The biology of micrometastases from uveal melanoma

The aim of this study was to investigate the possible causes of tumour latency in uveal melanoma primarily through the analysis of micrometastases in tissue obtained from donors postmortem. Various explanations have been proposed but there is no clear answer from animal studies and few human data. The main hypotheses may be divided into several areas--immunological control of metastatic cells, lack of angiogenesis within micrometastases and reduced cell turnover.

196 patients were recruited to the study between 2003 and 2007. Patients were invited to take part and their relatives agreed to postmortem examination of their liver and lungs in the event of their death, including tissue sampling to assess the presence of micrometastases and their biology. Metastatic cells were detected by immunohistochemistry using a pan-melanoma antibody reagent, and by quantitative reverse transcriptase (qRT)-PCR for three melanoma-associated genes (tyrosinase Melan-A, and gp100) and a housekeeping gene (HMBS/PBGD) in samples stored in RNAlater or as formalin-fixed paraffin-embedded tissue.

22 deaths were investigated at autopsy as part of the study. Sixteen patients died with large deposits of metastatic melanoma, while six patients died of other causes. In addition, a liver resection for hepatic adenoma provided further tissue from a case without clinical evidence of metastasis. Metastatic melanoma cells were identified by immunohistochemistry of the liver samples in one case and by qRT-PCR in two further cases without macrometastases. There was no evidence of multicellular micrometastases sufficiently large to require angiogenesis and no associated inflammation was observed.

The most likely explanation for latency in this setting is the inability of uveal melanoma cells in metastatic sites to grow.