Intratumor genetic heterogeneity in advanced human colorectal adenocarcinoma

Colorectal carcinogenesis is widely accepted as one of the best-characteriz ed examples of stepwise progression. The existing colorectal carcinogenesis model assumes genetic homogeneity of individual tumors for the main known genetic alterations: K-rcs and p53 genes point mutations and loss of hetero zygosity (LOH) of chromosome 5q and 18q, The object of the present study wa s to demonstrate the existence of an intratumor genetic heterogeneity in ad vanced sporadic colorectal carcinoma for these genetic alterations, Using i mproved tissue microdissection and DNA extraction, for each tumor, amplifia ble DNA was obtained from 15 to 20 areas, of which 1 to 2 concerned lymph n ode metastases (LNM), This study revealed that 10 of 15 (67%) analyzed tumo rs were heterogeneous for at least I genetic alteration, with between 2 and 6 genotypically different clones detected per tumor. No correlation was ob served between the genotype of these subclones and histological differentia tion or invasive propensity. Intratumor heterogeneity was more frequently o bserved for LOH than for point mutations, 67% and 58% for LOH at APC and DC C locus, and 20% for mutation of either the K-ros or p53 gene, In 5 of the 9(56%) heterogeneous cases with available LNM, the genotype observed in the LNM was different from that of the main clone in the primary tumor, and mo reover, 2 of the LNM displayed a genotype undetected in the primary tumor. In conclusion, intratumor genetic heterogeneity was demonstrated in advance d sporadic colorectal carcinoma and was represented as topographically dist inct genotypic subclones, Taking into account such a significant genetic he terogeneity of colorectal tumors, the use of genetic markers for prognosis management should be reconsidered, (C) 2001 Wiley-Liss, Inc.