rs1800734 — MLH1 -93G>A

MLH1 -93G>A — A Mismatch Repair Promoter Variant and Colorectal Cancer Risk

The MLH1 gene encodes a critical component of the DNA mismatch repair (MMR) system¹¹ DNA mismatch repair (MMR) system
A cellular proofreading mechanism that detects and corrects errors made during DNA replication, particularly insertions, deletions, and mismatched bases, the cell's proofreading machinery that corrects errors during DNA replication. When MMR fails, replication errors accumulate — especially in microsatellites²² microsatellites
Short repetitive DNA sequences (1-6 base pair repeats) scattered throughout the genome that are particularly prone to replication errors, producing a signature called microsatellite instability (MSI) that drives tumor formation. Germline mutations that disable MLH1 cause Lynch syndrome³³ Lynch syndrome
An inherited cancer predisposition syndrome (formerly HNPCC) caused by pathogenic mutations in MMR genes, conferring 40-80% lifetime colorectal cancer risk, the most common inherited colorectal cancer syndrome. The rs1800734 variant (-93G>A) is fundamentally different: it is a common, low-penetrance promoter polymorphism that does not disable MMR but subtly reduces MLH1 expression and predisposes the promoter to epigenetic silencing. It modestly increases colorectal cancer risk — primarily for the microsatellite-unstable subtype — without the severe clinical implications of Lynch syndrome.

The Mechanism

The -93G>A variant sits 93 base pairs upstream of the MLH1 transcription start site, within a region critical for promoter activity⁴⁴ promoter activity
The promoter is a DNA sequence that controls when and how much a gene is transcribed into mRNA; variants here affect gene expression levels rather than protein structure. The A allele disrupts binding of the transcription factor TFAP4⁵⁵ TFAP4
Transcription Factor AP-4, a basic helix-loop-helix protein that activates MLH1 expression by binding to a specific E-box motif in the promoter, reducing transcriptional activation of the MLH1 gene. Functional studies⁶⁶ Functional studies
Savio AJ and Bapat B. Modulation of transcription factor binding and epigenetic regulation of the MLH1 CpG island and shore by polymorphism rs1800734. Epigenetics, 2017 using chromatin immunoprecipitation demonstrated enriched TFAP4 occupancy at the G allele but not the A allele, directly linking the variant to reduced promoter activity.

Beyond acute transcriptional reduction, the A allele predisposes the MLH1 promoter to CpG island hypermethylation⁷⁷ CpG island hypermethylation
The addition of methyl groups to cytosine bases in CpG-rich regions near gene promoters, which silences gene expression; this is an epigenetic change acquired during tumor development, an epigenetic modification that completely silences the gene during tumor evolution. This two-hit process — reduced baseline expression from the germline variant followed by somatic methylation-driven silencing — mirrors the pathway seen in sporadic MSI colorectal cancers.

The Evidence

The association between rs1800734 and colorectal cancer has been replicated across multiple large studies, with a consistent pattern: the risk is concentrated in MSI-high tumors.

Raptis et al.⁸⁸ Raptis et al.
Raptis S et al. MLH1 -93G>A promoter polymorphism and the risk of microsatellite-unstable colorectal cancer. J Natl Cancer Inst, 2007 first demonstrated this in two independent Canadian populations, finding the AA genotype associated with dramatically increased risk of MSI-H colorectal cancer (OR 3.23 in Ontario, OR 8.88 in Newfoundland), while risk of microsatellite-stable tumors was unchanged.

Samowitz et al.⁹⁹ Samowitz et al.
Samowitz WS et al. The MLH1 -93 G>A promoter polymorphism and genetic and epigenetic alterations in colon cancer. Genes Chromosomes Cancer, 2008 studied 1,211 colon cancer cases and 1,968 controls, finding the AA genotype strongly associated with hallmarks of the MSI pathway: MLH1 promoter methylation (OR 4.16), BRAF V600E mutation (OR 4.26), and CpG island methylator phenotype (OR 3.44). These associations were absent in microsatellite-stable tumors, confirming the variant acts specifically through the MMR/MSI pathway.

The largest study to date, Whiffin et al.¹⁰¹⁰ Whiffin et al.
Whiffin N et al. MLH1-93G>A is a risk factor for MSI colorectal cancer. Carcinogenesis, 2011, genotyped 10,409 colorectal cancer cases and 6,965 controls. The per-allele OR for all colorectal cancer was 1.06 (P = 0.037), but when restricted to MSI-H cases, the OR rose to 1.39 (P = 1.45 x 10^-4). A meta-analysis combining this data with four prior studies reached genome-wide significance for the MSI-H association (P = 3.43 x 10^-12).

Notably, this association appears tissue-specific. Russell et al.¹¹¹¹ Russell et al.
Russell H et al. The MLH1 polymorphism rs1800734 and risk of endometrial cancer with microsatellite instability. Clin Epigenetics, 2020 found no association between rs1800734 and endometrial cancer with MSI, suggesting that MLH1 hypermethylation occurs through different mechanisms in different tissues.

Practical Implications

This variant is important to frame correctly. It is NOT a Lynch syndrome mutation and does not carry the high lifetime cancer risk (40-80%) associated with pathogenic MLH1 mutations. Instead, it is a common polymorphism (the A allele is found in 23% of Europeans and up to 48% of people of African descent) that modestly increases colorectal cancer risk, primarily for the MSI-H subtype.

For carriers of one or two A alleles, the key actionable step is enhanced colorectal cancer screening. Current guidelines recommend average-risk screening beginning at age 45, but carriers of this variant — particularly AA homozygotes — may benefit from initiating screening at age 40. Colonoscopy is preferred over stool-based tests because it directly visualizes and removes precancerous polyps.

Because the variant promotes MLH1 silencing through promoter methylation, maintaining robust one-carbon metabolism may help protect against aberrant methylation patterns. Adequate folate (as methylfolate), vitamin B12, and B6 support the methylation cycle that maintains normal DNA methylation homeostasis.

Interactions

rs1800734 operates in the broader landscape of colorectal cancer susceptibility. The 8q24 region variant rs6983267, one of the most consistently replicated colorectal cancer GWAS hits, acts through a distinct mechanism (enhancer-mediated MYC regulation) and contributes additive risk. The APC variant rs1801155 (I1307K) affects the Wnt signaling pathway. Individuals carrying risk alleles at multiple colorectal cancer loci accumulate risk multiplicatively, though no specific epistatic interaction between rs1800734 and these variants has been documented. A combined colorectal cancer polygenic risk profile incorporating MLH1, 8q24, and APC variants could stratify screening intensity more precisely than any single variant alone.