PI3K-Like Protein Kinases
Recombination can drive tumourogenesis
Prof James Haber - Brandeis University, Waltham, USA
Recombination can be good and bad for cells, can you elaborate on this?
It’s essential for life so every time your cell replicates its DNA it has to copy this much DNA, literally, which is two metres of DNA and it does so with incredible fidelity. It’s 99.999999% accurate but that’s not enough nines. So it turns out that every time cells copy their DNA that there are breaks in the DNA that have to be repaired or else the cells die. So if you deprive mammalian cells of a number of different recombination proteins they basically can’t live. So recombination is as essential for life as transcription or translation or any other of the basic cell processes.
In that sense it is for good because without it you simply cannot survive but some repair mechanisms lead to what is known as loss of heterozygosity which, among other things, takes certain cells and makes them homozygous for tumour-creating mutations. In those circumstances recombination is for bad because you are essentially depriving the cell of a wild-type copy of some allele and this is part of the progression of various tumours. In addition, there are other repair mechanisms which are less accurate and if you look in tumour cells what you find is that their chromosomes are grossly rearranged. They’ve lost their more accurate ways of putting the DNA back together and they use these other repair mechanisms that put fragments of DNA back together that normally should not be together. Some of those lead to translocations which again are associated with particular cancers.
What is your lab particularly interested in?
We’re interested in the mechanisms by which cells repair in the most accurate way the breaks in DNA. We don’t study mammalian cells, we study simple yeast cells and the idea of our experiments are to create very specific breaks in exact locations that we know using enzymes that will break the DNA and then be able to follow in minute by minute detail how the breaks are being processed and how they’re being repaired. So we’re interested in that and we’re interested in how the cells detect that they have DNA damage and how they regulate their growth in response to DNA damage.
It turns out that tumour cells frequently either have lost their ability to repair normally these accurate ways of repairing DNA or they’ve failed to recognise breaks and respond appropriately. So these two sides of responding to a break are both important and that’s what we work on.
What do you think the PI3 kinase meeting aims to achieve?
The core proteins in almost all of these processes are signalling proteins that are highly related at the structural level. They all are known as PI3-like kinases, they all have a very common protein architecture. They’re hard to work on because most of these proteins are huge by protein standards so they’re hard to purify and they’re hard to work on biochemically. But they each signal a very different process and so it’s not uncommon for someone to go to a meeting that would be entirely devoted to ATM, which is one of these proteins, and sometimes people working on ATR, which is an ATM related protein, will sneak into the ATM meeting. The people who work on TOR or work on DNA-PK or other related proteins tend not to go to the same meetings and since there seem to be biochemically and structurally and signal commonalities among all these proteins it seems like it would be a really good idea to get the people who work on them all together to share from their different perspectives how this is working.
In my lab we started out working on the ATM and ATR like proteins and discovered now that we’ve found a connection to the so-called TOR pathway that we didn’t know existed before. So there are ways in which these pathways are being integrated.
What is autophagy?
TOR is a basic mechanism to respond to starvation by regulating how the cell is going to produce enough nutrients. So if cells are starving they will chew up pieces of themselves, that’s what autophagy really means. They do that to recycle components so they can make new proteins and keep themselves going. It turns out that there’s a second or, in fact, several highly specialised versions of autophagy which target and degrade very specific proteins which don’t just chew up at random whatever the grab mechanism will grab. So it turns out that some very critical cell processes are regulated by autophagy so that key components are being targeted in some cases from the nucleus into the cytoplasm into the lysosome or vacuole where they’re being degraded. So we’ve gotten very interested in that because a key mitotic regulator that we study turns out to be degraded by this pathway. We’ve shown that there is a DNA damage specific sub-pathway that is different from all the previously described versions of this process.
