rs6742078 — UGT1A10

Intronic variant in the UGT1A gene cluster (chromosome 2q37) strongly associated with serum bilirubin levels; the T allele reduces glucuronidation capacity, causing mild hyperbilirubinemia and a dose-dependent increase in pigment gallstone risk

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UGT1A10 rs6742078 — The Bilirubin Setpoint Variant

Every red blood cell that reaches the end of its 120-day lifespan releases haemoglobin, which breaks down into biliverdin and then bilirubin11 bilirubin
a yellow pigment that must be conjugated with glucuronic acid by UGT1A enzymes to become water-soluble and excretable in bile. The UGT1A gene cluster on chromosome 2q37 encodes nine UDP-glucuronosyltransferase enzymes — UGT1A1 through UGT1A10 — all sharing exons 2–5 but each with a unique exon 1 that confers distinct substrate preferences. rs6742078 sits in an intronic region of UGT1A10, near the 3′ end of the cluster (chr2:233,763,993, GRCh38), but the variant's GWAS signal spans the entire cluster and tracks most strongly with UGT1A1-mediated bilirubin glucuronidation.

The T allele acts as a proxy marker for reduced-activity haplotypes at UGT1A1 — the enzyme responsible for ~95% of bilirubin conjugation in humans. Carrying one or two T alleles shifts the individual bilirubin setpoint upward, producing mildly elevated unconjugated bilirubin without liver damage. This condition, when sufficiently elevated, is termed Gilbert syndrome22 Gilbert syndrome
a benign constitutional hyperbilirubinemia affecting 3–10% of Western populations.

The Mechanism

The UGT1A cluster uses a unique splicing architecture: a shared promoter drives alternative exon 1 usage, so variants anywhere in the cluster can affect regulation of multiple family members simultaneously. rs6742078 lies 9,307 bp from the end of UGT1A10 and likely exerts its effects through linkage disequilibrium with functional variants in the UGT1A1 promoter or coding sequence — most prominently, the TA-repeat polymorphism (UGT1A1*28) and coding variants including Gly71Arg (rs4148323, UGT1A1*6). These functional variants reduce the enzyme's rate of bilirubin glucuronidation, causing unconjugated bilirubin to accumulate in plasma.

The downstream consequence in the biliary system is that elevated unconjugated bilirubin spills into bile at higher concentrations, exceeding its solubility threshold and promoting nucleation of calcium bilirubinate crystals33 nucleation of calcium bilirubinate crystals
the seed particles that grow into black pigment gallstones. This is a dose-dependent chemical process: the higher the bilirubin, the greater the supersaturation of bile, and the greater the stone-formation risk.

The Evidence

Bilirubin levels: The landmark meta-analysis of 9,464 participants from three European cohorts44 meta-analysis of 9,464 participants from three European cohorts
Johnson et al. Human Molecular Genetics, 2009 identified the UGT1A1 locus as the dominant genetic determinant of serum bilirubin, explaining ~18% of total variation — an exceptionally large proportion for a quantitative trait. The association p-value was <5×10⁻³²⁴, effectively impossible to obtain by chance. A subsequent GWAS in 10,282 Han Chinese55 GWAS in 10,282 Han Chinese
Dai et al. Genetic Epidemiology, 2013 confirmed rs6742078 at combined P=1.44×10⁻⁸⁹, explaining 9.2% of bilirubin variation in East Asians.

Gallstone disease (causal, Mendelian randomization): Stender et al. JAMA Internal Medicine, 201366 Stender et al. JAMA Internal Medicine, 2013 used rs6742078 as a Mendelian randomization instrument in 61,212 Danish participants followed for up to 34 years. The T allele raised bilirubin by 16% per copy, with TT homozygotes averaging 90% higher bilirubin than GG homozygotes. Over 34 years, 3,374 participants developed symptomatic gallstone disease: GT heterozygotes had HR 1.09 (95% CI 1.02–1.17) and TT homozygotes HR 1.22 (95% CI 1.09–1.36) versus GG. Because Mendelian randomization uses a randomly assigned genetic instrument, these hazard ratios are interpreted as causal — elevated bilirubin itself is lithogenic, not merely a marker.

Sex-specific gallstone risk: Buch et al. Gastroenterology, 201077 Buch et al. Gastroenterology, 2010 found a striking sex difference in 2,606 German cases and 1,121 controls: male TT homozygotes had OR 2.34 (P=2.1×10⁻⁷) for gallstone disease, while women showed no significant association (P=0.47). The sex-specific finding replicated in South American samples (OR 2.19 in men, P=0.046; no effect in women). Combined, the UGT1A and ABCG8 loci attributed 21.2% of population- attributable risk in men. The biological basis of sex specificity is not fully understood, but oestrogen independently promotes cholesterol saturation in bile, potentially masking the bilirubin-mediated pathway in women while amplifying it in men who lack this alternative lithogenic mechanism.

Circulating cell-free DNA: An unanticipated finding from a GWAS of cell-free DNA levels88 GWAS of cell-free DNA levels
Jylhävä et al. PLoS One, 2012 was that all 110 genome-wide significant SNPs for serum cf-DNA mapped to the UGT1A locus on 2q37, strongly implicating UGT1A1-associated glucuronidation pathways in the clearance of extracellular DNA from circulation.

Practical Actions

The TT genotype confers a clear, modifiable gallstone risk. Black pigment gallstones (the type driven by elevated unconjugated bilirubin) can cause biliary colic, cholecystitis, and pancreatitis when they obstruct the bile duct. Monitoring bilirubin and using targeted lifestyle and dietary measures to reduce gallstone nucleation is the evidence-based management approach.

For heterozygous GT carriers, the bilirubin increase (~16%) is modest. Awareness and periodic bilirubin monitoring during routine blood work is sufficient; no specific interventions are required unless other risk factors for gallstones are present.

Interactions

UGT1A1 coding variants (rs4148323): rs6742078 acts as a tag SNP for the broader UGT1A reduced-activity haplotype. Individuals who carry both rs6742078 T alleles and the UGT1A1*6 variant (rs4148323 A allele, common in East Asians) or UGT1A1*28 (rs887829, common in Europeans and Africans) have compounded reduction in bilirubin glucuronidation, producing more severe hyperbilirubinemia and greater gallstone risk. These combinations represent true Gilbert syndrome.

SLCO1B3 (rs2417940): This organic anion transporter on chromosome 12p12 handles hepatic uptake of bilirubin and other organic anions. In the same Chinese GWAS, rs2417940 explained an additional 0.9% of bilirubin variation. Combined reduced-activity genotypes at both loci may produce additive hyperbilirubinemia, though direct interaction data for this pair are limited.

Genotype Interpretations

What each possible genotype means for this variant.

GG Normal Bilirubin Normal

Normal UGT1A activity — typical serum bilirubin levels

You carry two copies of the reference G allele at rs6742078, which is associated with normal UGT1A glucuronidation capacity and typical serum bilirubin levels. About 45% of people in European and global populations share this genotype. Your baseline bilirubin is not elevated by this variant, and your risk of pigment gallstone disease from this locus is not increased.

GT Mild Hyperbilirubinemia Intermediate

One T allele — modestly elevated bilirubin and mildly increased gallstone risk

The T allele at rs6742078 tags reduced-activity haplotypes at UGT1A1, the enzyme responsible for conjugating bilirubin to make it water-soluble for biliary excretion. With one reduced-activity copy, bilirubin conjugation capacity is sufficient for normal conditions but operates with less reserve. Bilirubin levels may drift slightly higher during fasting, febrile illness, or intense exercise. In bile, unconjugated bilirubin can exceed solubility and nucleate calcium bilirubinate crystals — the seeds of black pigment gallstones. The 9% increase in hazard ratio is small enough that most GT carriers never develop gallstones, but awareness supports targeted monitoring.

TT Elevated Bilirubin High

Two T alleles — substantially elevated bilirubin and significantly increased gallstone risk

The TT genotype at rs6742078 represents the homozygous reduced-activity state of the UGT1A cluster. Both copies carry the haplotype that impairs bilirubin glucuronidation, producing sustained unconjugated hyperbilirubinemia. This is the genetic basis of Gilbert syndrome when caused by this locus.

Bilirubin at these levels is an antioxidant — it scavenges reactive oxygen species — which has led to speculation about cardiovascular protection. However, the prospective evidence is mixed and does not support a clinically meaningful cardioprotective benefit from the rs6742078 TT genotype alone.

The primary actionable concern is pigment gallstone formation. Unconjugated bilirubin that exceeds its solubility in bile precipitates as calcium bilirubinate crystals, which nucleate and grow into black pigment stones. This is a slow, chemical process: most TT carriers develop elevated bilirubin in their teens or twenties but do not develop symptomatic gallstones until their forties or beyond. The sex-specific data suggest men face roughly twice the gallstone risk of women with this genotype — the reason is not fully established but may relate to sex-hormone differences in bile composition.

Additionally, if you ever require chemotherapy with irinotecan or HIV therapy with atazanavir, inform your treating physician. These drugs are metabolised by UGT1A1, and reduced enzyme activity increases the risk of toxicity — particularly severe neutropenia with irinotecan. The UGT1A1 pharmacogenomic risk is primarily driven by the *28 and *6 coding variants, but the TT genotype at rs6742078 tags the same reduced-activity haplotype.