Investigating spontaneous sister chromatid exchange in uveal melanoma.

Sister chromatid exchange (SCE) occurs during S-phase of the cell cycle and is the physical exchange of genetic information between two sister chromatids. Although it has been used for many years as a marker of mutagenesis and genetic instability, the exact mechanism of SCE formation remains to be elucidated. However, it is known to be an end product of the DSB repair pathway, homologous recombination (HR), and defects in various repair proteins have been associated with altered levels of SCE, we therefore investigated the DNA damage repair response in uveal melanoma, using these cells as a model system for SCE formation. Uveal melanoma is a rare but aggressive cancer that arises in the uveal tract of the eye affecting the iris, ciliary body and choroid. It is characterised by non-random chromosomal alterations such as monosomy of chromosome 3 that is associated with metastasis and a poor prognosis. However, random chromosomal aberrations are rare suggesting that these tumours have low genetic instability. In addition these tumours have been found to have reduced levels of spontaneous SCE, which is also consistent with low genetic instability. In general, cancer is associated with high genetic instability and high SCE; this is therefore the only disease state known to date to exhibit this low SCE phenotype. Here, we have found that uveal melanoma cells have high levels of spontaneous DNA damage consistent with a defect in repair. Furthermore, endogenous HR is reduced and we postulate that it is caused by a reduction in the expression of the FA protein F ANCD2, and that this defect causes the reduced SCE frequency in these cells. Consistent with this, complementing uveal melanoma cell lines with F ANCD2 restores the spontaneous levels of SCE. In addition, we have shown that the high chemo-resistance of uveal melanoma to interstrand cross-link inducing agents such as Mitomycin C is due to a defect in metabolism rather than a defect in DNA repair.