Sickle Cell Anemia Risk and Your Genetics
Written by Scott Peeples, BS Biomedical Sciences · ExomeDNA Founder
Reviewed by ExomeDNA Editorial Process · [/methodology/editorial-process]
Last reviewed: 2026-05-29
This content is educational and informational. For health decisions, consult a clinician.
Sickle cell anemia is a hereditary red blood cell disorder caused by a structural change in hemoglobin, the protein that carries oxygen in blood. Beyond the primary mutation, a separate layer of genetic variation influences how severely the condition affects individuals — particularly through haemolysis, the breakdown of red blood cells. This page explains what research has found about those modifier genes, how they relate to hemoglobin biology, and what the genetics of haemolysis in sickle cell anemia looks like at a population level.
What is sickle cell anemia (haemolysis)?
Sickle cell anemia is caused by a variant in the HBB gene that produces an abnormal form of hemoglobin called HbS. When HbS molecules become deoxygenated, they polymerize and distort red blood cells into a rigid, crescent shape. These sickled cells are fragile and break down prematurely — a process called haemolysis. The rate of haemolysis varies considerably between people who carry the same primary HBB mutation, and that variation is partly explained by other genes acting as modifiers of disease severity.
Haemolysis in sickle cell anemia contributes to anemia, elevated bilirubin, pulmonary hypertension, and other complications. Understanding the genetics that influence haemolytic rate is an active area of research because modifier genes may represent targets for therapeutic strategies aimed at reducing disease burden.
The genetics behind sickle cell anemia (haemolysis)
The primary cause of sickle cell anemia — a glutamic acid to valine substitution at position 6 of the beta-globin chain — has been understood for decades. However, genome-wide association studies (GWAS) have identified additional loci that modify haemolysis severity in people who already carry the sickle cell mutation.
Several genes in the hemoglobin gene cluster stand out as biologically important modifiers:
HBG2 encodes gamma-globin 2, one of the two gamma-globin chains that form fetal hemoglobin (HbF). HbF is a well-established modifier of sickle cell severity: higher HbF levels inhibit HbS polymerization and reduce sickling. HBG2 variation contributes to differences in how much fetal hemoglobin individuals continue to produce into adulthood.
HBE1 encodes epsilon-globin, normally expressed in early embryonic development. Variants near HBE1 contribute to regulatory control of the broader hemoglobin gene cluster on chromosome 11.
BCL11A is a transcriptional repressor of gamma-globin expression. BCL11A suppresses HbF production after the fetal-to-adult hemoglobin switch; variants that reduce BCL11A function are associated with higher HbF and generally milder sickle cell phenotypes. BCL11A has become a major focus of gene therapy research for hemoglobinopathies precisely because of its role in this regulatory axis.
NPRL3 (nitrogen permease regulator-like 3) is the top gene by locus-to-gene scoring in the haemolysis GWAS dataset. NPRL3 lies near the alpha-globin cluster on chromosome 16 and has roles in cellular metabolism and the mTORC1 signaling pathway. Its precise mechanistic contribution to haemolysis in sickle cell anemia is an area of ongoing investigation.
Haemolysis variation: Studies of sickle cell populations show that markers of haemolytic rate — including lactate dehydrogenase (LDH), bilirubin, and reticulocyte count — vary substantially across individuals with the same primary HBB genotype, pointing to the importance of modifier loci.[1]
The gene cluster on chromosome 11 that includes HBG2 and HBE1 is regulated by a powerful upstream enhancer called the locus control region (LCR). Variation across this region influences the relative expression of embryonic, fetal, and adult globin genes throughout development and into adult life — making it a central focus for understanding why sickle cell phenotypes differ between individuals.
What the research says
Research base: Moderate.
Genetic studies of haemolysis in sickle cell anemia have identified associations with loci in and around the beta-globin gene cluster and the alpha-globin region. Milton et al. (2013) examined genetic determinants of haemolytic rate in sickle cell anemia using markers including LDH, bilirubin, and reticulocyte count as proxy measures of red blood cell breakdown.[1] The study identified associations pointing toward the role of fetal hemoglobin regulation and modifier gene effects on disease severity.
BCL11A variants associated with increased HbF persistence have been observed to correlate with reduced haemolytic markers in sickle cell populations. This relationship is consistent with the known biology: more HbF means less HbS polymerization, less sickling, and reduced red blood cell destruction.
NPRL3's presence as the top-ranked locus in the haemolysis GWAS is consistent with its chromosomal location near the alpha-globin gene cluster. Alpha-globin gene copy number and expression levels are known modifiers of sickle cell severity, as excess alpha-globin chains (in the absence of adequate beta-like chains) contribute to red blood cell instability.
BCL11A therapeutic relevance: BCL11A has been identified as a clinically validated target: gene therapy approaches that reduce BCL11A function in erythroid cells have demonstrated HbF induction and clinical benefit in trials for sickle cell disease and beta-thalassemia.[1]
It is important to note that this GWAS signal captures modifier effects — variation that influences the degree of haemolysis among people already affected by sickle cell anemia — rather than primary susceptibility to the condition itself. The research base for this specific phenotype is moderate, meaning associations have been reported but replication across diverse cohorts and full mechanistic characterization remain incomplete.
Because the primary HBB sickle variant is a simple Mendelian mutation, the GWAS context here is specifically about the genetic architecture of phenotypic variation in sickle cell anemia, not about predicting who develops the condition.
How sickle cell anemia (haemolysis) affects you
For people with sickle cell anemia, the rate of haemolysis — red blood cell breakdown — is a key determinant of which complications they experience and how severely. High haemolytic rate is associated with particular complication subtypes, including pulmonary hypertension, leg ulcers, priapism, and stroke risk through different mechanisms than vaso-occlusive pain crises.
The concept of "haemolysis-endothelial dysfunction" as a distinct complication pathway in sickle cell anemia has emerged from clinical research. When red blood cells break down rapidly, they release hemoglobin and other contents into circulation. Free hemoglobin scavenges nitric oxide — a molecule essential for blood vessel relaxation — and this process contributes to vascular complications.
Genetic modifier variants, including those in the HbF-regulating pathway, influence where on the haemolysis spectrum an individual falls. People with higher genetically-mediated HbF levels tend to have less haemolysis, fewer vascular complications, and generally milder disease overall.
For people without sickle cell anemia, this trait as reported through a population GWAS does not carry the same clinical context. The variant associations identified apply specifically in the context of sickle cell disease biology, not as a general risk factor for red blood cell problems in the broader population.
For those with a family history of sickle cell anemia or who are carriers of the HBB sickle variant, understanding the broader genetic landscape of disease modifiers is a legitimate area of educational interest. Genetic counselors and hematology specialists are the appropriate resources for clinical interpretation.
Working with your sickle cell anemia (haemolysis) profile
Genetic results related to haemolysis modifiers in sickle cell anemia are most meaningful when interpreted within a full clinical and family history context. A genetics report that identifies variants near BCL11A, HBG2, or NPRL3 does not stand alone as a clinical assessment — it represents one informational layer among many.
For individuals known to have sickle cell anemia, hematologists routinely track haemolysis markers including LDH, total bilirubin, reticulocyte count, and hemoglobin levels. These laboratory measures provide direct information about haemolytic rate that genetic variants can help contextualize but cannot replace.
For family members of individuals with sickle cell anemia, genetic carrier screening (which evaluates the primary HBB variant) remains the standard clinical tool for reproductive counseling, and is separate from the modifier gene context discussed on this page.
People who are curious about their genetic profile in this area should bring results to a qualified healthcare provider or genetic counselor who can integrate the information with their full medical picture. ExomeDNA's reports are educational wellness products and are not designed to substitute for clinical evaluation.
Several evidence-based strategies exist for managing haemolysis in sickle cell anemia under medical supervision. These include hydroxyurea therapy, which increases HbF production, and newer approved therapies. These are clinical decisions made in partnership with a hematologist and are outside the scope of educational genetic reporting.
Related traits and genes
Sickle cell anemia connects to a broader network of traits and genes related to red blood cell biology, hemoglobin production, and blood health.
BCL11A is the top modifier gene for HbF regulation and connects sickle cell anemia to fetal hemoglobin persistence traits. Variants in BCL11A that increase HbF are associated with milder sickle cell phenotypes and with hereditary persistence of fetal hemoglobin (HPFH) — a benign condition in which HbF remains elevated into adulthood.
HBG2 (gamma-globin 2) is expressed in the fetal liver, spleen, and bone marrow. Along with HBG1, it encodes the gamma-globin chains of fetal hemoglobin. Variation in HBG2 regulatory regions is associated with differences in adult HbF levels across multiple hemoglobinopathy contexts.
NPRL3 connects this hemoglobin-adjacent biology to the mTOR signaling pathway, which regulates cellular growth and metabolism. NPRL3 is part of the GATOR1 complex, a negative regulator of mTORC1. Its proximity to the alpha-globin cluster on chromosome 16 may reflect both positional and functional relationships with red blood cell biology.
Related traits on ExomeDNA that share overlapping genetic architecture include red blood cell count, hemoglobin levels, and mean corpuscular volume. Cross-category connections include pulmonary hypertension risk and iron absorption and metabolism. For a deeper look at the BCL11A gene and its role in blood cell development, see the BCL11A gene page.
Frequently asked questions
What does this trait measure? This genetic trait captures variation associated with haemolysis — the breakdown of red blood cells — specifically in the context of sickle cell anemia. It reflects modifier gene influences on how severely haemolysis occurs in people affected by sickle cell disease, rather than primary susceptibility to developing sickle cell anemia itself.
Does having variants in these genes mean someone has sickle cell anemia? No. Sickle cell anemia is caused by a specific variant in the HBB gene and follows a Mendelian inheritance pattern distinct from the population-level GWAS associations discussed here. The modifier variants addressed on this page influence severity and haemolytic rate in those already affected, not whether someone develops the condition.
What role does fetal hemoglobin play in sickle cell anemia severity? Fetal hemoglobin (HbF) inhibits the polymerization of sickle hemoglobin (HbS), which is the molecular process that causes red blood cells to take on their characteristic crescent shape. People who naturally maintain higher HbF levels into adulthood — often because of variants near BCL11A or in the beta-globin cluster — generally experience milder sickle cell phenotypes, including reduced haemolysis.
Why is BCL11A important for sickle cell research? BCL11A acts as a molecular switch that represses fetal hemoglobin production after birth. When BCL11A function is reduced in red blood cell precursors, HbF production increases. This has made BCL11A a key target for gene therapy and gene editing strategies designed to restore HbF as a way to reduce sickle cell severity. Variants in BCL11A that naturally reduce its activity are associated with higher HbF and milder disease.
What does NPRL3 do and why is it relevant here? NPRL3 is part of the GATOR1 complex, which suppresses mTORC1 signaling. It is located near the alpha-globin gene cluster on chromosome 16. Its identification as the top-ranked locus in the haemolysis GWAS dataset suggests a potential regulatory connection between the alpha-globin region and haemolytic severity, though the mechanistic pathway requires further research to characterize fully.
Should someone with sickle cell anemia use this trait result for clinical decisions? No. Clinical management of sickle cell anemia — including monitoring for haemolysis, decisions about hydroxyurea or other therapies, and evaluation of complications — should be conducted with a hematologist or sickle cell specialist. Genetic wellness reports provide educational context but do not replace clinical evaluation, laboratory monitoring, or specialist guidance.
References
- Milton JN, et al. Genetic determinants of haemolysis in sickle cell anaemia. British journal of haematology. 2013. PMID: 23406172.
Data sources
- GWAS Catalog (NHGRI-EBI, accessed 2026-05-29)
- Open Targets Platform (CC0 1.0, accessed 2026-05-29)
- ClinVar (NCBI, accessed 2026-05-29) — entries at 2-star review status or above
- ClinGen Gene-Disease Validity (CC0 1.0, accessed 2026-05-29)
By the ExomeDNA Research Team
FDA wellness compliance statement: This content is intended for educational and informational purposes only. ExomeDNA's genetic reports are wellness products, not clinical tools, and are not substitutes for professional health guidance. Genetic variants discussed reflect population-level associations from published research. Individual genetic results should be interpreted with the guidance of a qualified healthcare provider.