A CDKN2A Mutation in Familial Melanoma that Abrogates Binding of p16 super(INK4a) to CDK4 but not CDK6

The CDKN2A locus encodes two distinct proteins, p16 super(INK4a) and p14 super(ARF), both of which are implicated in replicative senescence and tumor suppression in different contexts. Here, we describe the characterization of a novel strain of human diploid fibroblasts (designated Milan HDFs) from an individual who is homozygous for the R24P mutation in p16 super(INK4a). As this mutation occurs in the first exon of INK4a (exon 1 alpha ), it has no effect on the primary sequence of p14 super(ARF). Based on both in vitro and in vivo analyses, the R24P variant is specifically defective for binding to CDK4 but remains able to associate with CDK6. Nevertheless, Milan HDFs behave as if they are p16 super(INK4a) deficient, in terms of sensitivity to spontaneous and oncogene-induced senescence, and the R24P variant has little effect on proliferation when ectopically expressed in normal fibroblasts. It can, however, impair the proliferation of U20S cells, presumably because they express more CDK6 than primary fibroblasts. These observations suggest that CDK4 and CDK6 are not functionally redundant and underscore the importance of CDK4 in the development of melanoma. [Cancer Res 2007; 67(19):9134-41]