Microsatellite instability and novel mismatch repair gene mutations in northern Chinese population with hereditary non-polyposis colorectal cancer.
OBJECTIVE
Hereditary non-polyposis colorectal cancer (HNPCC) syndrome is the most common cause of hereditary colorectal cancer with an early age of onset. Microsatellite instability (MSI) and germline mutation in one of the DNA mismatch repair (MMR) genes are found in the majority of HNPCC families and provide an opportunity for genetic diagnosis and prophylactic screening. The MMR gene mutation spectrum may vary across different populations and be influenced by founder mutations that prevail in specific ethnic groups. China is a big and ancient nation with enormous genetic diversity, which is especially notable between the northern and southern Chinese populations. A MMR gene mutation database for the southern Chinese population based in Hong Kong has been previously established. This study compares the MMR gene mutation spectrum and the MSI of HNPCC between the northern and southern Chinese populations.


METHODS
Twenty-five HNPCC families from northern China were systematically analyzed. The MSI analysis was performed using five loci in the USA National Cancer Institute (NCI) panel (D2S123, D5S346, BAT-25, BAT-26 and BAT-40) by PCR from the tumor and normal tissue. MSH2, MSH6 and MLH1 were performed using immunohistochemical staining. Two founder mutations of MSH2 and MLH1 were examined by PCR base analyses using primers flanking the two deletion sites (c.1452_1455delAATG in MSH2 and 1.8 kb deletion involving exon 11 of MLH1).


RESULTS
Of the 25 families collected, 19 met Bethesda guideline (BG) 1 and six met BG3. Twenty-two (15.7%) were extra-colonic cancers with gastric cancer (in seven patients) being the most common cancer type. Of the 25 tumors analyzed, 21 (84%) were high level microsatellite instability (MSI-H) and four (16%) were microsatellite stable (MSS). Eighteen (86%) of the 21 MSI-H tumors showed loss of either the MLH1 or the MSH2 protein. Three MSI-H tumors and all four MSS tumors showed no loss of expression of the three MMR proteins. Out of the 21 patients with MSI-H tumors, 12 (57%) showed pathogenic germline mutations in either MLH1 (n = 8) or MSH2 (n = 4). Overall, three novel mutations (in patients H22, H17 and H29) have been identified. One of them, c.503_4insA, caused a frameshift mutation in the MLH1 gene. The other two were found in the MSH2 gene, including a frameshift (c.899_890insAT) and a splice junction (IVS7-1G-->A, SA of Exon 8) mutation.


CONCLUSIONS
The results suggest a distinctly different mutation spectrum of MMR genes between northern and southern Chinese populations and call for a systematic, nationwide study to facilitate the design of a MMR gene mutation detection strategy tailored for individual populations in China.
Sheng
Jian Qiu
JQ
Hereditary non-polyposis colorectal cancer (HNPCC) group, Beijing Army General Hospital, Beijing, China.
Chan
Tsun Leung
TL
Chan
Yee Wai
YW
Huang
Ji Sheng
JS
Chen
Ji Gui
JG
Zhang
Ming Zhi
MZ
Guo
Xiu Lan
XL
Mu
Hong
H
Chan
Annie Sy
AS
Li
Shi Rong
SR
Yuen
Siu Tsan
ST
Leung
Suet Yi
SY
D048868:Adaptor Proteins, Signal Transducing; D000328:Adult; D020137:Base Pair Mismatch; D002352:Carrier Proteins / Q000235:genetics*; D002681:China; D003123:Colorectal Neoplasms, Hereditary Nonpolyposis / Q000235:genetics*; D053843:DNA Mismatch Repair; D004268:DNA-Binding Proteins / Q000235:genetics*; D005260:Female; D016368:Frameshift Mutation; D018095:Germ-Line Mutation; D006801:Humans; D008297:Male; D053842:Microsatellite Instability; D008875:Middle Aged; D000070957:MutL Protein Homolog 1; D051718:MutS Homolog 2 Protein / Q000235:genetics*; D009687:Nuclear Proteins / Q000235:genetics*
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D004268:DNA-Binding Proteins; C094443:G-T mismatch-binding protein; C501486:MLH1 protein, human; D009687:Nuclear Proteins; C497172:MSH2 protein, human; D000070957:MutL Protein Homolog 1; D051718:MutS Homolog 2 Protein