Malignant melanoma, the most deadly form of skin cancer, shares few if any environmental or phenotypic risk factors with kidney cancer. The main risk factors associated with melanoma are sun exposure and skin pigmentation, whereas kidney cancer risk is associated with common cancer-predisposing phenotypes such as obesity and smoking.

However, melanoma and renal cell carcinoma (RCC) have been observed in the same patients and families often enough for a common underlying genetic factor to be suggested.

One transcription factor, the microphthalmia-associated transcription factor (MITF) is believed both to act as an oncogene in melanoma and to stimulate the hypoxia inducible factor (HIF1A) pathway, which has been linked to kidney cancer. This gene is therefore a strong candidate for a genetic link between these two malignancies.

The gene MITF has a complex structure, yielding several different protein products through alternative splicing.

A large group of researchers connected to the French Familial Melanoma Study Group and led by Brigitte Bressac-de Paillerets from Service d'Oncologie Génétique, Caen, France, set out to probe the role of MITF in these tumours types by sequencing the gene in sixty-two patients who had all at some time been diagnosed with both melanoma and RCC.

Five patients were found to have the same missense mutation in this gene. This mutation (E318K), in which the negatively-charged wild type residue is substituted by a positively-charged one, was found to be significantly more common in melanoma-RCC patients than in cancer-free controls (p = 1.3 x 10-4); individuals who carry this mutation are at 14-fold greater risk of developing both cancers than those who do not.

The researchers then investigated the effect of this mutation on predisposition to either melanoma or renal cancer alone. The E318K mutation was found at significantly higher frequencies in patients with either melanoma or RCC than those without. When all patient groups studied were pooled, the group of individuals carrying the mutation were found to be at over 5-fold greater risk of developing either or both cancers than those who were wild type at this position.

The gene MITF is known to be expressed in a majority of melanomas studied and in a number of renal cell tumours. The researchers then explored the function of the protein product of this gene and the effect of the E318K mutation on this function.

MITF forms a covalent bond to the small-ubiquitin-like modifier (SUMO) and the glutamic acid residue (E) that is mutated in this variant is a critically important part of the consensus motif (IKQE) that is necessary for SUMO binding.

Western blot analysis showed that SUMO binding to MITF was greatly reduced in the E318K variant of the protein and removed completely in a double mutant including this variation (p.K182R;p.E318K).

Binding of SUMO to MITF is known to repress its signal transduction activity and the associated gene transcription; as expected, the researchers found the transcriptional activity of the E318K mutant to be increased and that of the double mutant to be further increased compared to the wild type.

Many of the genes seen to be differentially regulated by wild type and E318K MITF were shown through bioinformatics analysis to be involved in cell processes linked to cancer such as cell growth, proliferation and inflammation.

Chromatin immuno-precipitation followed by sequencing (ChIP-seq) showed the E318K mutant to bind to a larger number of its target sites than the wild type protein.

The mutant protein was also shown to promote an invasive phenotype in both melanoma and RCC cells. The researchers concluded that their success in identifying this rare genetic variant that confers a moderate risk of several tumours validates genetic screening of patient cohorts as a useful strategy for identifying cancer-associated mutations.

Reference

Bertolotto, C., Lesueur, F., Giuliano, S. and 53 others, with the French Familial Melanoma Study Group (2011). A SUMOylation-defective MITF germline mutation predisposes to melanoma and renal carcinoma. Nature, published online ahead of print 20 October 2011. doi:10.1038/nature10539