Chromosomal allele loss in primary cutaneous melanoma is heterogeneous and correlates with proliferation

Loss of heterozygosity at specific loci in neoplastic cells suggests the presence of a tumor suppressor gene within the deleted region. Using the microdissection technique, loss of heterozygosity has been identified in paraffin-embedded primary melanomas on chromosomes 1p and 9q. Our purpose was to determine loss of heterozygosity in primary cutaneous melanomas and to relate chromosomal alterations with cell morphology and proliferation of the tumor. Two to seven morphologic different areas in 12 primary cutaneous high risk melanomas (Breslow > 1.5 mm), as well as adjacent normal tissue, were microdissected and subjected to single step DNA extraction. Extracted genomic DNA was amplified by polymerase chain reaction using two polymorphic markers on chromosomes 1p (D1S450, between D1S548 and D1S228) and 9q (D9S12). Proliferation was evaluated by MIB-1 (Ki67) immunoreactivity and cell morphology, pigmentation and inflammation surrounding microdissected areas were investigated by microscopic inspection of hematoxylin and eosin stained sections. Twelve of 34 different areas (35%) showed loss of heterozygosity with at least one marker. Two of 32 areas showed loss of heterozygosity with D1S450 (two areas noninformative), seven of 30 with D9S12 (four areas noninformative), and three areas showed loss of heterozygosity at both loci. In three of these cases, analysis of different tumor foci revealed areas with/without loss of heterozygosity. In these cases, the percentage of MIB-I-positive cells was at least four times higher in areas with loss of heterozygosity compared with areas without loss of heterozygosity. Most areas with loss of heterozygosity consisted of small cuboidal to epitheloid cells. Spindle shaped and large anaplastic cells showed loss of heterozygosity less frequently. Neither melanization of tumor cells nor the presence of inflammation had an influence on the frequency of loss of heterozygosity. Primary cutaneous melanomas show intratumoral morphologic and chromosomal heterogeneity. Loss of heterozygosity on chromosomes 1p and 9q correlated with cell proliferation, suggesting that selected cell clones are responsible for tumor progression.