CLONAL OVEREXPRESSION OF METALLOTHIONEIN IS INDUCED BY SOMATIC MUTATION IN MORPHOLOGICALLY NORMAL COLONIC MUCOSA

Metallothionein (MT) overexpression occurs frequently in human rumours but the underlying mechanism is unknown. Morphologically normal-appea ring mucosa from human colorectal carcinoma resection specimens and of the colons of ageing laboratory mice contains scattered single crypts whose cells show uniformly increased MT immunostaining, suggesting th at MT overexpression arises directly from random crypt stem cell somat ic mutation, followed by colonization of the clonal unit by the mutate d progeny. This hypothesis has now been tested by quantifying the freq uency of immunocytochemically detectable monocryptal colorectal MT ove rexpression, 5 and 30 days after injection of 8-week-old mice with a s ingle dose of the mutagen dimethylhydrazine (DMH, 30 mg/kg subcutaneou s). Otherwise normal-appearing MT-positive crypts were recorded as eit her wholly or partially involved by the overexpressing phenotype. Five days after DMH injection, the median frequency of partially involved MT-positive crypts was 11.7 x 10(-4), declining significantly to 1.8 x 10(-4) at 30 days (Mann-Whitney U, P<0.01). In contrast, the median f requency of wholly involved crypts was 0.2 x 10(-4) at 5 days, increas ing significantly (P<0.005) to 12.9 x 10(-4) at 30 days. The frequency of MT-positive crypts and the time course of evolution of partially i nvolved to wholly involved forms were similar to those described for m utation-induced crypt-restricted loss of glucose-6-phosphate dehydroge nase activity in mice treated with an identical DMH regimen. The findi ngs indicate that cellular MT overexpression can occur as a direct con sequence of somatic mutation, either cis-activating mutation(s) of the MT gene itself, or trans-activating mutation(s) of other genes involv ed in controlling MT expression. This is likely to be an important mec hanism underlying MT overexpression in neoplasia. Such mutation-induce d aberrant MT expression may be involved in the acquisition of selecti ve cellular growth or survival advantage during tumour progression. (C ) 1998 John Wiley & Sons, Ltd.