APOL1 G2 — The Trypanosome Shield That Costs the Kidneys

The APOL1 gene encodes apolipoprotein L1¹¹ apolipoprotein L1
a component of HDL particles that functions as the human innate immune system's weapon against African trypanosomes. APOL1 protein forms ion channels in the trypanosome membrane, killing the parasite. But Trypanosoma brucei rhodesiense evolved a countermeasure — the serum resistance-associated (SRA) protein²² serum resistance-associated (SRA) protein
binds to wild-type APOL1 and neutralizes it, allowing the parasite to survive in human blood.

The G2 variant (rs71785313) is a 6-base-pair in-frame deletion³³ 6-base-pair in-frame deletion
c.1164_1169del, removing asparagine-388 and tyrosine-389 in the C-terminal domain of APOL1. This deletion alters the SRA-binding site, preventing the trypanosome's defense protein from neutralizing APOL1. The result: carriers of even one G2 allele can kill T. b. rhodesiense⁴⁴ carriers of even one G2 allele can kill T. b. rhodesiense
five-fold dominant protective association against infection. This is the evolutionary advantage that drove G2 to high frequency in sub-Saharan Africa over the past 5,000-10,000 years.

The trade-off is kidney disease. In the homozygous state (G2/G2) or compound heterozygous with G1 (G1/G2)⁵⁵ homozygous state (G2/G2) or compound heterozygous with G1 (G1/G2)
a recessive model where two risk alleles are required, the altered APOL1 protein becomes toxic to kidney podocytes — the specialized cells that form the filtration barrier. This toxicity drives focal segmental glomerulosclerosis (FSGS), HIV-associated nephropathy (HIVAN), and hypertension-attributed end-stage kidney disease at dramatically elevated rates. The pattern mirrors sickle cell trait: one copy protects against a parasite, two copies cause disease.

The Mechanism

APOL1 is expressed in kidney podocytes, vascular endothelium, and circulating HDL particles⁶⁶ kidney podocytes, vascular endothelium, and circulating HDL particles
with highest expression in the kidney. The G2 deletion removes two amino acids from the C-terminal domain that normally interacts with the SRA protein of trypanosomes. While this disruption is beneficial for parasite killing, the altered protein also gains cytotoxic properties in kidney cells⁷⁷ cytotoxic properties in kidney cells
depleting cellular potassium and activating stress-activated protein kinases (SAPK/JNK).

In podocytes, the G2 variant forms aberrant ion channels in cell membranes and mitochondria⁸⁸ aberrant ion channels in cell membranes and mitochondria
opening the mitochondrial permeability transition pore and inducing cell death. Podocyte loss is irreversible — these cells do not regenerate — and progressive podocyte depletion leads to proteinuria, glomerulosclerosis, and eventual kidney failure. The disease requires a "second hit" (infection, inflammation, interferon signaling) to manifest, which explains why only 15-20% of individuals with two risk alleles develop kidney disease⁹⁹ 15-20% of individuals with two risk alleles develop kidney disease
the remaining ~80% never progress.

The Evidence

The landmark discovery by Genovese et al. (2010) in Science¹⁰¹⁰ landmark discovery by Genovese et al. (2010) in Science
mapping APOL1 risk variants after decades of attributing the association to nearby MYH9 identified G1 and G2 as the true causal variants for kidney disease disparities in African Americans. The original study found odds ratios of 10.5 for FSGS and 7.3 for hypertension-attributed ESKD in those with two risk alleles.

Kopp et al. (2011)¹¹¹¹ Kopp et al. (2011)
JASN study of 271 African American cases and 939 controls refined the risk estimates: OR=17 for FSGS, OR=29 for HIVAN, with APOL1-associated FSGS showing earlier age of onset and faster progression to ESKD. The recessive model was confirmed — heterozygous carriers showed no significant kidney disease risk.

The AASK/CRIC analysis by Parsa et al. (2013) in the New England Journal of Medicine¹²¹² AASK/CRIC analysis by Parsa et al. (2013) in the New England Journal of Medicine
693 Black patients with hypertensive CKD in AASK and 2,955 patients in CRIC demonstrated that APOL1 high-risk genotype accelerated CKD progression with a hazard ratio of 1.88 (P<0.001) for the composite endpoint of ESKD or doubling of serum creatinine. The effect was present regardless of diabetes status.

A breakthrough in treatment came with inaxaplin (VX-147)¹³¹³ inaxaplin (VX-147)
a small molecule that directly inhibits APOL1 channel function. The Phase 2a trial in 13 patients with two APOL1 risk variants and biopsy-proven FSGS showed a 47.6% mean reduction in proteinuria at 13 weeks, providing the first genotype-targeted therapy for APOL1 nephropathy.

Practical Actions

APOL1 G2 kidney risk follows a recessive pattern — one copy confers trypanosome protection with minimal kidney risk, while two copies (G2/G2 or G1/G2 compound heterozygous) create significant kidney disease susceptibility. If you carry two risk alleles, proactive monitoring is essential because early detection dramatically improves outcomes.

The key biomarkers are urine albumin-to-creatinine ratio (UACR) and estimated glomerular filtration rate (eGFR)¹⁴¹⁴ urine albumin-to-creatinine ratio (UACR) and estimated glomerular filtration rate (eGFR)
early proteinuria and declining filtration are the first signs of APOL1 nephropathy. Annual screening can catch disease years before symptoms appear. Blood pressure control is critical — hypertension accelerates podocyte loss in APOL1 high-risk kidneys.

For those with two risk alleles considering kidney donation, APOL1 genotyping is now recommended before living donation, as donors with high-risk genotypes face accelerated post-donation kidney function decline.

Interactions

APOL1 G2 and G1 (rs73885319 + rs60910145)¹⁵¹⁵ G1 (rs73885319 + rs60910145)
the other APOL1 risk haplotype comprising S342G and I384M missense variants interact through compound heterozygosity. G1/G2 compound heterozygotes carry the same kidney disease risk as G1/G1 or G2/G2 homozygotes — any combination of two risk alleles activates the recessive disease mechanism.

The recently discovered APOL1 p.N264K variant¹⁶¹⁶ APOL1 p.N264K variant
a missense change in the pore-lining domain acts as a powerful modifier. When inherited in cis with G2 (on the same chromosome), N264K abolishes the cytotoxic effect, strongly protecting against FSGS and kidney disease even in compound heterozygotes. This modifier is co-inherited with G2 through a proximity recombination event and explains some of the incomplete penetrance.

HIV infection is a critical environmental modifier — HIVAN occurs almost exclusively in individuals with two APOL1 risk alleles¹⁷¹⁷ HIVAN occurs almost exclusively in individuals with two APOL1 risk alleles
OR=29 for HIVAN vs. controls. Interferon signaling upregulates APOL1 expression, and the amplified production of toxic G2 protein in podocytes drives rapid kidney destruction. Effective antiretroviral therapy substantially reduces but does not eliminate this risk.

Proposed compound actions for supervisor review:

1. APOL1 G2/G2 homozygous (rs71785313 DD)

• Genotypes: rs71785313 DD
• Combined effect: Two copies of G2 deletion — high-risk genotype for APOL1 nephropathy
• Evidence: OR=10-17 for FSGS, OR=29 for HIVAN, HR=1.88 for CKD progression
• Recommendation: Annual UACR + eGFR screening, strict BP control <130/80, nephrology referral if proteinuria detected, discuss inaxaplin eligibility
• Note: This is a single-genotype action, not compound — included here for visibility

2. APOL1 G1/G2 compound heterozygous

• Genotypes: rs73885319 (G1 component, if added to system) + rs71785313 DI
• Combined effect: Same high-risk phenotype as G2/G2 homozygous — any two APOL1 risk alleles
• Evidence: Identical ORs to homozygous risk genotypes (Genovese 2010, Kopp 2011)
• Recommendation: Same as G2/G2 — annual screening, BP control, nephrology awareness
• Note: Requires G1 SNPs (rs73885319, rs60910145) to be added to system first