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From the Molecular and Population Genetics Laboratory,* London Institute, Cancer Research United Kingdom, London; the Colorectal Cancer Unit,
Cancer Research United Kingdom, St. Marks Hospital, Harrow; the Department of Clinical Genetics,
Guys Hospital, London; and the Cancer and Immunogenetics Laboratory,
Cancer Research United Kingdom, Institute of Molecular Medicine, John Radcliffe Hospital, Oxford, United Kingdom
| Abstract |
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1 to 3% of unselected colorectal cancers, but appear always to be associated with multiple adenomas. Somatic inactivation of the DNA glycosylases involved in the BER pathway however does not appear to be involved in colorectal tumorigenesis.
10% of cancers of the colorectum;4
and the transforming growth factor-ß pathway member, SMAD4, causes juvenile polyposis and is mutated in
20% of sporadic colorectal cancers. In contrast, the genes responsible for other Mendelian diseases that predispose to colorectal cancer appear to be rarely involved in the pathogenesis of sporadic bowel cancers. Examples include the Peutz-Jeghers hamartoma syndrome gene, LKB1/STK115
and the second juvenile polyposis gene, BMPR1A/ALK3.6
Al Tassan and colleagues7
recently found a previously unrecognized autosomal recessive condition comprising multiple colorectal adenomas and cancer. They reported that three affected siblings from a single UK family were compound heterozygotes for nonconservative sequence variants, Y165C and G382D, in the base excision repair (BER) gene MYH. 8-Oxo-G is a stable product of oxidative damage and it misrepairs readily with adenine residues resulting in an excess of G:C -> T:A transversion mutations. Analysis of homologous variants in Escherichia coli revealed that the defective proteins had significantly decreased ability to repair 8-oxoG:A defects. Adenomas from the individuals carrying the Y165C and G382D germline sequence variants had a higher than expected incidence of G:C
T:A, somatic mutations in the APC gene, consistent with defective BER. We have recently confirmed these data in a large series of unrelated patients with multiple colorectal adenomas8
and found that approximately one-third of patients with between 15 and
100 adenomas had bi-allelic germline MYH mutations. We also screened the related BER genes, MTH1 and OGG1, but found no pathogenic germline changes.
The BER pathway repairs mutations caused by reactive oxygen species that are generated during aerobic metabolism.9 Oxidative DNA damage has been previously implicated in the etiology of degenerative diseases, aging, and cancer.10 Levels of 8-oxo-dG have been found to be significantly elevated in carcinomas of the breast,11 lung,12,13 and kidney.14 Although no pathogenic MYH mutations have been found to date in sporadic forms of these cancers,15 mutations in OGG1 have been found in human lung and kidney tumors.16,17 Thus, it appears that genes involved in the BER pathway are good candidates for involvement in the pathogenesis of sporadic tumors of the large bowel.
In this study, we screened 75 unselected, sporadic cancers of the colorectum for mutations in and allelic loss at MYH. Forty-eight of these cancers were also screened for mutations in MTH1 and OGG1.
| Materials and Methods |
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Loss of heterozygosity (LOH, allelic loss) analysis was performed at the microsatellite marker D1S2677 (located
2.5 kb from the MYH locus) in the cancer samples, using standard protocols, dye-labeled oligonucleotides, and the ABI377 sequencer. Allelic loss was scored if the dosage of one allele in the tumor decreased by 50% or more relative to the other allele, after correcting for the relative allele peak areas using constitutional DNA. LOH was also assessed by direct inspection of sequence electropherograms for samples with mutations or polymorphisms within the MYH gene.
In selected samples, we screened for mutations in APC (exon 15 regions D-I), ß-catenin (exon 3), and K-ras (exon 1) by direct sequencing (forward and reverse). p53 was screened by fluorescent single-strand conformational polymorphism analysis essentially as described above. The oligonucleotides and polymerase chain reaction conditions used are available from the authors. LOH at APC was assessed as above using the microsatellites D5S346 and D5S656.
A panel of 35 colorectal cancer cell lines (C10, C106, C32, C70, C75, C80, C84, C99, CACO2, COLO205, COLO320, COLO678, COLO741, GD2D, GP5D, H716, HCA46, HCA7, HCT8, HRA19, HT29, HT55, LOVO, LS123, LS174T, PC/JW, RKO, SKCO1, SW1222, SW1417, SW403, SW480, SW620, SW837, SW948) was screened for loss of MYH protein expression using Western blots and the muty11-second and muty12-second antibodies (Autogen Bioclear). The same cell lines were screened for MYH mRNA expression using reverse transcriptase-polymerase chain reaction (Muty-cdna oligonucleotides: forward primer, CAGAGGCTTTGAAGGCTACC; reverse primer, TGCAGCATGACCTCTGAGAC).
| Results and Discussion |
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TGT, of a type in keeping with defective oxidative repair. This finding clearly supports the previous evidence that recessive germline MYH mutations cause multiple colorectal adenomas and cancer.
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The other heterozygous MYH change (tumor no. 338) was a novel mutation, V61E. This change does not involve a conserved amino acid and the human mutant residue is actually the wild type in rat and mouse; it seems unlikely, therefore, that this variant has pathogenic effects. The patient presented at age 86 with a left-sided, moderately differentiated, Dukes C cancer. No personal history of multiple adenomas or any family history of note was reported. Although this patient was not informative at D1S2677, direct inspection of the raw sequence showed clear loss of the germline wild-type MYH allele in the cancer (Figure 2)
. Screening for APC mutations in this cancer revealed a G
T transversion at 1317 (E1317X) in exon 15G. There was no LOH at D5S346 or D5S656. No mutations were found in either K-ras or p53. Overall, allelic loss was uncommon close to MYH. Sixty-nine tumors were both informative and did not show microsatellite instability at D1S2677, of which 16 (23%) showed LOH.
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Although no somatic mutations were found in MYH, we also addressed the possibility that MYH mRNA or protein was lost from sporadic colorectal cancers by other mechanisms, such as transcriptional silencing. Although some subtle quantitative effects were not excluded we found that all 35 cell lines showed expression of MYH mRNA and protein.
No clearly pathogenic mutations were found in MTH1 or OGG1. The previously described polymorphisms, V83 mol/L and D119D in MTH119 and G308E20 and S326C21 in OGG1, were found with respective allele frequencies of 1%, 15%, 1%, and 24%. It has been suggested that the S326C polymorphism plays an important role in the risk for smoking- and alcohol-related orolaryngeal cancer22 and is associated with an increased risk of lung cancer.23 All of the polymorphisms found in this panel of tumors occurred at rates not significantly different from those found in the normal population,7,19,23 suggesting that, subject to the limitations of our sample size, they do not play a major role in the risk of colorectal cancer (details not shown).
Our data clearly support the existence of the recessive MYH-associated polyposis syndrome. It is interesting that our patient with bi-allelic MYH mutations had no detected APC mutations or LOH. Although we did not screen the entire APC gene, our data suggest the entirely plausible possibility that MYH mutations do not specifically lead to APC hypermutation, but can also lead to increased mutations of other, unknown genes involved in the pathogenesis of colorectal tumors. Based on an estimated frequency of 2 to 3% for pathogenic MYH alleles8
and a lifetime risk of colorectal cancer of 3 to 4%, we expect that
1 to 3% of all colorectal cancers would result from MYH changes, a frequency that is higher than familial adenomatous polyposis and probably comparable with hereditary nonpolyposis colon cancer. Our finding that 1 of 75 unselected colorectal cancer patients has bi-allelic mutations is consistent with this expectation. Our data do indicate, however, that colorectal cancer in the absence of multiple adenomas is unlikely to result from bi-allelic MYH mutations, thereby emphasizing the need for accurate reporting of polyps synchronous with carcinoma.
There is little suggestion from our data that MYH mutant/wild-type heterozygotes are at an increased risk of colorectal cancer. Our V61E carriers cancer had lost the wild-type MYH allele and had a somatic mutation at APC typical of those associated with MYH-associated polyposis,7 but this was most likely to be coincidental. Given our findings, it appears unlikely that MYH mutations act as subpolymorphic, low penetrance susceptibility alleles as we proposed for APC E1317Q in colorectal tumors24 and has recently been shown for CHEK2 in breast cancers.25 A large case-control study is however required to confirm that there is no increased risk in MYH heterozygotes.
We found no somatic MYH mutations (except a moderate frequency of LOH) and no absence of mRNA or protein. On the one hand, these findings are to some extent expected, because it is evident that a colorectal tumor will, by chance, usually acquire two APC mutations before it has acquired two MYH mutations and two APC mutations. MYH-associated polyposis and hereditary nonpolyposis colon cancer have a similar prevalence and both involve defective DNA repair of mismatched bases. It is therefore a little surprising given these parallels and that MLH1 function is lost in
10% sporadic colorectal cancers26,27
that MYH appears to have no such role. Future analyses are, we suggest, likely to reveal that mismatch repair and oxidative repair genes predispose to bowel tumors through fundamentally different mechanisms.
| Acknowledgements |
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| Footnotes |
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S. H. is a Cancer Research UK Translational Fellow.
Accepted for publication February 3, 2003.
| References |
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