Uveal melanoma is the most frequent primary intraocular tumour in Caucasian adults, having an annual incidence rate of 0.7 per 100000 people (Prescher et al, 1996). The eye is the most common site for noncutaneous melanomas, accounting for approximately 80% of such lesions (Sisley et al, 2000) and accounting for 13% of all deaths from melanoma because of its very high mortality rate (Albert et al, 1992). Both uveal and cutaneous melanomas originate from the melanocyte, but little is known about the underlying molecular pathogenesis of uveal melanoma. This is in contrast to cutaneous melanoma where there have been more substantial advances in detecting mutations (Chin et al, 1998). Both tumours differ significantly in their aetiology, with UV light appearing to play little or no part in the causation of uveal melanoma, unlike skin melanoma (Dolin et al, 1994). Uveal melanomas spread haematogenously leading to liver metastasis, whereas cutaneous melanoma spreads mainly via the lymphatics (Seftor et al, 1999) with skin metastases a more common problem. Unlike cutaneous melanoma, no genes or tumour-suppressor pathways have so far been convincingly linked to uveal melanoma (Edmunds et al, 2002).
It has recently been reported that a large proportion of cutaneous melanoma tumours contain activating oncogenic mutations in the BRAF gene (Davies et al, 2002). This is an oncogene in the RAS–RAF–MEK–ERK–MAP kinase pathway that mediates cellular response to growth signals. Genetic alterations to key components of this pathway are known to contribute to the development of many cancers (Pollock and Meltzer, 2002). Activating RAS point mutations are known to be found in more than 30% of human tumours, predominantly pancreatic, colonic, and in up to 36% of cutaneous melanomas (Demunter et al, 2001). BRAF is a gene that is regulated by RAS binding, and was shown to have missense mutations in 66% of primary melanoma tumours, 59% of melanoma cell lines, and 80% of melanoma short-term cultures (Brose et al, 2002; Davies et al, 2002). Mutations have also been detected in up to 82% of cutaneous melanocytic nevi (Pollock et al, 2003). Activation of this pathway has been noted in uveal melanoma tumours although mutations have not been detected in any of the RAS genes (H-, K, and N-RAS) (Mooy et al, 1991; Soparker et al, 1993). This makes BRAF an interesting candidate gene to screen in uveal melanoma tumours because of BRAF mutation being a potential mechanism for the activation of this pathway, and the fact that BRAF mutations are not thought to be related to the effects of UV light (Davies et al, 2002).
BRAF mutations were predominantly found in two small regions of the kinase domain of the BRAF molecule. The majority of the mutations were a single T→A base substitution at nucleotide 1796 in exon 15 of the BRAF gene, and in some of the adjacent codons. A smaller number of mutations were also found in a region of exon 11, and other lower levels of mutations have been reported in these codons in cancers including ovarian, sarcomas, lung (Brose et al, 2002), and colorectal tumours (Rajagopalan et al, 2002).