rs36053993 — MUTYH G396D

MUTYH G396D — Guarding Against Oxidative DNA Damage

Every day, reactive oxygen species assault your DNA, creating a specific form of damage called 8-oxoguanine¹¹ 8-oxoguanine
8-oxo-7,8-dihydroguanine (8-oxoG), one of the most common and mutagenic forms of oxidative DNA damage; it can mispair with adenine during replication, causing G:C to T:A transversion mutations (8-oxoG). Left uncorrected, 8-oxoG pairs with adenine instead of cytosine during DNA replication, producing permanent G:C to T:A transversion mutations²² transversion mutations
A type of point mutation where a purine is replaced by a pyrimidine or vice versa; transversions are more disruptive than transitions because they swap the chemical class of the base. The MUTYH gene encodes a DNA glycosylase that sits on the front line of base excision repair³³ base excision repair
A DNA repair pathway that fixes small, non-helix-distorting lesions; a glycosylase removes the damaged base, then downstream enzymes cut the backbone and fill in the correct nucleotide (BER), removing adenines that have been misincorporated opposite 8-oxoG. Without functional MUTYH, these transversion mutations accumulate — particularly in the APC tumor suppressor gene — setting the stage for colorectal cancer.

The Mechanism

The G396D variant (rs36053993) substitutes glycine with aspartate at position 396 of the MUTYH protein, located within the nudix hydrolase domain⁴⁴ nudix hydrolase domain
A catalytic domain found in a superfamily of enzymes that cleave nucleoside diphosphates; in MUTYH, this domain is critical for recognizing and excising mismatched adenines essential for substrate recognition and catalytic activity. This missense change substantially reduces the enzyme's ability to recognize and excise adenine mismatched with 8-oxoguanine. Functional studies show the G396D protein retains roughly 2% of normal glycosylase activity in vitro.

MUTYH-Associated Polyposis (MAP) follows autosomal recessive inheritance⁵⁵ autosomal recessive inheritance
Both copies of the gene must carry a pathogenic variant for the full disease phenotype; carriers with one mutant copy are largely protected by their remaining functional allele. Individuals with two pathogenic MUTYH alleles (biallelic carriers) develop tens to hundreds of colorectal adenomatous polyps, typically presenting between ages 40 and 60. Heterozygous carriers retain one fully functional copy and have near-normal DNA repair capacity.

The Evidence

The landmark 2002 discovery⁶⁶ landmark 2002 discovery
Al-Tassan N et al. Inherited variants of MYH associated with somatic G:C→T:A mutations in colorectal tumors. Nat Genet, 2002 by Al-Tassan and colleagues first linked biallelic MUTYH mutations to familial adenomatous polyposis with a characteristic excess of somatic G:C to T:A transversions in the APC gene. This established a novel mechanism for colorectal cancer: defective base excision repair leading to a specific mutational signature.

A large-scale meta-analysis by Theodoratou et al.⁷⁷ large-scale meta-analysis by Theodoratou et al.
Theodoratou E et al. A large-scale meta-analysis to refine colorectal cancer risk estimates associated with MUTYH variants. Br J Cancer, 2010 pooling data from multiple cohorts found that biallelic MUTYH carriers have a 28-fold increased risk (95% CI 6.95-115) for colorectal cancer, while monoallelic (heterozygous) Y179C carriers have an OR of 1.34 — a modest elevation that varies by variant.

A retrospective cohort study by Nieuwenhuis et al.⁸⁸ retrospective cohort study by Nieuwenhuis et al.
Nieuwenhuis MH et al. Evidence for accelerated colorectal adenoma-carcinoma progression in MUTYH-associated polyposis. Gut, 2012 calculated a cumulative colorectal cancer risk of 63% by age 60 for biallelic MUTYH carriers in a retrospective cohort, underscoring the critical importance of early and regular colonoscopy.

For heterozygous carriers, a multisite case-control study by Cleary et al.⁹⁹ multisite case-control study by Cleary et al.
Cleary SP et al. Germline MutY human homologue mutations and colorectal cancer: a multisite case-control study. Gastroenterology, 2009 found an adjusted OR of 1.48 (95% CI 1.02-2.16) for CRC, confirming that heterozygous carrier status confers a small but real increase in risk beyond the general population.

G396D and Y179C (rs34612342)¹⁰¹⁰ Y179C (rs34612342)
The most common MUTYH pathogenic variant in Europeans, accounting for roughly 50-55% of all pathogenic MUTYH alleles; G396D accounts for approximately 25-30% together account for approximately 75-85% of all pathogenic MUTYH alleles in European populations, making them the primary targets for clinical screening.

Practical Implications

For GG individuals: both copies of MUTYH function normally. Your base excision repair pathway handles oxidative DNA damage effectively at this locus.

For AG (heterozygous carrier) individuals: you carry one non-functional copy of MUTYH. Your remaining functional allele provides adequate DNA repair capacity. The primary concern is reproductive — there is a risk of passing the variant to children. If your partner also carries a MUTYH pathogenic variant, each child has a 25% chance of being biallelic. A modest CRC risk elevation (OR ~1.2) has been observed in carriers. Standard-age colonoscopy screening is sufficient, though starting at age 40 rather than 45 is reasonable given the carrier status.

For AA (biallelic) individuals: you have MUTYH-Associated Polyposis. Current ACG/NCCN guidelines¹¹¹¹ ACG/NCCN guidelines
Syngal S et al. ACG clinical guideline: Genetic testing and management of hereditary gastrointestinal cancer syndromes. Am J Gastroenterol, 2015 recommend colonoscopy every 1-2 years starting at age 25-30. If polyps are found, annual colonoscopy with polypectomy is indicated. Colectomy may be necessary if polyp burden becomes unmanageable endoscopically. Upper endoscopy for duodenal adenomas should begin at age 30-35 and be repeated every 1-5 years depending on findings.

Interactions

The most important interaction is with Y179C (rs34612342), the other common MUTYH pathogenic variant. Compound heterozygosity — carrying one G396D allele and one Y179C allele — produces the same MAP phenotype as homozygosity for either variant alone. If a user carries AG at rs36053993 (G396D carrier) and is also heterozygous for rs34612342 (Y179C carrier), they are effectively biallelic for MUTYH and should follow the full MAP surveillance protocol. This compound heterozygous state accounts for a significant proportion of MAP cases, since many affected individuals carry one of each variant rather than two copies of the same one.