Allelic loss is heterogeneous throughout the tumor in colorectal carcinoma

Loss of heterozygosity (LOH) at 17p and 18q in colorectal carcinoma has been depicted as a potential prognostic marker for the disease. However, conclusions vary among reports, and evidence of clinically useful genetic prognostic markers is still lacking. As a rule, single biopsies are used. In this study, the authors hypothesized that an important cause of earlier contradictory results was the heterogeneity of colorectal neoplasms.

In this study, DNA originating in each quadrant of tumors from 64 patients with colorectal carcinoma was analyzed. Microsatellite markers for chromosome 18q and 17p were amplified by polymerase chain reaction and automatically analyzed.

The authors found that, regardless of stage, LOH and non-LOH in both 17p and 18q varied among biopsies within the tumors in a random fashion. LOH in 18q was detected in all 4 quadrants in 22% and in 1 of 4, 2 of 4, or 3 of 4 quadrants in 56% of the tumors, whereas 22% of the tumors were homogeneously without LOH in 18q. LOH 17p was distributed similarly throughout the tumors and was present in 1 of 4, 2 of 4, or 3 of 4 of the quadrants in 44%. The authors also reexamined a subset of tumors by subdividing one biopsy from each into four. Analysis of the microsatellite markers then yielded identical results. No correlation between the degree of LOH status and patient survival was observed.

LOH status within a colorectal tumor is extensively heterogeneous. However, it is more homologous on a lower macroscopic level. For relevant genetic analysis, multiple biopsies and DNA sampling preceded by careful morphologic examination must be standard in the preparation of DNA.