Gene therapy for sickle cell disease

Abiola Olowoyeye; Charles I Okwundu

doi:10.1002/14651858.CD007652.pub7

. 2020 Nov 30;2020(11):CD007652. doi: 10.1002/14651858.CD007652.pub7

Gene therapy for sickle cell disease

Abiola Olowoyeye ^1,^✉, Charles I Okwundu ²

Editor: Cochrane Cystic Fibrosis and Genetic Disorders Group

PMCID: PMC8275984 PMID: 33251574

Abstract

Background

Sickle cell disease encompasses a group of genetic disorders characterized by the presence of at least one hemoglobin S (Hb S) allele, and a second abnormal allele that could allow abnormal haemoglobin polymerisation leading to a symptomatic disorder.

Autosomal recessive disorders (such as sickle cell disease) are good candidates for gene therapy because a normal phenotype can be restored in diseased cells with only a single normal copy of the mutant gene. This is an update of a previously published Cochrane Review.

Objectives

The objectives of this review are:

‐ to determine whether gene therapy can improve survival and prevent symptoms and complications associated with sickle cell disease;

‐ to examine the risks of gene therapy against the potential long‐term gain for people with sickle cell disease.

Search methods

We searched the Cochrane Cystic Fibrosis and Genetic Disorders Group Haemoglobinopathies Trials Register, which comprises of references identified from comprehensive electronic database searches and searching relevant journals and abstract books of conference proceedings. We also searched online trial registries,

Date of the most recent search of the Group's Haemoglobinopathies Trials Register: 21 September 2020.

Selection criteria

All randomised or quasi‐randomised clinical trials (including any relevant phase 1, 2 or 3 trials) of gene therapy for all individuals with sickle cell disease, regardless of age or setting.

Data collection and analysis

No trials of gene therapy for sickle cell disease were found.

Main results

No trials of gene therapy for sickle cell disease were reported.

Authors' conclusions

No randomised or quasi‐randomised clinical trials of gene therapy for sickle cell disease were reported. Thus, no objective conclusions or recommendations in practice can be made on gene therapy for sickle cell disease. This systematic review has identified the need for well‐designed, randomised controlled trials to assess the benefits and risks of gene therapy for sickle cell disease.

Keywords: Humans; Anemia, Sickle Cell; Anemia, Sickle Cell/genetics; Anemia, Sickle Cell/therapy; Genetic Therapy

Plain language summary

Gene therapy for sickle cell disease

Review question

We reviewed the evidence about the effect and safety of gene therapy on survival and on preventing symptoms and complications associated with sickle cell disease.

Background

Sickle cell disease results when a child inherits faulty genes for producing haemoglobin from both parents. It is a disease that is linked with frequent illness from early life and often results in death earlier than in the general population. Gene therapy replaces these faulty genes with normal ones.

Search date

The evidence is current to: 21 September 2020.

Key results

We looked for trials that used this approach of replacing faulty genes for producing haemoglobin with normal ones in the treatment of sickle cell disease. We found no trials to provide reliable evidence about the risks or benefits of gene therapy for this condition. There is a need for trials that assess the benefits or risks of gene therapy for sickle cell disease.

Background

Description of the condition

Sickle cell disease (SCD) is a group of genetic conditions that result from the inheritance of abnormal haemoglobin genes, thereby resulting in the production of abnormal haemoglobin in red blood cells (Akinyanju 1989). Haemoglobin is responsible for transporting oxygen around the body packaged in red blood cells. In people with SCD, the red blood cells contain abnormal haemoglobin and change from their normal round disk shape to narrow sickle forms. The sickle shape is the end result of a series of complex biochemical and biophysical events within the red cell after deoxygenation of Hb S. The sickle‐shaped cells do not flow smoothly through small blood vessels the way disk‐shaped red blood cells do; they block the vessels, causing pain and organ damage.

About 5% of the world's population carry genes responsible for haemoglobinopathies with most of the people living in, or originating from sub‐Saharan Africa (WHO 2006). Each year about 300,000 infants are born with major haemoglobin disorders, including more than 200,000 cases of sickle cell anaemia in Africa (WHO 2006). Some parts of Africa have up to 10% to 30% of the population with the sickle cell trait and approximately 8% of people of African descent carry one sickle gene in the USA and the Caribbean (Ohene‐Frempong 1994; Serjeant 1992).

Although inheriting a single abnormal sickle gene may protect against the severe form of malaria caused by Plasmodium falciparum, inheritance of two abnormal sickle genes (one from each parent) leads to sickle cell anaemia and confers no such protection. Malaria is a major cause of ill‐health and death in children with sickle cell anaemia. In addition to its health implications, SCD also impacts on individuals and their families both socially and psychologically when trying to meet the demands of this chronic illness. SCD often also interferes with educational development due to its unpredictable and debilitating nature. There is a higher incidence of school absenteeism and hospital costs in families that have a child with SCD (Manci 2003; Platt 1991; Powars 2002; Serjeant 1993). Life expectancy is on the rise for people with SCD, but is still less than that of the general population (Platt 1994).There are no firm data on the survival of individuals with sickle cell anaemia on the African continent (Platt 1991).The median survival was estimated in 1994 to be 42 years for men and 48 years for women in the USA, whereas comparable figures for Jamaica published in 2001 suggested 53 years for men and 58.5 years for women (Platt 1991). The mortality rate is higher amongst people with SCD, and is usually caused either by chronic organ failure consequent on the sickling process, or as a result of an acute catastrophic event, such as a stroke (Powars 1983); acute sickle chest syndrome; splenic sequestration (Steinberg 2003); sudden death or other complications (Gray 1991).

Management of SCD is multidisciplinary. Penicillin prophylaxis to prevent pneumococcal infections, appropriate use of blood transfusions and other supportive measures have improved survival of people with SCD (Gaston 1986; Lezcano 2006). Hydroxycarbamide (hydroxyurea) made a major impact on sickle cell therapy when it was shown to reduce morbidity and mortality (Steinberg 2003). Bone marrow transplantation remains the only possible curative therapy (Mankad 2001). Although palliative therapies and bone marrow transplantation therapy have been developed for these disorders, treatment still remains sub‐optimal and many individuals suffer significant morbidity and early mortality (Persons 2003). Therefore, development of a gene therapy approach has been sought for many years (Nathan 2001; Persons 2003).

Description of the intervention

Gene therapy holds a great deal of promise for the future of medical treatment. It involves the introduction of genetic material into a cell to treat disease. The momentum was generated in 2000 when it was demonstrated that lentivirus vectors containing normal human globin genes and regulatory regions from the beta‐globin locus control region could be faithfully transduced into mouse hematopoietic stem cells and expressed in erythroid cells (Bodine 2003; May 2000).

How the intervention might work

Gene therapy is the introduction of genes into an individual's cells and tissues in order to treat a disease. Most of the conditions treated in this way are genetic disorders that result from gene mutation malfunctions (Postnote 2005). Viruses are the most widely used vectors in gene therapy. Some of the different types of viruses used as vectors in gene therapy include: retroviruses; adenoviruses; adeno‐associated virus; parvovirus; herpes simplex virus; hepatitis virus; and vaccinia virus (Postnote 2005). The viral vectors are made harmless by removing the viral gene and replacing them with the therapeutic gene. This approach is known as gene addition (the transfer of normal beta globin gene into hematopoietic cells via retroviral vectors or adeno‐associated viruses that have been modified or crippled so they do not become infective). In theory, the viral vectors used in gene therapy are made harmless to prevent them from replicating and causing unwanted effects. However, these viruses could trigger unwanted immune responses and cause unwanted side effects.

Other approaches of gene therapy that have been developed include gene replacement (targeted insertion of the transferred gene into the endogenous globin locus by homologous recombination) and gene repair or chimeroplasty (introduction of chimeric oligonucleotides composed of DNA and modified RNA residues into stem cells to direct correction of the mutation in the sickle gene).

Good candidates for gene therapy include autosomal recessive disorders (such as SCD) because a normal phenotype can be restored in diseased cells with only a single normal copy of the mutant gene (Goncz 2002). Gene therapy, if successful, will provide a viable alternative for permanent correction of the abnormal gene in SCD. It will circumvent the problem of donor shortage and avoid complications related to graft versus host rejection that are involved with bone marrow or other forms of stem cell transplant. Gene therapy has been used to correct SCD in mice (Nathan 2001). Despite the technical problems faced, major progress in the globin gene therapy field has been achieved. This progress has advanced the possibility of gene therapy for haemoglobin disorders in the near future (Persons 2003; Chang 2006). The first successful use of gene therapy in humans for SCD has been reported; the investigators used a lentivirus to achieve integration of a normal beta globin gene into the participants hematopoetic cells (Ribeil 2017). A further clinical trial using the lentivirus is ongoing by other researchers (Kohn 2014).

This Cochrane Review is an update of previously published versions (Olowoyeye 2010; Olowoyeye 2012; Olowoyeye 2014; Olowoyeye 2016; Olowoyeye 2018).

Why it is important to do this review

SCD causes significant morbidity and mortality. Identifying therapies that can be potentially curative, with a clear description of the evidence around these interventions, is vital to the care of individuals with SCD.

Objectives

The objectives of this review are to:

determine whether gene therapy can improve survival and prevent symptoms and complications associated with SCD;
examine the risks of gene therapy against the potential long‐term gain for people with SCD.

Methods

Criteria for considering studies for this review

Types of studies

Randomised control trials or quasi‐randomised trials. We plan to include any relevant phase I, II and III trials.

Types of participants

People of any age and sex with SCD of all phenotypes and regardless of geographic or healthcare setting.

Types of interventions

Gene therapy (any approach) plus standard treatment versus standard treatment or other 'curative' type treatment such as bone marrow transplantation, stem cell transplantation or cord blood transplantation.

Types of outcome measures

We will assess the following outcome measures.

Primary outcomes

Event‐free survival (individuals alive and free of SCD symptoms)
Change in mean life expectancy of individuals with SCD attributable to gene therapy

Secondary outcomes

Changes in haematocrit (as measured by the primary trial)
Severity as assessed by changes in complications associated with SCD (e.g. acute chest syndrome, strokes, chronic renal failure, liver and gallbladder diseases, leg ulcers, sequestration crises, etc.)
Number of hospital visits and admissions as a result of pain crises and complications
Change in use of blood transfusion, hydroxyurea and other treatment modalities
Cost effectiveness of gene therapy

Adverse events

As measured by the primary trials.

Search methods for identification of studies

We searched for all relevant published and unpublished trials without restrictions on language, year or publication status.

Electronic searches

We searched the Group's Haemoglobinopathies Trials Register using the terms: (sickle cell OR (haemoglobinopathies AND general)) AND gene therapy.

The Haemoglobinopathies Trials Register is compiled from electronic searches of the Cochrane Central Register of Controlled Trials (CENTRAL) (updated each new issue of the Cochrane Library) and weekly searches of MEDLINE. Unpublished work is identified by searching the abstract books of five major conferences: the European Haematology Association conference; the American Society of Hematology conference; the British Society for Haematology Annual Scientific Meeting; the Caribbean Public Health Agency Annual Scientific Meeting (formerly the Caribbean Health Research Council Meeting); and the National Sickle Cell Disease Program Annual Meeting. For full details of all searching activities for the register, please see the relevant section of the Cochrane Cystic Fibrosis and Genetic Disorders Group's website.

Date of the most recent search of the Cystic Fibrosis and Genetic Disorders Group's Haemoglobinopathies Trials Register: 21 September 2020.

We searched the following trials registries;

The World Health Organization International Clinical Trials Registry Platform (www.who.int/trialsearch) (August 2018, attempted 03 November 2020, but the site was not available to search due to the Covid‐19 pandemic);
ClinicalTrials.gov (www.clinicaltrials.gov) (03 November 2020).

Searching other resources

Reference lists of all identified papers were to be screened for relevant trials.

Data collection and analysis

Two authors, with the assistance of the Cystic Fibrosis and Genetic Disorders Group, undertook searches for eligible trials.

We were unable to identify any randomised controlled trials eligible for inclusion in this review, thus we could not perform the selection process and data analyses that we had planned. If, in the future, we identify any trials and include them in the review, we will adhere to the methods described in detail below in the remainder of this section.

Selection of studies

Two authors (OA, OC) plan to independently undertake a critical appraisal of any identified trial to establish the possible relevance of each article for inclusion in the review.

Data extraction and management

We plan to design a data extraction form and independently extract data using the agreed form. Both authors will verify the extracted data. Extracted information will include the following.

Trial details: citation; trial population demographics; trial design; time period; population size; attrition rate; and source of funding.
Outcome details: as outlined in Types of outcome measures.

In the event that the authors disagree on the abstraction of trial details, they will contact the editor of the review to resolve the conflict.

OA and OC will extract and enter all the data and both authors will re‐check all the entries. We will aim to resolve disagreements by discussion.

Assessment of risk of bias in included studies

The risk of bias will be assessed using the following domains:

sequence generation;
allocation concealment;
masking of participants, personnel and outcome assessors;
completeness of data for each main outcome;
selective outcome reporting;
other sources of bias.

These will be judged as being 'Yes', 'No' or 'Unclear' by using the Cochrane's tool for assessing the risk of bias (Higgins 2008).

Measures of treatment effect

For binary outcome measures, we aim to calculate a pooled estimate of the treatment effect for each outcome across trials using the risk ratio (RR) where appropriate. For continuous outcomes, we plan to record either mean relative change from baseline for each group or mean post‐treatment or intervention values and standard deviation (SD). If standard errors are reported, and where possible, these will be converted to SDs. We will calculate a pooled estimate of treatment effect by calculating the mean difference (MD) and 95% confidence intervals (CIs)

Unit of analysis issues

Cross‐over trials

We do not expect to find cross‐over trials as successful gene therapy is irreversible.

Dealing with missing data

We will attempt to reach the principal investigators of the trials in the event of any missing data or the need for any clarification about the trials.

In order to allow an intention‐to‐treat analysis, we will seek data on the number of participants with each outcome event, by allocated treated group, irrespective of compliance and whether or not the participant was later thought to be ineligible or otherwise excluded from treatment or follow up.

Assessment of heterogeneity

We will calculate whether statistical heterogeneity is present using the Chi² test for homogeneity (P < 0.1). The impact of statistical heterogeneity will be quantified using the I² statistic, which describes the percentage of total variation across trials that is due to heterogeneity rather than sampling error (Higgins 2003).

Assessment of reporting biases

We will attempt to assess whether our review is subject to publication bias by using a funnel plot. If asymmetry is detected, causes other than publication bias will be explored.

Data synthesis

If the trials are found to be clinically homogenous, meta‐analysis will be conducted. For comparable trials, we will summarize their findings using a fixed‐effect model. We shall consider the appropriateness and value of a meta‐analysis if there is significant clinical heterogeneity among the included trials.

Subgroup analysis and investigation of heterogeneity

In the event of significant heterogeneity, we plan to explore the possible causes by undertaking the following subgroup analyses: gene therapy technique; age; sex; and type of SCD.

Sensitivity analysis

We plan to perform a sensitivity analysis based on the generation of the allocation sequence within the trials (including and excluding quasi‐randomised trials).

Summary of findings and assessment of the certainty of the evidence

In a post hoc change, we aim, in future updates when trials are included, to present summary of findings tables for each comparison included.

We aim to report on all seven outcomes as pre‐defined in 'Types of outcome measures'.

We will determine the quality of the evidence using the GRADE approach; and downgrade evidence in the presence of a high risk of bias in at least one trial, indirectness of the evidence, unexplained heterogeneity or inconsistency, imprecision of results, high probability of publication bias. We will downgrade evidence by one level if we consider the limitation to be serious and by two levels if very serious.

Results

Description of studies

We did not identify any trials that were eligible for inclusion in this review.

Results of the search

We did identify 26 studies by searching trial registries, however, none of these required further investigation.

Included studies

We did not identify any trials that were eligible for inclusion in this review.