Genetic Instability Caused by Loss of MutS Homologue 3 in Human Colorectal Cancer

Abstract

Microsatellite instability (MSI) is a hallmark of mismatch repair (MMR) deficiency. High levels of MSI at mononucleotide and dinucleotide repeats in colorectal cancer (CRC) are attributed to inactivation of the MMR genes, hMLH1 and hMSH2. CRC with low levels of MSI (MSI-L) exists; however, its molecular basis is unclear. There is another type of MSIelevated microsatellite alterations at selected tetranucleotide repeats (EMAST)-where loci containing [AAAG] n or [ATAG] n repeats are unstable. EMAST is frequent in non-CRCs; however, the incidence of EMAST and its cause in CRC is not known. Here, we report that MutS homologue 3 (MSH3) knockdown or MSH3-deficient cells exhibit the EMAST phenotype and low levels of mutations at dinucleotide repeats. About 60% of 117 sporadic CRC cases exhibit EMAST. All of the cases defined as MSI-H (16 cases) exhibited high levels of EMAST. Among 101 non-MSI-H cases, all 19 cases of MSI-L and 35 of 82 cases of MSS exhibited EMAST. Although non-MSI-H CRC tissues contained MSH3-negative tumor cells ranging from 2% to 50% of the total tumor cell population, the tissues exhibiting EMAST contained more MSH3-negative cells (average, 31.5%) than did the tissues not exhibiting EMAST (8.4%). Taken together, our results support the concept that MSH3 deficiency causes EMAST or EMAST with low levels of MSI at loci with dinucleotide repeats in CRC. [Cancer Res 2008;68(20):8465-72]

MutSβ exceeds MutSα in dinucleotide loop repair

“…HCT116 cells, which are deficient in MLH1 and MSH3, complemented with MLH1 through the addition of chromosome 3 have been shown to demonstrate mononucleotide repeat stability but still a low level of dinucleotide and a high level of tetranucleotide repeat instability suggesting a problem in MSH3. Although, the tetranucleotide repeat markers represented a level of instability five times higher than dinucleotide repeats supporting the functional overlap of MutSb and MutSa in IDL2 repair, low dinucleotide repeat instability was caused by defected MutSb (MSH3) (Haugen et al, 2008). The microsatellite stability was indeed reversible by complementing HCT116 cells with both chromosomes 3 and 5, hence expressing both lost proteins MLH1 and MSH3 (Haugen et al, 2008).…”

“…However, the degree and type of MSI differ from high to low and between mono-, di-, tri-and tetranucleotide instability or elevated microsatellite alterations at selected tetranucleotide repeats (EMAST) (Peltomäki and Vasen, 2004;Plaschke et al, 2004;Haugen et al, 2008) depending on the MMR gene affected. MLH1-and MSH2-deficient tumours are characterised by both mono-and dinucleotide repeat instability, whereas the level of MSI is lower in MSH6-deficient tumours (Bhattacharyya et al, 1995;Papadopoulos et al, 1995).…”

“…MSH6-deficient cells are unable to repair single base mismatches, whereas they retain proficiency to repair two, three and four base loops (Drummond et al, 1995;Risinger et al, 1996;Umar et al, 1997), thus, causing only mononucleotide repeat instability in tumours (Wagner et al, 2001;Plaschke et al, 2004). Recently, EMAST and also low dinucleotide repeat instability have been associated with MSH3 deficiency both in tumour cell lines and in sporadic colorectal tumours (Haugen et al, 2008). The type of MSI seems to be dependent on the substrate specificities of the MMR protein affected.…”

973 Relationship of EMAST and Microsatellite Instability Among Patients With Rectal Cancer

“…While the etiology of EMAST is still not clear, general clues point toward some epigenetic relaxation of DNA MMR as one possibility for its cause. It has been shown that (a) colon cancers have heterogeneous expression of the DNA MMR protein MSH3, 8 suggesting an acquired loss and (b) oxidative stress has been shown to reduce the expression of MSH6 and PMS2, causing faulty DNA MMR that can be corrected when the stress is removed. 9,28 Additionally, we show in the present study a linkage between EMAST and chronic inflammation, further suggesting that inflammation may fuel the occurrence of EMAST in rectal and possibly other tumors.…”

“…8 Although the underlying mechanism behind EMAST remains unknown, the authors suggests that MSH3, an MMR gene involved in repair of longer repeat sequences such as those greater than dinucleotide repeats, may be linked to EMAST due to its "heterogeneous" immunohistochemical expression in some colon cancers. 8 This type of pattern suggests an acquired defect, as no germ line mutation in MSH3 has ever been demonstrated. 2 Some MMR genes, in particular MSH6 and PMS2, can be downregulated in the setting of inflammation, 9 suggesting a potential mechanism for "relaxation" of DNA MMR function.…”