Linkage and association analysis of nevus density and the region containing the melanoma gene CDKN2A in UK twins

Germline Variation at CDKN2A and Associations with Nevus Phenotypes among Members of Melanoma Families

Germline mutations in CDKN2A are frequently identified among melanoma kindreds and are associated with increased atypical nevus counts. However, a clear relationship between pathogenic CDKN2A mutation carriage and other nevus phenotypes including counts of common acquired nevi has not yet been established. Using data from GenoMEL, we investigated the relationships between CDKN2A mutation carriage and 2-mm, 5-mm, and atypical nevus counts among blood-related members of melanoma families. Compared with individuals without a pathogenic mutation, those who carried one had an overall higher prevalence of atypical (odds ratio = 1.64; 95% confidence interval = 1.18-2.28) nevi but not 2-mm nevi (odds ratio = 1.06; 95% confidence interval = 0.92-1.21) or 5-mm nevi (odds ratio = 1.26; 95% confidence interval = 0.94-1.70). Stratification by case status showed more pronounced positive associations among non-case family members, who were nearly three times (odds ratio = 2.91; 95% confidence interval = 1.75-4.82) as likely to exhibit nevus counts at or above the median in all three nevus categories simultaneously when harboring a pathogenic mutation (vs. not harboring one). Our results support the hypothesis that unidentified nevogenic genes are co-inherited with CDKN2A and may influence carcinogenesis.

Cutaneous melanoma primary site is linked to nevus density

There are at least two pathways driving cutaneous melanoma; one is linked to an inherent melanoma susceptibility to nevi development and the second to environmental cumulative ultraviolet light exposure. In this study, we examined the relation between nevus density, accrued sun damage and the site of primary melanoma excision. In a series of 888 consecutive cutaneous melanoma patients, melanomas appearing in skin areas with a high relative nevus density were most prominent in men, with an elevated nevus count, at sites without solar elastosis, but with an epidemiological history of previous sunburn. The present study associates melanoma development to sites with high nevus density. Our study supports more careful surveillance of body areas with increased nevus density in patients with high total body number of nevi, especially when they report a history of sunburns at these sites.

The dysplastic nevus: from historical perspective to management in the modern era: part I. Historical, histologic, and clinical aspects

Since its description in the 1970s, the dysplastic nevus has been a source of confusion, and whether it represents a precursor to melanoma remains a controversial subject. Although a Consensus Conference in 1992 recommended that the term "dysplastic nevus" no longer be used, the histologic diagnosis continues to present a therapeutic quandary for dermatologists and other physicians, and there remains significant variation in clinical management. In part I of this continuing medical education review, we will discuss the historical origins of the term, the evidence for its distinct histologic basis, and its clinical significance.

Melanoma epidemiology

Melanoma is a complex, heterogeneous cancer that continues to increase in incidence. Multiple studies have consistently identified major host and environmental risk factors for melanoma. Nevi, particularly dysplastic nevi, confer much higher risks than most pigmentary characteristics. Ultraviolet radiation exposure is the predominant environmental risk factor for melanoma. Recently, both rare high risk susceptibility genes and common polymorphic genes contributing to melanoma risk have been identified.

A genome-wide scan for naevus count: linkage to CDKN2A and to other chromosome regions

High numbers of melanocytic naevi (moles), and mutations in the p16 gene (CDKN2A), are two strong risk factors for cutaneous malignant melanoma. We have previously reported linkage of mole count to the CDKN2A locus. Here, we report genome-wide scans for mole counts (differentiated into flat, raised and atypical subtypes) with a total of 796 microsatellite markers for 424 families with 1024 twins and siblings, plus genotypes for 690 parents. Inclusion of 221 pairs of MZ twins enabled separation of shared environmental and polygenic influences, so placing an upper limit to estimates of QTL variance. Maximum likelihood multipoint variance component methods were used to assess linkage of naevus count. Sex, age, body surface area, skin colour, hair colour, sunburn and facial freckles were included as covariates. Peak linkage of flat mole count was to regions on chromosomes 2, 9, 8 and 17 with lod scores 2.95, 2.95, 2.50 and 2.15, respectively. The support for linkage to the CDKN2A gene region (9p21) increased to 3.42 when additional fine mapping markers were added. For raised mole count, there was suggestive evidence of linkage in our sample to chromosome 16 (lod=1.87), and for atypical mole count on chromosomes 1, 6 and X with lod scores of 2.20, 2.00 and 2.00, respectively. The multivariate linkage peaks generally match those from individual trait analyses, with the exception of a new peak on chromosome 4 (point-wise empirical P-value=0.001). We replicate our earlier finding of linkage to CDKN2A and discovering linkage to several novel regions that may also influence risk of the development of malignant melanoma.

Genome-wide search for nevus density shows linkage to two melanoma loci on chromosome 9 and identifies a new QTL on 5q31 in an adult twin cohort

The density of acquired melanocytic nevi represents an important risk factor for malignant melanoma. Total body nevus counts were collected in a cross-sectional study of 1730 healthy females from the UK Adult twin registry comprising 709 dizygous and 156 monozygous pairs. Nevus density (ND) increased up to the age of 35 years and then gradually declined. Quantitative genetic analysis showed a smaller genetic influence (36%) on ND up to 35 years, compared with after 35 years where it rose to 59%. Using a sub-sample of 1238 genotyped individuals, we performed distinct genome-wide scans for individuals above and below 35 separately. In the younger sub-sample, we confirmed a quantitative trait locus (QTL) for ND on chromosomes 9p21 (LOD=2.54), a region already linked to both familial melanoma and ND. We also observed a linkage signal on 9q21 (LOD=2.55) overlapping a recently reported susceptibility locus for ocular and cutaneous melanoma in Danish families. The strongest evidence of linkage identified a novel QTL on chromosome 5q31-32 (LOD=3.47). None of these linkages was observed in the group aged 35 years and over, which showed suggestive linkage on chromosome 2p24 (LOD=2.75). To the best of our knowledge, this is the first genome-wide search for ND in a large sample of healthy adults. The results suggest that different sets of genes are likely to influence the processes leading to the appearance of nevi and their involution. They provide both novel and replicated QTLs for nevus development, some of which might overlap with those for melanoma and warrant detailed investigation.

The effect of sun exposure in determining nevus density in UK adolescent twins

We report a study of 221 teenage twin pairs to examine the genetic and environmental determinants of nevi representing the most potent phenotypic risk factor for melanoma. Our published heritability analysis estimated that nevi are mainly genetically determined. In this paper we examine the role of sun exposure. We report a correlation between nevus density and sun exposure, particularly that acquired in hotter countries than in the UK (mean nevus density 41 per m2 in those in the highest quartile of exposure vs 24 per m2 in those with no exposure, p<0.0001). We were not able to demonstrate a protective effect for either sun protection cream or shirt wearing. By including phenotypic variables and reported sun exposure into the heritability analysis, we conclude that 66% of the total variance of nevus count is attributable to genetic effects: 7% associated to eye color, 6% to hair color, and 1% to reported skin type, which leaves 52% as to yet unidentified genetic factors. Of the 25% of variation attributable to environmental influences, one-third is estimated to be because of sun exposure on hot holidays.