rs2361502 — MROH2A

MROH2A rs2361502 — The Bilirubin Advantage: When Your Body's Waste Product Protects Your Heart

The yellowing of the skin in jaundice has made bilirubin notorious as a toxin. But at physiological concentrations, bilirubin functions as one of the body's most potent endogenous antioxidants — and your genome partly determines how much of it you carry. The rs2361502 variant in the MROH2A gene, situated directly adjacent to the UGT1A gene cluster¹¹ UGT1A gene cluster
A family of enzymes on chromosome 2q37.1 responsible for glucuronidating bilirubin — conjugating it so the liver can excrete it in bile on chromosome 2q37.1, is one of the genetic markers most consistently associated with circulating bilirubin concentrations in genome-wide association studies.

The Mechanism

rs2361502 is an intronic variant within MROH2A (maestro heat-like repeat family member 2A, also annotated as HEATR7B1), a gene whose protein product is poorly characterised at the functional level. The variant itself is unlikely to alter MROH2A protein function. Rather, because MROH2A sits immediately adjacent to the UGT1A locus, rs2361502 is a tag SNP²² tag SNP
A tag SNP is a marker variant in strong linkage disequilibrium with a functional variant; it tracks the functional variant across populations even though it has no effect itself that captures regulatory variation in UGT1A1 — specifically variation that modulates how efficiently the liver conjugates bilirubin for excretion. Carriers of the C allele have lower hepatic UGT1A1 activity in this region, resulting in slower bilirubin clearance and thus higher steady-state serum concentrations. Each copy of the C allele raises total serum bilirubin by roughly 0.096–0.098 mg/dL.

At physiological concentrations (0.2–1.2 mg/dL), bilirubin scavenges reactive oxygen species³³ reactive oxygen species
Unstable molecules such as superoxide, hydroxyl radical, and peroxynitrite produced during normal metabolism and during inflammation; excessive ROS damages DNA, lipids, and proteins and inhibits LDL oxidation, a key early step in atherosclerosis. It also suppresses the NADPH oxidase system and blunts inflammatory cytokine signalling.

The Evidence

The strongest direct evidence for rs2361502 comes from a genome-wide association study of 430 subjects with metabolic syndrome⁴⁴ genome-wide association study of 430 subjects with metabolic syndrome
Coltell O et al. Genome-Wide Association Study (GWAS) on Bilirubin Concentrations in Subjects with Metabolic Syndrome. Nutrients, 2019 drawn from the PREDIMED Plus-Valencia cohort. rs2361502 reached genome-wide significance (p=4×10⁻⁸), with each C allele increasing bilirubin by 0.096 mg/dL. A larger signal (p=7×10⁻²³, beta=0.098 mg/dL per C allele) emerged from the eMERGE network analysis by Bielinski et al., confirming the locus in a population with thousands of individuals. A PheWAS in HIV-positive individuals⁵⁵ PheWAS in HIV-positive individuals
Moore CB et al. Phenome-wide Association Study Relating Pretreatment Laboratory Parameters With Human Genetic Variants in AIDS Clinical Trials Group Protocols. Open Forum Infect Dis, 2015 from AIDS Clinical Trials Group protocols (n=2,547) independently replicated the bilirubin association (p=3×10⁻⁶).

The clinical significance of chronically elevated bilirubin is well-established. A meta-analysis of 16 observational studies in 175,911 participants⁶⁶ meta-analysis of 16 observational studies in 175,911 participants
Nano J et al. Association of circulating total bilirubin with the metabolic syndrome and type 2 diabetes. Diabetes Metab, 2016 found those in the highest bilirubin tertile had roughly 30% lower odds of metabolic syndrome (OR 0.70, 95% CI 0.62–0.78) and 23% lower odds of type 2 diabetes (OR 0.77, 95% CI 0.67–0.87) compared to those with the lowest bilirubin. Similarly, both retrospective and prospective studies consistently show an inverse relationship between bilirubin and cardiovascular disease events, with individuals carrying UGT1A1*28 — the canonical low-clearance allele for which rs2361502 serves as a marker — having significantly lower CVD risk⁷⁷ significantly lower CVD risk
Schwertner HA, Vitek L. Gilbert syndrome, UGT1A1*28 allele, and cardiovascular disease risk. Atherosclerosis, 2008 than wild-type carriers.

It should be noted that Mendelian randomization has not yet confirmed a strong causal role for bilirubin in cardiovascular protection: a 2015 analysis using rs6742078 as an instrumental variable found OR 1.03 (95% CI 0.98–1.09) for coronary heart disease per SD increase in bilirubin, suggesting some of the observational benefit may reflect confounding. The protective signal is robust observationally but causal confirmation awaits larger MR studies.

Practical Actions

Carriers of the CC genotype maintain chronically higher bilirubin concentrations, which appear to confer modest metabolic protection. The main clinical value of knowing this genotype is interpreting blood test results accurately: mildly elevated bilirubin (up to ~2 mg/dL in the absence of symptoms) is a benign, genetically-driven phenotype rather than a sign of liver disease. Additionally, behaviours and substances that increase oxidative stress — smoking, very high alcohol intake, or chronic exposure to environmental oxidants — will consume bilirubin's antioxidant capacity, eroding the genetic advantage.

CT heterozygotes have intermediate bilirubin and a similar but attenuated benefit.

TT homozygotes have the reference (lower) bilirubin. This does not imply disease risk; average bilirubin concentrations are already within the protective range for most people. Maintaining low oxidative stress through diet rich in antioxidant micronutrients (vitamins C and E, polyphenols) partially compensates for the reduced endogenous bilirubin buffering.

Interactions

rs2361502 is in linkage disequilibrium with several UGT1A1 variants, including rs887829 and rs6742078, which are the canonical functional markers for Gilbert syndrome and the UGT1A1*28 promoter haplotype. Carriers of multiple C-raising alleles at this locus (e.g., rs2361502 CC combined with rs887829 minor-allele homozygosity) likely experience additive bilirubin elevation. No published compound-genotype analysis of rs2361502 combined with other UGT1A1 variants has been reported, but the biological logic of additive UGT1A1 suppression is well established.