rs73885319 — APOL1 G1 S342G

APOL1 G1 — Trypanosome Resistance at the Cost of Kidney Disease

The APOL1 gene encodes apolipoprotein L1¹¹ apolipoprotein L1
APOL1 is a serum protein that kills trypanosomes — the parasites causing African sleeping sickness — by forming pores in their membranes, a critical component of the innate immune defense against Trypanosoma brucei²² Trypanosoma brucei
The single-celled parasite transmitted by tsetse flies that causes African sleeping sickness, fatal if untreated. The rs73885319 variant (c.1024A>G) produces a serine-to-glycine change at position 342, located in the SRA-interacting domain³³ SRA-interacting domain
The serum resistance-associated (SRA) domain is where the trypanosome protein binds to neutralize APOL1; G1 variants alter this binding site. This variant is one half of the G1 risk haplotype — the other being rs60910145 (p.Ile384Met) — which are in near-complete linkage disequilibrium and almost always inherited together.

The Mechanism

APOL1 normally circulates in trypanosome lytic factor (TLF)⁴⁴ trypanosome lytic factor (TLF)
HDL-like particles in human blood that kill non-human-infective trypanosomes by forming ion channels in parasite membranes complexes and kills trypanosomes by inserting into their endosomal membranes and forming cation-selective pores. The human-infective subspecies T.b. rhodesiense evolved a serum resistance-associated (SRA) protein⁵⁵ serum resistance-associated (SRA) protein
A virulence factor that binds APOL1's SRA-interacting domain, neutralizing its trypanolytic activity and enabling the parasite to survive in human blood that binds and neutralizes wild-type APOL1. The G1 variant (S342G + I384M) alters the SRA-binding site so the parasite can no longer neutralize APOL1, restoring trypanolytic activity against T.b. rhodesiense.

The kidney disease mechanism involves the same pore-forming function turned against host cells. In podocytes — the specialized kidney cells that maintain the glomerular filtration barrier⁶⁶ glomerular filtration barrier
The three-layer filter in the kidney that allows waste to pass into urine while retaining proteins and blood cells — G1-variant APOL1 causes excessive cation flux, cell swelling, and ultimately podocyte injury and death⁷⁷ podocyte injury and death
G1/G2 APOL1 expressed in cell culture causes potassium efflux, sodium influx, cell swelling, and cytotoxicity at rates far exceeding wild-type APOL1. This damage is recessive: one copy of G1 is tolerated because sufficient wild-type APOL1 maintains normal podocyte function, but two risk alleles (G1/G1, G1/G2, or G2/G2) overwhelm the protective capacity.

The Evidence

The landmark 2010 discovery⁸⁸ landmark 2010 discovery
Genovese et al. Association of trypanolytic ApoL1 variants with kidney disease in African Americans. Science 2010; 329:841-845 identified APOL1 G1 and G2 as the causal variants behind the excess kidney disease burden in African Americans, with odds ratios of 10.5 (95% CI: 6.0-18.4) for focal segmental glomerulosclerosis (FSGS) and 7.3 (95% CI: 5.6-9.5) for hypertension-attributed end-stage kidney disease in those carrying two risk alleles. Subsequent studies expanded the associations: OR 29 for HIV-associated nephropathy (HIVAN)⁹⁹ OR 29 for HIV-associated nephropathy (HIVAN)
Kopp et al. JASN 2011 — APOL1 genetic variants in FSGS and HIV-associated nephropathy, OR 5.4 for lupus collapsing glomerulopathy, and a hazard ratio of 1.88¹⁰¹⁰ hazard ratio of 1.88
Parsa et al. NEJM 2013 — APOL1 risk variants, race, and progression of CKD; HR 1.88 (95% CI 1.2-2.9) for composite renal endpoint for CKD progression in the AASK and CRIC cohorts.

The risk is strongly recessive¹¹¹¹ strongly recessive
Approximately 10-15% of African Americans carry two APOL1 risk alleles (G1/G1, G1/G2, or G2/G2), conferring substantially elevated kidney disease risk; one copy confers trypanosome resistance without kidney risk. Heterozygous carriers have the evolutionary advantage — trypanosome resistance — without the kidney cost, a classic example of balancing selection¹²¹² balancing selection
Similar to sickle cell trait: heterozygous carriers are protected against malaria, while homozygotes develop sickle cell disease comparable to sickle cell and malaria.

The G1 risk allele frequency is approximately 23% in African Americans and up to 40% in West African populations (Yoruba), but is essentially absent (<0.01%) in European, East Asian, and South Asian populations. This means APOL1-associated kidney disease is almost exclusively a condition of African-ancestry populations — a critical factor in the 3-4 fold higher rate of end-stage kidney disease among African Americans compared to European Americans.

Practical Implications

The clinical impact depends entirely on how many risk alleles you carry. One copy of the G1 variant (AG genotype) provides the trypanosome resistance benefit with no measurable kidney risk — this is the evolutionary "sweet spot." Two risk alleles (GG, or compound heterozygous with G2) create substantial kidney disease susceptibility, though penetrance is incomplete: most two-risk-allele carriers never develop kidney disease, suggesting additional triggers (infections like HIV, inflammatory conditions, or other genetic modifiers) are required.

For carriers of two risk alleles, the most important actions are kidney function monitoring via regular estimated GFR and urine albumin-to-creatinine ratio (UACR) testing, aggressive blood pressure management to protect glomerular function, and avoiding nephrotoxic agents when possible. The N264K protective modifier¹³¹³ N264K protective modifier
Gupta et al. Nat Commun 2023 — APOL1 p.N264K variant on a G2 haplotype reduces FSGS risk by nearly 100% (rs2239785) can ameliorate G2-associated risk, though it does not modify G1 risk.

Interactions

The G1 risk haplotype requires BOTH rs73885319 (S342G) and rs60910145 (I384M) — these two missense variants are in near-complete linkage disequilibrium and together define the G1 allele. APOL1 kidney risk follows a recessive model where any combination of two risk alleles is pathogenic: G1/G1 homozygosity, G1/G2 compound heterozygosity (with rs71785313, the G2 6-bp deletion), or G2/G2 homozygosity all confer similar risk. A compound action covering the G1+G2 interaction (rs73885319 + rs71785313) should be created when both variants are in the database, as the combined genotype assessment is critical for risk stratification.

The N264K modifier variant (rs2239785) is co-inherited exclusively with the G2 allele and reduces G2-associated FSGS risk by approximately 100%, effectively converting a G2 haplotype to behave like G0 (wild-type). This modifier does not affect G1-associated risk. HIV infection, lupus, and other inflammatory conditions act as "second hits" that dramatically increase penetrance in two-risk-allele carriers.