clinical trials.

Often there is an assumption that the identical conditioning regimen from a successful multicenter trial can be modified to include a different stem cell source with limited prior evidence of the clinical utility. If such adjustments are made at a local site, we encourage the transplant team to acknowledge that there is a major departure from previous protocols, to register their protocol in clinicaltrials.gov, and to have appropriate research oversight with appropriate stopping rules for futility and safety.

●Conditioning regimen – We advocate pursuing complete donor-derived myeloid chimerism as a therapeutic goal of allogeneic hematopoietic cell transplantation (HCT), unless in a setting of a clinical trial with stopping rules for safety and futility with an independent data safety monitoring board. Studies have shown that full donor chimerism can be achieved with reduced intensity conditioning, especially in adults with SCD [4].

General details about conditioning regimens are presented separately. (See "Preparative regimens for hematopoietic cell transplantation".)

Testing and interventions prior to transplant — Considerations include the following, which are summarized in separate topic reviews:

●Infectious disease evaluation of donor and recipient. (See "Evaluation for infection before hematopoietic cell transplantation".)

●Consultation regarding fertility preservation, with cryopreservation of tissues or embryos as appropriate. (See "Fertility preservation: Cryopreservation options" and "Fertility and reproductive hormone preservation: Overview of care prior to gonadotoxic therapy or surgery".)

●GvHD prophylaxis. (See "Prevention of graft-versus-host disease".)

●Attention to pre-transplantation percent Hb S and medication management.

●Evaluation and optimization of psychosocial conditions.

CLINICAL EXPERIENCE/TRANSPLANT OUTCOMES — The first patient with SCD to be treated with an allogeneic hematopoietic stem cell transplant (HCT) was an eight-year-old who had recurrent vaso-occlusive pain episodes and developed acute myeloid leukemia (AML); her case history was published in 1984 [24]. She received bone marrow from her matched related four-year-old brother, who had sickle cell trait. In addition to curing the AML, the transplant led to resolution of vaso-occlusive pain episodes, with full donor chimerism; she remained alive decades later [24,25].

Experience with allogeneic HCT has continued to accrue in the ensuing years. However, there are no randomized trials comparing HCT with medical therapy or directly comparing different transplantation protocols or stem cell sources. Randomized trials comparing transplantation with medical therapy may be especially challenging to conduct since the two approaches differ dramatically in their short- and long-term risks and benefits. The preferred approach may differ in adults and children, since children have a higher risk of graft rejection.

Survival — Outcomes with HLA-identical sibling donors are excellent in children and even in adults with SCD. The use of less toxic nonmyeloablative and reduced intensity conditioning regimens has significantly reduced transplantation-related mortality while achieving good rates of disease-free survival [4,26-28].

Observational studies have demonstrated overall survival of ≥95 percent using matched sibling donors in children and adolescents with SCD [29,30]. Rates of event-free survival (which typically includes graft failure) and disease-free survival (eg, with persistent vaso-occlusive complications or recurrent strokes) are slightly less than rates of overall survival.

A general overview can be obtained from the Center for International Blood and Marrow Transplant Research (CIBMTR) registry, which provides data on large numbers of patients [22,29]. However, these data have accumulated over multiple decades during which there have been major changes in conditioning regimens, supportive care, graft-versus-host disease prophylaxis, and other aspects of transplantation, making comparisons challenging.

Graft failure — Unlike overall survival, which has remained relatively stable, freedom from graft failure (which correlates with event-free survival) has continued to improve, likely due to improvements in HLA matching, conditioning regimens, GvHD prophylaxis, and supportive care for HCT over time [20,22,31-33].

Assessment of engraftment and the degree of chimerism needed for clinical cure is discussed below. (See 'Assessing engraftment and donor chimerism' below.)

GvHD — Risk factors for graft-versus-host disease (GvHD) include patient age, donor type, stem cell source, conditioning regimen, GvHD prophylaxis, and recipient characteristics (inflammatory environment) [4,20,22,27,29,30,34-36]. Robust lymphodepletion (eg, using pre-transplant anti-thymocyte globulin [ATG] and post-transplant cyclophosphamide) has led to a significant decrease in the incidence of severe GvHD [19,37-40]. (See "Pathogenesis of graft-versus-host disease (GVHD)".)

SCD-related complications — Results are summarized as follows:

●Vaso-occlusive pain is eliminated after successful engraftment. However, some patients might continue to have chronic pain, especially those with a greater history of pain prior to transplant and older age [41,42].

●Organ dysfunction seems to improve, although systematic evaluations are lacking, and fixed deficits often are not reversible [31-33,43-48]. Some organ dysfunction may be worsened due to transplantation-related toxicities.

●Rates of ovarian insufficiency are reduced [27,49-53]; all patients should be offered age-appropriate fertility preservation strategies [53,54].

●Quality of life improves, although research is limited [55-57].

●Myeloid malignancy has been reported, particularly in adults following graft failure or with very low donor myeloid chimerism [22,58-62]. A mechanism involving oxidative stress and inflammation, with cytotoxic therapy leading to expansion of clonal hematopoiesis, has been suggested [63].

Alternative donors — Several options are available for individuals who do not have a matched sibling donor.

Alternative donors include haploidentical related donors, matched unrelated donors, mismatched unrelated donors, and umbilical cord blood units from cord blood banks.

●Experience with haploidentical transplantations is increasing rapidly with promising results being reported [4,20]. (See 'Haploidentical related donor' below.)

●Experience with matched unrelated donors and unrelated cord blood units is limited, mainly due to the very low donor availability, but these approaches may be appropriate for selected individuals as part of a clinical trial. (See 'Matched unrelated donor (bone marrow or umbilical cord blood)' below.)

Haploidentical related donor — Haploidentical donors are related donors (typically parents, siblings, or children) who share one haplotype (50 percent of their HLA loci) with the recipient. Use of haploidentical donors is a means of expanding the donor pool as haploidentical donors are the most common alternative donor source.

Another benefit of haploidentical donors is that repeat collections and large cell dose collections are possible. The experience for haploidentical transplant with nonmyeloablative conditioning and post-transplant cyclophosphamide continues to demonstrate promising results, particularly for adults with SCD [4,20]. (See "HLA-haploidentical hematopoietic cell transplantation".)

Haploidentical platforms differ; the Johns Hopkins platform has shown remarkable outcomes and is the basis for the National Institutes of Health (NIH) Blood & Marrow Transplant Clinical Trials Network (BMT CTN) protocol [35,36].

A 2024 multicenter study involving 70 children and adults with SCD who underwent haploidentical transplantation using nonmyeloablative conditioning with thiotepa and post-transplant cyclophosphamide reported overall survival of 96 percent at one year and 94 percent at two years [4]. There were five deaths, all due to infections. Graft failure occurred in eight individuals (11 percent), all of whom were <18 years. Addition of thiotepa to the conditioning regimen was reported to dramatically reduce the risk of graft failure.

For haploidentical transplant, the recipient should not have donor-specific HLA antibodies for the mismatching HLA foci of the potential donor, as these are associated with an increased risk of graft rejection, especially when the mean fluorescence intensity (MFI) level is >5000 [4].

Patients with donor-specific HLA antibodies have been successfully transplanted following lowering the antibody titers with a desensitization protocol [64]. However, this is considered experimental, and using alternative donors to whom the recipient has no donor-specific antibodies is preferred. (See "Donor selection for hematopoietic cell transplantation", section on 'Donor-specific HLA antibodies'.)

Most haploidentical regimens for SCD include post-transplant cyclophosphamide, which decreases the risk of GvHD. While post-transplant cyclophosphamide reduced GvHD, additional myelosuppression, immunosuppression, or both is required to improve engraftment and SCD-free survival [20].

While further improvements are needed in haploidentical transplantation protocols in children with SCD, results in adults appear comparable with those in myeloablative transplantations in children [4]. Many trials evaluating haploidentical transplant for patients with SCD are planned or ongoing.

Matched unrelated donor (bone marrow or umbilical cord blood) — The first pediatric multicenter matched unrelated donor (MUD) trial was published in 2016 and included a high rate of mortality and chronic GvHD [18]. A pilot study that used the same conditioning plus abatacept in seven patients reported a two-year overall survival of 100 percent and SCD-free survival of 93 percent, with a low incidence of moderate to severe GvHD [65].

Additional trials are ongoing, although a trial comparing HLA-matched related and HLA-matched unrelated donors was halted due to slow accrual.

The role of unrelated umbilical cord blood (UCB) transplants in patients with SCD remains experimental [66]. Most transplant centers do not favor this strategy.

POST-TRANSPLANTATION CARE

Slow weaning of GvHD prophylaxis — In contrast to hematologic malignancies, graft-versus-malignancy effect is not required in transplantation protocols for SCD. Furthermore, the risk of graft rejection is higher in SCD patients than in patients with hematologic malignancies.

Therefore, to further limit the risk of graft-versus-host disease (GvHD) and graft rejection, immunosuppressive therapy is used for longer periods than in transplantations in patients with hematologic malignancies [67]. As an example, whereas in malignant diseases immunosuppression is tapered starting between two to three months following transplantation, in SCD the taper starts at least six months after transplantation. In haploidentical transplantations, the immunosuppression is stopped 12 months post-transplantation [4].

The combination of pre-transplant anti-thymocyte globulin (ATG) and post-transplant cyclophosphamide as rejection and GvHD prophylaxis (immune-ablative therapy) in non-malignant diseases has made it possible to use less toxic (nonmyeloablative) chemotherapy in the conditioning regimens [38]. This combination probably decreases the risk of both GvHD and graft rejection.

However, the use of double lymphodepletion might render the patients more susceptible to viral reactivation and infections following transplantation. (See "Overview of infections following hematopoietic cell transplantation".)

The immunosuppressive medications most commonly used as GvHD prophylaxis, in addition to lymphodepletion with ATG and post-transplant cyclophosphamide, are:

●Sirolimus (mTOR inhibitor).

●Cyclosporine (calcineurin inhibitor).

●Tacrolimus (calcineurin inhibitor).

●Mycophenolate mofetil (MMF, inhibits DNA synthesis in lymphocytes) is often used as a second prophylactic drug until day +35, especially in haploidentical transplantations [36].

Cyclosporine has been associated with increased risk of neurotoxicity in children with SCD undergoing myeloablative HCT [68]. While sirolimus seems to be less neurotoxic, it is associated with a higher incidence of bone aches in transplanted SCD patients [28].

General details of GvHD prevention and treatment in hematologic malignancies are presented separately. (See "Prevention of graft-versus-host disease" and "Treatment of acute graft-versus-host disease".)

Assessing engraftment and donor chimerism — Engraftment of the transplanted cells is assessed according to the degree of donor chimerism (percentage of hematopoietic cells derived from the donor rather than the recipient).

This is generally done by DNA testing, most commonly with short tandem repeat (STR) testing by polymerase chain reaction (PCR) and next generation sequencing (NGS) [69]. Details are similar to testing in thalassemia. (See "Thalassemia: Management after hematopoietic cell transplantation", section on 'Engraftment'.)

●Donor chimerism of >95 percent is considered full donor chimerism.

●Donor chimerism of <5 percent is considered graft rejection.

●Donor chimerism between 5 and 95 percent is called mixed chimerism.

•Donor myeloid chimerism of ≥20 percent is considered sufficient for clinical cure as it provides sufficient hemoglobin A (non-sickle hemoglobin) to prevent sickling [70].

•Donor T-cell chimerism of >50 percent is considered essential to maintain a stable mixed donor chimerism [27].

Methods of donor chimerism assessment that use red blood cell phenotyping and/or cytogenetic differences (eg, Y chromosome when donor and recipient are of different sex) are considered insufficient to accurately monitor donor chimerism.

Post-transplant immunosuppression (GvHD and rejection prophylaxis) is mostly discontinued based on chimerism assessments [4,27].

Options for individuals with graft failure include a second transplant or medical therapy, depending on the patient's clinical status and donor availability.

Supportive care — Care is provided according to institutional protocols. Important aspects soon after transplant include the following:

●Hematologic support

•Transfusions – Transfusions are given during the early engraftment period. Thresholds and special modifications related to ABO blood type discrepancies are discussed separately. (See "Hematopoietic support after hematopoietic cell transplantation".)

•G-CSF – While granulocyte-colony stimulating factor (G-CSF) should be avoided in non-transplant patients with SCD, G-CSF stimulation has been shown to be safe and well tolerated after allogeneic HCT in patients with SCD [71]. Stimulation with G-CSF can be used to enhance neutrophil recovery following transplantation. (See "Overview of the management and prognosis of sickle cell disease", section on 'Avoidance of G-CSF'.)

●Infection prevention and treatment – Immunizations after transplantation are discussed separately. (See "Immunizations in hematopoietic cell transplant candidates, recipients, and donors".)

Infections are treated with appropriate antibiotics. (See "Overview of infections following hematopoietic cell transplantation".)

●Iron stores – Iron overload, if present, is treated with phlebotomy, provided the individual has a sufficient hemoglobin level. The approach is similar to that used in individuals with thalassemia (eg, magnetic resonance imaging [MRI] to assess iron burden and ferritin monitoring to assess iron removal). (See "Thalassemia: Management after hematopoietic cell transplantation", section on 'Iron stores'.)

●Psychosocial support – Adults with SCD mostly need psychological and/or occupational support following transplantation, as being cured after a life-long disease brings new challenges [56]. Children may require school support and/or accommodations.

●Rehabilitation and management of chronic and recurrent pain – Health care utilization for painful episodes decreases significantly, but patients with severe chronic pain continue to experience pain [41,42,72]. These individuals require multidisciplinary rehabilitation focused on chronic disabling pain.

Long-term follow-up is similar to that used in individuals with thalassemia, with multidisciplinary review of growth and development (for children), endocrinology, cardiology, pulmonary and kidney function, ophthalmology, and others. (See "Thalassemia: Management after hematopoietic cell transplantation", section on 'Long-term management'.)

Donor lymphocyte infusion — Donor lymphocyte infusion (DLI) is used in hematologic malignancies as a means of increasing graft-versus-tumor effect. (See "Immunotherapy for the prevention and treatment of relapse following allogeneic hematopoietic cell transplantation", section on 'Donor lymphocyte infusion (DLI)'.)

Experience with DLI to improve engraftment in nonmalignant diseases is extremely limited. Most centers do not use DLI to treat ongoing graft rejection, as inducing GvHD by DLI is considered unacceptable.

However, given the risks of graft failure (delayed or absent autologous regeneration and myeloid malignancies), application of low dose DLI might be a useful addition that needs to be evaluated in the setting of a well-designed clinical trial, especially when using nonmyeloablative or reduced intensity conditioning regimens [73].

GENE THERAPY AND GENE EDITING

Overview of gene therapy and gene editing — Gene therapy (introducing a new gene) and gene editing (altering the sequence of an endogenous gene) have the potential to cure SCD [74,75]. Unlike transplantation of allogeneic hematopoietic stem cells (HSCs) from a donor, these approaches modify the person's own HSCs, and concerns about GvHD (and the need for immunosuppression) do not apply.

Typically, the autologous HSCs are harvested, manipulations are done in the laboratory, myeloablative chemotherapy (busulfan) is delivered, and the autologous HSCs are reinfused as part of an autologous transplant.

CRISPR (clustered regularly interspaced short palindromic repeats)-Cas9 is one of the most studied tools for gene editing. (See "Overview of gene therapy, gene editing, and gene silencing", section on 'Gene editing'.)

In December of 2023, the US Food and Drug Administration (FDA) approved two autologous cell-based therapies, one using gene therapy and one using gene editing [76-79]:

●Lovotibeglogene autotemcel (lovo-cel, Lyfgenia) is a gene therapy construct that introduces an anti-sickling beta globin variant into autologous HSCs (the T87Q variant, which is thought to block Hb S polymerization). (See 'Anti-sickling beta globin gene (lovo-cel, Lyfgenia)' below.)

●Exagamglogene autotemcel (exa-cel, Casgevy) is a gene editing construct that disrupts the transcription factor BCL11A in autologous HSCs. (See 'Gamma globin upregulation (including exa-cel, Casgevy)' below.)

Other general conceptual strategies include:

●Replacing the abnormal allele at the beta globin gene locus (HBB^S) with the wild-type HBB^A allele or providing a corrected beta globin gene. (See 'Beta globin gene correction' below.)

●Other genetic methods of increasing Hb F expression. (See 'Gamma globin upregulation (including exa-cel, Casgevy)' below.)

●Expressing delta globin, which lacks the Hb S mutation. (See 'Delta globin gene' below.)

In vitro studies and animal models have provided evidence for the feasibility of these approaches [80-85]. However, issues remain related to the safety and efficacy of the delivery viruses (or other delivery methods). Myeloid malignancy is a particularly concerning outcome that is incompletely understood. (See 'Concern about myeloid malignancy in gene therapy studies' below.)

Other areas of active research include whether alternatives to myeloablative therapy can be used to prepare the individual for infusion of the autologous HSCs, or whether the gene therapy or gene editing construct can be delivered directly to the patient rather than to their HSCs in the laboratory [86-88]. (See "Overview of gene therapy, gene editing, and gene silencing", section on 'In vitro versus in vivo transduction'.)

As with other autosomal recessive conditions, it is not necessary to completely eliminate expression of the disease gene. Induction of 40 to 50 percent Hb F can be sufficient to eliminate SCD-related vaso-occlusive pain episodes. However, the effects on gene therapy or gene editing on the rheologic behavior of modified red blood cells and on chronic SCD-related organ complications are not well known.

Beta globin gene correction — The sickle cell variant is a single point mutation, and reversion to the wild-type sequence at one or both alleles of the beta globin gene (HBB) locus could convert SCD to sickle cell trait, a benign carrier condition, or to normal adult hemoglobin (Hb A).

Gene correction (which takes advantage of homology-directed repair) and gene editing are processes by which a wild-type beta globin gene is used as a template for endonucleases to "repair" the mutant sequence. (See "Genetics: Glossary of terms", section on 'Gene editing'.)

A preclinical model using this approach was able to correct the sickle mutation in cultured bone marrow cells from patients with SCD, and cells manipulated in this manner could differentiate and restore hematopoiesis in a mouse model [89].

Nulabeglogene autogedtemcel (nula-cel, formerly GPH101) is a CRISPR-Cas9 gene editing system used to correct the sickle cell mutation. In January 2023, a clinical trial was voluntarily paused due to unexpected cytopenias that were attributed to the therapy [90,91].

Anti-sickling beta globin gene (lovo-cel, Lyfgenia) — An artificial version of the beta globin gene has been developed that creates an amino acid substitution of glutamine for threonine at position 87 (beta globin T87Q) [80]. This amino acid switch replicates the region of the gamma globin sequence that is thought to block polymerization of sickle hemoglobin.

Gene therapy with lovotibeglogene autotemcel (lovo-cel, Lyfgenia, previously called LentiGlobin) consists of autologous CD34-enriched hematopoietic stem and progenitor cells transduced with the lentiviral vector BB305 that expresses Hb A^T87Q. Clinical experience includes the following:

●In a 2017 case report, a 13-year-old boy with SCD who had multiple vaso-occlusive pain episodes and other SCD complications that did not improve with hydroxyurea or chronic transfusions was treated with an autologous transplantation using this construct [92]. Following engraftment, he had increasing expression of the modified beta globin until it reached stable levels of approximately 50 percent of total hemoglobin at nine months, with a reciprocal decline in Hb S expression. He had no vaso-occlusive events and required no further analgesics during the post-transplant observation period of more than 15 months. There were no major adverse events other than the expected cytotoxicity of the conditioning regimen.

●In individuals in group A of a study using this construct, two of seven patients developed acute myeloid leukemia (AML) that could not be directly related to the gene therapy construct and was attributed to the increased risk of myeloid malignancy due to the underlying SCD, exposure to cytotoxic chemotherapy, and a suboptimal treatment protocol with insufficient number of transduced hematopoietic stem and progenitor cells [93,94]. Details are discussed below. (See 'Concern about myeloid malignancy in gene therapy studies' below.)

●In a 2021 report describing outcomes in 35 individuals who underwent gene therapy with this construct in the more optimized protocol in group C of this study, using plerixafor-mobilized peripheral blood stem cells and myeloablative conditioning, all 35 had engraftment, with a median follow-up of 17.3 months (range 3.7 to 37.6) [95]. The following post-transplant outcomes were reported:

•Vaso-occlusive events decreased from a mean of 3.5 per year to a mean of 0 (in 25 evaluable patients).

•Median hemoglobin increased from 8.5 g/dL to ≥11 g/dL.

•Hb A^T87Q accounted for ≥40 percent of hemoglobin and was present in 85 percent of RBCs.

•Hb S decreased to approximately 50 percent.

•Two patients developed anemia of unknown cause, without evidence of MDS.

•There was one death, 20 months after transplant, in an individual with pulmonary hypertension, left ventricular hypertrophy, and cardiac interstitial fibrosis.

Gamma globin upregulation (including exa-cel, Casgevy) — The gamma globin gene is the source of gamma chains (beta-like chains) for fetal hemoglobin (Hb F), the predominant hemoglobin expressed in late gestation and early infancy. It is a separate gene from beta globin and thus does not contain the Hb S mutation (figure 1). (See "Structure and function of normal hemoglobins", section on 'Hb F'.)

The switch from gamma globin to beta globin expression leads to a reduction in Hb F and an increase in Hb A in early infancy. This switch is controlled in large part by the BCL11A gene, which encodes a transcriptional repressor of gamma globin expression. (See "Fetal hemoglobin (Hb F) in health and disease", section on 'BCL11A'.)

Manipulations that block the function of BCL11A reverse the switch and can dramatically increase Hb F levels, in turn reducing sickling and vaso-occlusive complications. These genetic manipulations have the potential to raise Hb F levels (and decrease Hb S) to a much greater extent than hydroxyurea. (See "Hydroxyurea use in sickle cell disease".)

Studies have validated the approach of targeting BCL11A using different methods:

●Gene editing using CRISPR-Cas9 – Gene editing has been used to increase Hb F expression via targeting the BCL11A gene or the gamma globin promotor.

•BCL11A editing – Exagamglogene autotemcel (exa-cel, Casgevy) is a gene editing construct that uses the CRISPR-Cas9 system to disrupt BCL11A expression in autologous HSCs followed by myeloablative conditioning (busulfan) and autologous transplantation. A 2024 report described 44 patients with SCD ages 12 to 35 years who had at least two vaso-occlusive episodes per year (eg, pain, acute chest syndrome, priapism) who were treated with this therapy [96]. Of the 30 participants who had a follow-up duration of at least 12 months, 29 (97 percent) had no vaso-occlusive episodes and none were hospitalized for vaso-occlusive events. Among all 44 patients, the mean Hb was 11.9 g/dL at month 3 and 12.5 g/dL at month 6. The mean percent Hb F was 37 percent at month 3 and 44 percent at month 6. Toxicities were as expected with myeloablative conditioning regimens, and no cancers occurred. One patient who had preexisting lung disease and busulfan-induced lung injury died of COVID-19 at day 268.

•Gamma globin promotor editing – OTQ923 is a CRISPR-Cas9-based system for disrupting the gamma globin promotor region (HBG1/HBG2). In a mouse model, targeting this region led to induction of Hb F [97]. Follow-up of the first three patients with SCD treated with OTQ923 (using myeloablative busulfan conditioning and autologous HSCs treated with OTQ923) demonstrated clinical improvements (substantial reduction in vaso-occlusive episodes, discontinuation of transfusions) as well as marked increases in Hb (range, 10 to 12 g/dL), Hb F (range, 19 to 27 percent), and F cells (range, 70 to 88 percent) [98,99]. (See "Fetal hemoglobin (Hb F) in health and disease", section on 'Gamma globin genes (HBG2 and HBG1)'.)

●Gene editing using zinc finger nucleases – Zinc finger nucleases (ZFN) are also used for gene editing. (See "Overview of gene therapy, gene editing, and gene silencing", section on 'Gene editing'.)

BIVV003 uses ZFN to target BCL11A. In a study of four patients with SCD treated with BIVV003, Hb F increased from 12 to 41 percent, and three had complete resolution of vaso-occlusive events [100].

Since patients received myeloablative chemotherapy prior to transplantation of genetically modified hematopoietic cells, transplant-related complications attributable to the myeloablative conditioning regimen were observed.

●RNAi – One approach used RNAi from a lentiviral vector encoding a short hairpin RNA (shRNA) targeting BCL11A mRNA [101,102]. The shRNA was embedded in a microRNA to promote erythroid-specific knockdown in autologous HSCs followed by autologous HCT. Autologous HCT used HSCs transfected with this construct. Gene therapy was ineffective in one individual; in the remaining patients, Hb F was increased from 21 to 42 percent. Five patients with >6 months of follow-up had a median of 6 (3 to 13) vaso-occlusive events prior to gene therapy and a median of 1 (0 to 1) vaso-occlusive events following gene therapy.

Details of Hb F regulation, gene editing techniques, and RNAi are discussed separately [103]. (See "Overview of gene therapy, gene editing, and gene silencing", section on 'RNA interference' and "Overview of gene therapy, gene editing, and gene silencing", section on 'Gene editing' and "Fetal hemoglobin (Hb F) in health and disease", section on 'Hemoglobin switching and downregulation of Hb F expression'.)

Delta globin gene — The delta globin gene is the source of beta globin-like chains for Hb A2, a minor adult hemoglobin that typically accounts for only 2 to 3 percent of total hemoglobin in children and adults. It is a separate gene from beta globin and thus does not contain the Hb S mutation. Like Hb F, Hb A2 inhibits the polymerization of Hb S [104,105]. (See "Structure and function of normal hemoglobins", section on 'Hb A2'.)

Studies in transgenic SCD mice highlight the possible value of increasing delta globin gene expression [106,107].

Preclinical models using gene therapy to introduce the delta globin gene have not been reported, but patients with SCD who have especially high Hb A2 levels appear to have a milder clinical phenotype, suggesting that approaches to increase delta globin expression might be worthwhile [104].

Plerixafor for mobilization — The use of plerixafor for mobilizing CD34 positive stem and progenitor cells in SCD remains investigational.

In a study from 2020 involving 15 patients from two clinical sites who underwent autologous HSC collection for gene therapy, 13 had adequate numbers of CD34 positive cells after a single subcutaneous dose of plerixafor (240 mg/kg) followed by apheresis, while two patients required a second dose [108]. Eleven of these patients experienced pain, with three requiring hospitalization. Plerixafor-mobilized HSCs were enriched for an engrafting population, suggesting relative superiority over other mobilization methods.

An analysis of 23 participants with SCD undergoing plerixafor mobilization and apheresis in a phase I safety and efficacy study found that severity of SCD, the number of medications used against chronic pain, and the period of hydroxyurea being held prior to the mobilization influenced the CD34 positive yields following plerixafor mobilization [109].

Plerixafor has been used in other studies [96].

Concern about myeloid malignancy in gene therapy studies — In early 2021, gene therapy studies using lentiviral vectors were suspended temporarily after 2 of 47 individuals with SCD who were participating in a study using the BB305 lentiviral vector subsequently developed myeloid malignancies (myelodysplastic syndrome [MDS] that later transformed to acute myeloid leukemia [AML] in one participant, AML in another participant) [93,94].

●For the first participant, the investigative team concluded that MDS/AML was not related to the viral vector but was a result of the conditioning regimen in combination with a higher risk of mutational burden due to the underlying SCD [93].

●Evaluation of the second patient, who had very limited engraftment of the modified HSCs and developed AML more than five years later, also determined that the gene therapy construct was unlikely to be responsible [94]. The insertion site was in a gene not known to be associated with oncogenesis and was present in most patients with SCD who received the same construct and did not develop AML, and several somatic mutations were present in the leukemic blasts unrelated to the vector but commonly seen in monosomy 7, which is known to complicate alkylating agent therapy. Low transgene expression, insufficient therapeutic response, and persistent hematopoietic stress may have contributed to somatic mutation evolution.

●Two other patients who were originally suspected of having MDS had transfusion-dependent anemia and trisomy 8, but there was no evidence of dysplasia or blasts on bone marrow evaluation [95,110,111].

Studies are indicated to assess for genetic risk factors for MDS and AML development after gene therapy for SCD. Hypotheses for the mechanism of leukemogenesis include insertional mutagenesis, transplant conditioning regimens, and expansion of preexisting premalignant clonal populations driven by regeneration stress of autologous hematopoiesis [112]. The observation of increased risk of myeloid malignancy after graft failure following allogeneic HCT suggests a higher prevalence of preexisting clonal hematopoiesis in SCD patients that might expand following exposure to the toxic conditioning regimen [59].

The optimal strategy to identify individuals with SCD who are at increased risk for transplant-related malignancy is unknown. Several strategies have been suggested but not proven to have clinical utility. Screening for clonal hematopoiesis with sensitive deep sequencing methods might detect patients who are at increased risk for transplant-related myeloid malignancy. The gene(s) and variant allele frequencies associated with an increased risk of MDS/AML following any curative therapy in SCD are unknown.

Individuals with thalassemia treated with the same lentiviral construct have not shown evidence of AML or MDS. However, no conclusive evidence exists to determine which features of disease or therapy predispose to myeloid malignancy, from among myeloablative therapy, the lentiviral vector, SCD-related genetic predisposition, stress erythropoiesis, or some combination of these factors. MDS and AML have been observed in allogeneic transplant studies in SCD, especially after graft failure, that did not involve gene therapy [22,59,113]. The cause(s) in individuals with SCD are under investigation.

Choice of gene therapy/gene editing approaches — Since approximately 4 percent of participants in one gene therapy trial developed MDS/AML, coupled with the high rate of mortality, we recommend caution in advising children with severe SCD to pursue gene therapy or gene editing.

For those who do wish to pursue one of these approaches, the contemporaneous availability of two different therapies, lovotibeglogene autotemcel (Lyfgenia) and exagamglogene autotemcel (exa-cel, Casgevy) poses a therapeutic dilemma. In the absence of side-to-side comparison of the two approaches, no conclusions can be drawn regarding their comparative safety or efficacy [114].

●Following treatment with Lyfgenia, patients with alpha thalassemia trait may experience anemia with erythroid dysplasia that may require chronic red blood cell transfusions. Lyfgenia has not been studied in patients with more than two alpha globin gene deletions. (See 'Anti-sickling beta globin gene (lovo-cel, Lyfgenia)' above.)

●Patients with high baseline Hb F expression prior to the initiation of optimal hydroxyurea therapy should be carefully evaluated to exclude a prior mutation affecting the genetic target of Casgevy. (See 'Gamma globin upregulation (including exa-cel, Casgevy)' above.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Sickle cell disease and thalassemias".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topics (see "Patient education: Allogeneic bone marrow transplant (The Basics)" and "Patient education: Sickle cell disease (The Basics)" and "Patient education: When your child has sickle cell disease (The Basics)")

●Beyond the Basics topics (see "Patient education: Hematopoietic cell transplantation (bone marrow transplantation) (Beyond the Basics)")

PATIENT PERSPECTIVE TOPIC — Patient perspectives are provided for selected disorders to help clinicians better understand the patient experience and patient concerns. These narratives may offer insights into patient values and preferences not included in other UpToDate topics. (See "Patient perspective: Sickle cell disease".)

SUMMARY AND RECOMMENDATIONS

●Definitions – Definitions used in curative therapies for sickle cell disease (SCD), including types of hematopoietic stem cell (HSC) donors, conditioning regimens, and gene therapy and gene editing, are discussed above and separately. (See 'Terminology' above and "Overview of gene therapy, gene editing, and gene silencing".)

●Decision to consider curative therapy – The decision to consider or pursue transplantation is highly individualized. Consultation for transplantation does not necessarily imply that transplantation is imminently indicated; it is an opportunity for discussion and shared decision-making. (See 'Decision to consider curative therapy' above and 'Opportunity for discussion and shared decision-making' above.)

We are most likely to consider curative therapy in individuals with SCD complications associated with early mortality or severe morbidity, such as stroke, frequent pain, or acute chest syndrome. Pain interfering with activities or resulting in disability is the most common indication. For children, the approach must have the potential to improve upon the already very high rate of survival to adulthood. For adults, the lifespan of approximately 48 years has not improved, and we believe adults wanting a cure should receive a formal consultation with a hematologist and transplant physician. (See 'Indications and eligibility' above and 'Unique considerations for children' above and 'Unique considerations for adults' above.)

Transplantation is most likely to be successful in children using myeloablative conditioning and an HLA-matched sibling donor, but such donors are frequently unavailable. Alternate donor options include a haploidentical related donor (parent, child, sibling) with nonmyeloablative conditioning, unrelated donor, or umbilical cord blood. Of these, haploidentical related donors are most promising. Gene therapy and gene editing have been approved and appear promising, although long-term outcomes need further study. (See 'Considerations for people without a matched related donor' above and 'Haploidentical related donor' above.)

●Preparation – Relatives with sickle cell trait can donate. Bone marrow and peripheral blood stem cells have not been directly compared; we endorse the HSC source used in multicenter peer reviewed trials. Other considerations include infectious disease evaluation (donor and recipient), fertility preservation, and graft-versus-host disease (GvHD) prophylaxis. (See 'Planning and preparation' above and "Sickle cell trait" and "Evaluation for infection before hematopoietic cell transplantation" and "Fertility and reproductive hormone preservation: Overview of care prior to gonadotoxic therapy or surgery" and "Fertility preservation: Cryopreservation options" and "Prevention of graft-versus-host disease".)

●Survival – Overall survival in children and adolescents with matched sibling donors is ≥95 percent. Disease-free survival is impacted by donor type, conditioning, and patient age. Center for International Blood and Marrow Transplant Research (CIBMTR) is a rich source of outcome information but is subject to the limitations of registry data. (See 'Survival' above.)

●Other outcomes – Rates of graft failure continue to decrease. Severe GvHD has decreased due to robust lymphodepletion (pre-transplant anti-thymocyte globulin and post-transplant cyclophosphamide). Rates of pain episodes are dramatically reduced. SCD-related organ damage stabilizes, and complications improve. Myeloid malignancy has been reported, particularly in adults with graft failure or low donor chimerism. (See 'Graft failure' above and 'GvHD' above and 'SCD-related complications' above.)

●Post-transplantation care – Graft-versus-malignancy effect is not required, and the risk of graft rejection is higher in SCD. Therefore, immunosuppressive therapy is used for longer periods than with hematologic malignancies. Engraftment and percent donor chimerism is generally assessed by DNA testing. Support must be provided (hematologic, infection, psychosocial) along with multidisciplinary care for chronic pain. Excess iron stores can generally be treated with phlebotomy. (See 'Post-transplantation care' above.)

●Gene therapy and gene editing – Several approaches to modifying the person's own HSCs are under study. One gene therapy and one gene editing approach were approved by the FDA in late 2023. Long-term studies of safety and efficacy including the risk of myeloid malignancy are ongoing. (See 'Gene therapy and gene editing' above.)

●Medical therapies – Separate topics discuss approved and investigational medical therapies. (See "Hydroxyurea use in sickle cell disease" and "Disease-modifying therapies to prevent pain and other complications of sickle cell disease" and "Red blood cell transfusion in sickle cell disease: Indications and transfusion techniques" and "Investigational pharmacologic therapies for sickle cell disease".)

ACKNOWLEDGMENTS — UpToDate gratefully acknowledges Stanley L Schrier, MD, who contributed as Section Editor on earlier versions of this topic and was a founding Editor-in-Chief for UpToDate in Hematology.

The UpToDate editorial staff also acknowledges extensive contributions of Donald H Mahoney, Jr, MD, Shakila Khan, MD, and Griffin P Rodgers, MD, to earlier versions of this topic review.