Ho, Ho, Hox

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Fruit flies are a fascinating scientific resource to consider if you can get beyond your annoyance when they appear in one of those lovely boxes of ripe fruit you receive from a relative this time of year. (Just be thankful it wasn’t fruitCAKE!). For some great reading on the topic, I highly recommend a book, “Time, Love, and Memory“, the story of Seymour Benzer and how his graduate students figured out how different genes are involved in these creatures’ sense of time, or how they do their mating dance or remember whether they shouldn’t put their little leg down into a beaker and get a shock.

As with their behavior, there are wonderfully complex genes that also control how they develop from a single fertilized egg into an adult fly. These are called homeobox or “Hox” genes and it turns out their analogues are conserved throughout the animal kingdom. In this nice review of their functions, the following picture shows how the gene family controls development in the anterior – posterior development of the fly AND the mouse embryo.

When things go wrong in the fruit fly (Drosophila), you can get a fascinating mutation that makes the fly look like this, with legs appearing where there should be antennae. In humans, analogous mutations can result in having extra fingers or malformations. You can read in more depth about how the Hox (a subset of the master homeotic regulator) genes are regulated at the Kahn academy in this article.

OK, you say, but what could this possibly have to do with prostate cancer? Ah, that’s what I find fascinating. Cancer is a superb example of dysregulation of the genetic programs that make cells behave. By the time you get to an animal developing a prostate gland, there are countless regulatory genes that must each turn on or off at the right time in embryogenesis. And just as “ontogeny recapitulates phylogeny“, oncology recapitulates ontogeny. One of these homeobox genes, HOXB13 was discovered to be mutated in studies of families with hereditary risk for prostate cancer by Johns Hopkins investigators several years ago. This gene interacts with the androgen receptor, so it makes some sense that the prostate gland would be affected by mutations. Further studies of families with this mutation indicate that if you inherit one copy of the G48E mutation, your risk of developing prostate cancer is 2.6 fold increased.

Whereas testing for such genetic mutations (and many others) used to be the provenance  of research labs, we are entering a time in medicine when genetic testing is becoming “mandatory” for best practice care. The following criteria are now used to help discern who might benefit from such testing:

This table comes from a company, Myriad, that is now advertising for its own cancer risk gene panel, but there are several such companies and panels of genes. Although we (I) still don’t send off a genetic panel test to Myriad, Foundation Medicine, Invitae or the other companies in all patients, we are rapidly approaching the time when that will be standard. The challenges (as outlined in this article) are which genes should be tested, and what to do with the results. Some mutations such as those involving DNA damage repair, are already recognized as useful in directing therapy. For now, it is a topic best discussed with a genetics counsellor, and I fear, even more importantly one with an interest in prostate cancer if you can find one. Most of us physicians are struggling to keep up with which panel (if any) to order and when to order it.

So just remember when you see that little fly emerge from your fruit box this season, he/she/it has made immeasurable contributions to cancer research, and be thankful for all the science that is helping us to understand our amazing world.