Study On The Myh Genovariation Associated With Colorectal Cancer Susceptibility

Background:MutY homologue(MYH),a base excision repair (BER) gene with 16 exons (1641 Base pairs, 1641 BP) is located within lp32.1-lp34.3. MYH glycosylase, the MYH protein reside in caryon and chondriosome. If a adenine in nascent chain mismatched to 8-oxo-7,8-dihydroguanine(8-oxodG)that was the oxidative damage product of reactive oxygen species (ROS) in parent chain, the mismatched adenine would be exsected by the MYH glycosylase. Homozygotic MYH variation could induce a kind of autosomal recessive inherited disease that is MYH-associated polyposis (MAP). It is similar to MAP that the three autosomal dominant inherited disease, familial adenomatous polyposis (CFA), attenuated familial adenomatous polyposis (AFAP) and Peutz–Jeghers syndrome (P-J syndrome) are all polyposis. The possible mechanism in MAP is including co-participation and interaction of BER system, mismatch repair (MMR) system and proliferating cell nuclear antigen (PCNA). Heterozygous MYH variation with genetic predisposition could also induced colorectal cancer. Onset probabilit of filial generation would be 25% theoretically if parents were all heterozygous MYH variation carriers.Objective:Many devices, such as genic detection, are proposed to acquire the high-risk group. This study is designed to approach the relationship between heterozygous MYH gene variation (The variations mentioned in this study are all heterozygous variation except special discription) and colorectal cancer susceptibility. It is expected that the results and conclusions will be guidance of clinic practice. It is also hoped that the tri-early code, early discovery, early diagnosis and early treatment will be complied by the outcome of this study.Methods:Peripheral blood of 140 colorectal cancer patients and 280 healthy controls was drew randomly to extract DNA, and then the polymerase chain reaction(PCR)was performed to amplify 7 in 16 exon districts of MYH gene that they were Exon 1, Exon 7, Exon 9, Exon 11, Exon 13, Exon 14, Exon 16. Denaturing high performance liquid chromatography(DHPLC)was presented to detect the variation samples with abnormal eluting peak. Finally, DNA sequencing was proposed to authenticate the variations. Chi square test(Fisher's Exact Test)& matched-pairs t-test (Homogeneity test & test of normality for variance had been performed before t-test) were required to be the statistic methods. All data was analyzed by the SPSS(v. 13. 0)software.Results:Variations were detected in the following 5 exon districts.They were Exon 1 district, Exon 11 district, Exon 13 district, Exon 14 district and Exon 16 district. Above all, Four variations, Exon1-316 G>A,Exon1-292 G>A and Intron1+11, were detected in Exon 1 district simultaneously in all variants of cases and controls. The 3 variations were appearanced simultaneously in all variation samples. Genovariation frequency of these 3 sites in cases was significantly higher than that in controls, genovariation risk in cases was 8.21-fold more than controls(p=0.04;OR=8.21,95%CI为1.63—41.22). A novel deletion variation,nt 1678-80 del GTT, was found in exon 16. In cases, the rectal cancer patients genovariation frequency was higher than colon cancer patients, genovariation risk in rectal cancer patients was 7.26-fold more than colon cancer patients(p=0.04;OR=7.26,95%CI was 1.17—45.27).The result was the benefit for choosing modus operandi, operation on anus and prognosis evaluation. Genovariation risk between cases and controls had no significant difference. A missense variation, Exon14+74 T>A, p.V463E, was found in exon 14. Electroneutral amino-isovaleric acid was replaced by electropositive aminoglutaminic acid. The heteropolarity and structure of protein might be transformed by the replacement. Genovariation risk between cases and controls had no significant difference. The variations Intron10-2 A>G and Intron13+12 C>T were found in Exon11 district and Exon 13 district. Genovariation risk between cases and controls had no significant difference in the both two disticts. In both Exon11 and Exon16 districts, difference between mean age of cases and controls had no significance(p>0.5). But mean age of variations in cases was significant higher than variations in controls(t11=4.91,p11<0.0005; t16=9.31,p16<0.0005, homogeneity test & test of normality for variance had been performed before t/t'-test). And difference between mean age of non-variations in cases and non-variations in controls had no significance(p>0.1). Although advanced age was still a predisposing factor for tumor, detection and monitoring of MYH variations were beneficial to clinical practice. Results of Exon16 district suggested that genovariation risk in males is significant higher than in females on a background level(p<0.05). It was supposed that the reason might be associated with BER system itself according to some reports about BER system. The mechanism of variation Intron10-2 A>G in Exon11 district was confirmed. The abnormality of mRNA was induced. At last, MYH-glycosylase fixing failed in caryon because of protein decurtation. Similar variations at the other sites in this study were detected, however whether those variations would cause function change or not still need further studies. No variation was detected in the two districts, Exon 7 and Exon 9. But Exon 7 district was one of most frequently variated exon districts in Caucasian population. Combining with the reports from all over the world, the difference above implied that the variety of MYH variations may relate to different races.Conclusion: This result raises the possibility that MYH variation may function as a factor of increasing the colorectal cancer onset risk. Most canceration patients are suffering rectal cancer. It is similar to other Asian races that the MYH variation frequency in Chinese is lower than Caucasian. The most obvious difference from reports about Caucasian is that no variation is found in Exon 7 district which is one of the most frequently variated exons in westerners. All these phenomena indicate that the differences of MYH variations in different countries and region may relate to races difference.