Gene amplifications characterize acral melanoma and permit the detection of occult tumor cells in the surrounding skin.

Acral melanoma (AM) is commonly distinguished from superficial spreading melanoma (SSM), the most common type of melanoma, by its clinical presentation as well as its ethnic distribution. However, justification for such a distinction is controversial because of histological overlap and lack of prognostic significance. We analyzed chromosomal aberrations of 15 AMs and 15 SSMs that were comparable for tumor thickness and patient age, using comparative genomic hybridization. All AMs had at least one (mean, 2.0) gene amplification, significantly more than the SSMs, in which only 2 of 15 (13%) had one amplification each (P < 0.0001). At least 15 different genomic regions were amplified in AM. These involved small portions of chromosomal arms, sometimes including known oncogenes implicated in melanoma. The most frequently amplified regions in AMs occurred at 11q13 (47%), 22q11-13 (40%), and 5p15 (20%). Comparison of the amplification levels of invasive and noninvasive portions of the tumors using fluorescence in situ hybridization suggested that amplifications occurred before the formation of the invasive portion. The finding of amplifications of 11q13 in three of five additional cases of AM in situ further supports the notion that amplifications arise early in the progression of AM. Very significantly, we found isolated melanocytes with amplifications in the epidermis up to 3 mm beyond the histologically recognizable extent of the melanomas in 5 of 15 invasive AMs. In conclusion, our data show that AM is a distinct type of melanoma characterized by focused gene amplifications occurring early in tumorigenesis, and that malignant cells are present beyond the histologically detectable boundary, thereby revealing one mechanism of local recurrence.