Screening for CDKN2A mutations in hereditary melanoma.

Abstract

Cutaneous malignant melanoma (CMM) is a potentially fatal form of skin cancer whose etiology is heterogeneous and complex. Approximately 10% of the cases of CMM occur in persons with a familial predisposition (1), often in association with clinically dysplastic or atypical nevi (2). In 1993, a low-molecular-weight protein p16 was shown to inhibit the activity of the cyclin D1–cyclin-dependent kinase 4 or 6 (CDK4 or CDK6) complex (3). This complex phosphorylates the retinoblastoma protein, allowing the cell to pass through the G1 cell-cycle checkpoint. The CDKN2A gene that encodes p16 was localized to chromosome 9p21 (4,5), a region that has been implicated in melanoma by linkage, cytogenetic, and loss-of-heterozygosity studies (6-11). Somatic mutations in this gene have frequently been detected in many melanoma cell lines (4,5). Although initial reports of germline mutations in melanomaprone families (selected for genetic analyses) suggested that the p16 protein might be important in approximately one half of inherited melanoma (12-14), subsequent studies have revealed lower frequencies of mutations. To date, germline CDKN2A mutations have been detected in approximately one fourth of melanoma-prone kindreds (defined as having two or more first-degree relatives with confirmed CMM) from North America, Europe, and Australia (12-21). In this issue of the Journal, Platz et al. (22) present the first report of screening for germline mutations in a population-based cohort of melanoma-prone families. The results provide further evidence that CDKN2A mutations may not be as prevalent as originally thought. Platz et al. (22) identified 177 melanoma-prone families, defined by the occurrence of two or more relatives (including first-, second-, and third-degree relatives) with histopathologically confirmed CMM, in the County of Stockholm, Sweden. A representative selection of 181 individuals with CMM, from 100 of these families, was screened for germline mutations in CDKN2A. Of the 100 families, 64 kindreds had at least two cases of melanoma in first-degree relatives; 24 kindreds had melanoma in two or more second-degree relatives; and 12 kindreds had melanomas in third-degree or more distant relatives. Both direct sequencing and single-strand conformation polymorphism (SSCP) analyses of exons 1 and 2 were conducted. Of the 64 families with two or more first-degree relatives with CMM, five (7.8%) had potentially disease-related mutations. Four of the five families with mutations had more than the mean (2.3) number of melanoma cases. Ten kindreds without detectable mutations also had more than two cases of melanoma in first-degree relatives. No mutations were detected in the 36 families with melanoma in more distant relatives (22). The frequency of CDKN2A mutations detected by Platz et al. (22) was lower than that in the initial reports of families selected for genetic analyses, yet the observations are consistent with numerous later results (13,15,17,20,23). There are several possible explanations for the divergent frequencies. First, part of the difference may be due to chance, since several studies examined small numbers of families. Second, the families that were studied likely represented a heterogeneous mix of hereditary and nonhereditary melanoma-prone kindreds. Certain investigations required three or more first-degree relatives with CMM, others required only two such relatives, and still others required only two relatives with CMM not restricted to first degree. Third, germline mutations may have been present in some families but undetected either because only one patient per family was screened or because the mutation was not revealed by direct sequencing or SSCP analysis. Mutations outside the CDKN2A coding region that interfere with RNA splicing or expression would not have been detected (24). In addition, alternative processes linked to transcriptional and/or translational regulation including methylation might also inactivate p16 (25). The frequency of such undetected mutations and alternative mechanisms of inactivation is difficult to estimate. Finally, the complexity and heterogeneity of CMM likely contributed to the divergent frequencies. Previous studies (12,19,26) have demonstrated statistically significant evidence for genetic heterogeneity to the chromosome 9p21 region as well as the identification of a second melanoma gene CDK4 in two melanoma-prone families (27). All of these alternatives likely contribute to the divergent estimates of CDKN2A mutations across populations and make it difficult to determine precisely the proportion of hereditary melanoma cases associated with p16 alterations. Even with the uncertainty about the frequency of CDKN2A