Microsatellite Instability In Colorectal Adenomas

Colorectal adenomas are the early stage of colorectal tumorigenesis , and is considered to be precancerous lesions of CRC. It can be hereditary, as in familial adenomatous polyposis(FAP), and sporadic, it can be histopathologically divided into the following types as tubular, villous, tubulovillous and flat adenoma, in which the villous type is most ready to develop into carcinoma. The "multi-gene, multi-step" progression has been determined by studies of molecular mechanisms in tumorigenesis. Such a progression from normal mucosa to aberrant crypt foci (ACF), and adenomas to eventually carcinoma .furthur invasion and metastasis involves mutation and/or inactivation of many oncogenes.such as K-ras.C-myc and tumor suppressor genes.such as APC,p53, and also 1p deletions. Such metastasis suppressor genes as nm23 also play a role. Some of these genetic alterations can also be detected in adenomas.Generally speaking, there are two ways to malignancy : microsatellite instability (MSI) and chromosome instability(ClN).Mismatch repair genes are considered to be "caretaker genes", which are in charge of the stability of the genome. Their mutations are not of necessities to cause CRC, but can cause instability of the whole genome, which in turn would greatly increase the mutation risk of other vital genes to CRC, such as ARC gene. MSI is the phenotype of MMR mutations, and is generally accepted as molecular bio-markers for tumor screening at early stages. Such numerical alterations of small repeated sequences in microsatellite loci are caused by replication error of DMA, and tumors with microsatellite instability are called RER+ tumors, which is also called MSI-H in many documents recently. There are three criteria for MSI-H that are in common use:(l)scholars at National Cancer Institute Workshop hold that 2 altered microsatellite loci of the selected 5 microsatellite markers (BAT25. BAT26, D5S346. D2S123. D17S250) is MSI-H; (2)some consider MSI-H as >30% of all tested markers have MSI compared to normal mucosa; (3)BAT26 is an appropriate indicator. MSI in adenomas is within alteration rate ranging from 0-10%.BAT26 is in the 5th intron of hMSH2 gene with rare polymorphism. The accuracy rate of BAT26 in the place of MSI-H is up to 99.4% by some researchers. We use normal mucosa as control for each BAT26 detection, and the accuracy rate is 96.8%, which also holds the above opinion.Two signal transduction pathways of ARC?P -catenin桾cf4 and TGFP are of great importance in colorectal carcinogenesis. Physiologically, they would regulate the growth and differentiation of10colon epithelial cells and maintein homostasis. Mutation of APC is the key step of tumor development; and D5S346 is a microsatellite locus 30-70kb downstream the APC gene, which could be a convenient and liable marker for APC detection. TGFJ3RII is the first gene to be discovered MSI within coding sequence[(A)io. (01)3], whose mutations deprive the functions of TGF(3 signal transduction pathway.1p deletions are becoming more attractive, and is said to be another early event in CRC tumorigenesis. There may be locations of one or more tumor suppressor genes especially in the end of 1p chromosome.This study detected 62 adenomas from 59 parients with 16 microsatellite loci. Microdissection allows for specific examinations of particular cells, so as to avoid distractions from other cells. It helps to improve the accuracy of the result and determine the original stage of the lesion. We have successfully set up this method of microdissection-PCR by making it optimal concerning the number of cells, of PCR cycles, and the concentration of primers. It turns out that the overall MSI is 14.7%; 6 adenomas are of BAT26+; Loci as D1S228, D1S548. D2S123. D5S346. IGFIIR(G)8. IGF1IR(CT)5. TGF PRII(GT)3. TGF p Rll(A)10. hMSH3(A)8?Tcf4(A)9 have comparatively higher alteration rate. D2S123 locates near hMSH2 gene, and D5S346 could reflect APC changes. So it can be concluded that there are alterations of 1p, APC, MMR genes, and also...