Detection of submicroscopic lymph node metastases with polymerase chain reaction in patients with malignant melanoma

The presence or absence of lymph node metastases in patients with malignant melanoma is the most powerful prognostic factor for predicting survival. If regional nodal metastases are found, the 5-year survival for the patient decreases approximately 50%. If the presence or absence of regional nodal metastases will determine which patients receive formal dissections or which patients enter adjuvant trials, then a technique is needed to accurately screen lymph node samples for occult disease. Routine histopathologic examination routinely underestimates the number of patients with metastases. This study was initiated to develop a highly sensitive clinically applicable method to detect micrometastases by examining lymph nodes for the presence of tyrosinase messenger RNA (mRNA). The hypothesis was that if mRNA for tyrosinase is found in the lymph node preparation, that finding is good evidence that metastatic melanoma cells are present.

The assay is accomplished using the combination of reverse transcription and double-round polymerase chain reaction (RT-PCR). The amplified samples are examined on a 2% agarose gel and tyrosinase cDNA is seen as a 207 base pair fragment. Lymph node preparations from 29 patients who were clinically stage I and II and undergoing elective node dissections were analyzed both by standard pathologic staining and RT-PCR.

Eleven of 29 lymph node (38%) samples from 29 patients with intermediate thickness melanoma were pathologically positive. Nineteen of the 29 lymph node preparations (66%) were RT-PCR-positive, and these included all of the pathologically positive samples, so that the false-negative rate was 0. In a spiking experiment, one SK-Mel-28 melanoma cell in a background of one million normal lymphocytes could be detected, thus indicating the sensitivity of this method. In addition, analysis by restriction enzyme mapping showed that the amplified 207-bp PCR product produced is part of the tyrosinase gene sequence.

The RT-PCR method is an extremely sensitive, reproducible, and efficient technique for the identification of micrometastases in patients with melanoma and could be widely applicable. If clinical correlation is obtained, staging of the melanoma patient becomes more accurate, and treatment becomes more standardized and rational, because all those patients who have evidence of nodal disease can be identified so that they may benefit from more extensive surgery (formal node dissections) or adjuvant therapies. Based on these results, RT-PCR could be a powerful tool to detect micrometastatic melanoma.