3.6 years ago by
Genes are not "for causing cancer". Genes are all evolved to have some particular role, which can cause cancer when they go awry. Some "cancer genes" are growth factors, which cause cancer when their activity is increased. Some "cancer genes" are growth restricting, which cause cancer when they become non-functional or under-expressed. They all perform a vital function when behaving normally.
If we look at the GO terms associated with BRCA1 we can see that it is involved in DNA replication and repair – vital functions. But it's also clear how damage to these processes would lead to cancer, as faulty DNA-repair mechanisms would increase the mutation rate.
BRAF is mutated in 80% of melanomas. Indeed, the simultaneous mutation of BRAF and TP53 can trigger melanoma with no other mutation, and this has been modelled in zebrafish, so this is not just a human mechanism. But BRAF isn't just a "cancer gene", it's necessary for signal transduction and development. TP53 is, arguably, a "cancer gene" because its role is to keep a check on cell division, causing apoptosis in cancerous cells.
The only reason humans get more cancer than other animals is that technology has led to better nutrition and medicine, meaning we live longer and don't tend to die of other things, like starvation and infection. You live long enough, you get cancer.