Monday, May 25, 2009

Pondering tumor suppressors

Now that I'm back in the cancer field full-time, I spend a lot of that time pondering the mysteries of the disease. Despite an explosion of knowledge about the disease during my lifetime, we truly don't understand how it works. In many ways we're still at the stage of the old story of seven blind men, not having figured out the elephant in front of us.

Sometimes when genes acquire mutations this moves a cell on the road to cancer. Such genes fall into two general categories. Oncogenes acquire activating mutations or are amplified and then play an active role in cancer. Tumor suppressors lead to disease when they are inactivated by mutations. A handful of genes have a very murky status, seemingly able to play both roles.

Many tumor suppressors were discovered through rare hereditary syndromes characterized by tumors. For example, RB1 is the retinoblastoma gene; inactivation of this gene in the retina leads to horrific tumors of the eye. NF1 is the neurofibramatosis gene; inactivation leads to benign tumors from nerves. Perhaps the best known in the popular space are BRCA1 and BRCA2, which greatly raise the risk of breast and ovarian cancer.

A great mystery for many such genes is why the tissue specificity of the tumor syndrome? In each of the genes mentioned above, the tumor syndrome appears to be very specific to a tissue type, yet in each of these cases the genes involved have been shown to be parts of cellular machinery used by every cell. Why does a failure of a general part manifest itself so specifically?

As we dig deeper into the genes and cancer, some of these distinctions do start smudging. BRCA1 mutations, for example, do also raise the risk of pancreatic cancer -- but not nearly to the extent as for breast cancer. If we look not at known hereditary links to cancer but the genes mutated in any cancer, we see these same players showing up. For example, RB1 is frequently mutated in a variety of cancers, including lung cancers.

Here's an interesting further bit to ponder. BRCA1 and BRCA2 are in a pathway together, so it is not surprising that mutating either one would have a similar effect. But again, mutations in other members of the pathway lead to other genetic disorders with different spectra of cancers.

Now a new bit of the puzzle that continues the puzzling. One of the physical partners of BRCA1 is BARD1. A lot of effort has gone into finding variants in BARD1 and attempting to demonstrate their relevance to breast cancer risk. While many variants have been found in BARD1, the linkage to breast cancer is weak if it exists at all. But a new paper now links germline variation in BARD1 to the risk of aggressive neuroblastomas.

The one clear thread in this is that continuing to cross-reference these known tumor suppressors and their partners (such as this recent report on PALB2, a physical partner of BRCA2 with links now to breast and pancreatic cancer) with emerging genetic information will yield fruit. There are probably many more such associations to be found and perhaps additional proteins in these pathways to be uncovered. But when will we finally conceptualize the elephant? That remains to be seen

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