Etiology of MNU-induced melanomas in Xiphophorus hybrids

Genetic hybrids of the genus Xiphophorus have historically been useful models for study of the genetic aspects of tumor formation. In the most studied Xiphophorus tumor model, two-gene loci, XMRK and DIFF, are implicated as critical both to UV-induced and spontaneous melanoma formation in BC1 hybrids of crosses between X. maculatus and X. helleri, with X. helleri as the recurrent backcross parent. In addition to UV, the direct-acting carcinogen N-methyl-N-nitrosourea (MNU) has been used to induce tumors in Xiphophorus BC1 hybrids from several cross types. In the present study, we address the hypothesis that excess melanomas in MNU-treated BC1 hybrids may have been generated by direct mutation of CDKN2AB, a candidate gene for DIFF. MNU treatment of F1 and BC1 hybrid fish significantly increased tumor incidence at 6 months; however, no association was found between MNU-induced tumor formation and zygosity of the candidate tumor tumor-suppressor CDKN2AB in BC1 hybrids, consistent with previously reported results. Sequence analysis of the X. maculatus CDKN2AB locus of heterozygous individuals (both BC1 and F1 hybrids) did not reveal any mutations caused by MNU, suggesting that the mechanism of MNU-induced melanoma formation in this Xiphophorus model does not involve direct mutation of CDKN2AB but may result from mutation of other critical genes.