The melanoma-associated 24 base pair duplication in p16INK4a is functionally impaired

Melanoma-associated germline mutations affecting the tumor suppressor and cyclin-dependent kinase (CDK) inhibitor, CDKN2A/p16INK4a, have been identified in over 100 melanoma-prone families worldwide. To predict the melanoma risk for carriers of specific mutations, mutant p16INK4a can be tested in biochemical and cellular assays. In most cases, p16INK4a mutations with predicted disease relation, due to segregation with melanoma, are functionally impaired in such assays. The N-terminal 24 base pair duplication of CDKN2A, however, encodes a p16INK4a variant previously shown to have wild-type function, despite segregating with melanoma in at least 5 melanoma families. To clarify whether the duplication mutation has a cell cycle regulatory defect or behaves like wild-type p16INK4a, we reanalyzed the cell cycle-inhibitory activity of this mutation. Stable cell clones of the p16-null WMM1175 melanoma cell line inducible for ectopic p16INK4a were used in this study. In these cells, p16INK4a expression can be controlled at physiologic levels. Our results show that in comparison to wild-type p16INK4a, the duplication mutant induced weaker S-phase inhibition and cells expressing this mutant form of p16INK4a retained colony formation ability. We also show that the cell cycle-regulatory defect of the p16INK4a duplication mutant was associated with decreased inhibition of pRb phosphorylation even though it retained significant binding to CDK4.