Disease relevance of MLH3

• Little evidence for involvement of MLH3 in colorectal cancer predisposition [1].
• Analysis of hMLH3 expression in colon cancer cell lines indicated that the protein levels vary substantially and independently of hMLH1 [2].
• A role for MLH3 in hereditary nonpolyposis colorectal cancer [3].
• MLH3 mutation in endometrial cancer [4].
• Mutation screening of mismatch repair gene Mlh3 in familial esophageal cancer [5].

High impact information on MLH3

• We identified ten different germline MLH3 variants, one frameshift and nine missense mutations, in 12 patients suspected of HNPCC [3].
• Cells in culture stably expressing a dominant-negative MLH3 protein exhibit microsatellite instability [6].
• MLH3 is more similar to mismatch repair proteins from yeast, plants, worms and bacteria than to any known mammalian protein, suggesting that its conserved sequence may confer unique functions in mice and humans [6].
• The DNA repair picture in humans becomes more complete with the identification of MLH3, a homologue of MutL and a heterodimeric partner of MLH1 [7].
• The DNA mismatch-repair MLH3 protein interacts with MSH4 in meiotic cells, supporting a role for this MutL homolog in mammalian meiotic recombination [8].

Biological context of MLH3

• To further evaluate the role of MLH3 in CRC predisposition, we analyzed 30 Finnish CRC cases for germline mutations by sequencing [1].
• MLH1 and MLH3 have been shown to mark late-meiotic nodules that correlate well with--and are thought to give rise to--the sites of reciprocal recombination between homologous chromosomes, which suggests a possible 10-fold variation in the processing of nascent recombination events [9].
• Furthermore, MLH3 nonsense mutations were identified in two of 12 microsatellite stable (MSS) tumors with 14q24 loss of heterozygosity [10].
• There was no evidence of MLH3 promoter methylation based on combined bisulfite restriction analysis [4].
• The results of the various methods are self-consistent and the equilibrium model includes the species MLH4, MLH3, MLH2, MLH, ML, MLH(-1), MLH(-2) and several hydrolysis products (where H4L denotes oxidized glutathione); individual formation constants and spectra are given [11].

Anatomical context of MLH3

• However, at pachynema, when chromosomes are fully paired, we find significant heterogeneity in the localization of the MutL homologs, MLH1 and MLH3, among human oocyte populations [9].
• To investigate whether MLH3 also acts during meiotic recombination, we analyzed its expression in mammalian germ cells [8].
• The MLH3 gene is expressed in mouse meiotic cells and in human testis, and, as revealed by immunoprecipitation assays, the MLH3 protein is found in mouse spermatocytes [8].

Other interactions of MLH3

• We now show that the hMLH1/hMLH3 heterodimer, hMutLgamma, can also assist in the repair of base-base mismatches and single extrahelical nucleotides in vitro [2].
• We further demonstrate that the meiosis-specific MSH4 protein, known to participate to meiotic recombination, is co-immunoprecipitated with MLH3 from mouse meiotic cell extracts [8].
• Similar to MSH3 and MSH6, MLH3 harbors mononucleotide repeats, ie, (A(6))-(A(9)), in its coding region, which makes it a putative target for somatic mutations in MSI-positive tumors [12].
• Two additional MMR genes, MLH3 and PMS1, have also been proposed to play a role in Lynch syndrome predisposition, but the clinical significance of mutations in these genes is less clear [13].
• Interestingly, these residues in hPMS2 and hMLH3 may form coiled-coil structures as predicted by the MULTICOIL program [14].

Analytical, diagnostic and therapeutic context of MLH3

• We screened 70 index patients suggestive of a genetic predisposition for germ-line mutations in hMLH3 with denaturing high-performance liquid chromatography [15].
• Our immunofluorescence analyses indicated that when all the three MutL homologues are natively expressed in human cells, endogenous hMLH1 and hPMS2 localize in the nucleus, whereas hMLH3 stays in the cytoplasm [16].

References