The role of the Tousled Like Kinases in genome stability and mammalian development

The human Tousled-like kinases 1 and 2 (TLKs) are predicted serine/threonine kinases that show maximal activity in S phase and are transiently inhibited by the DNA damage response (DDR). Both TLKs interact with and phosphorylate each other and the histone chaperone Asf1. Asf1 plays a critical role in regulating histone pools during several cellular processes, suggesting that the primary function of TLKs could be in the regulation of chromatin assembly during transcription, replication and repair processes. Thus, we hypothesize that reduction of TLK activity will impact on the function of Asf1, and perhaps other chromatin modulators, affecting key cellular processes that maintain genome integrity and proliferative capacity. In order to examine the in vivo consequences of TLK1 or TLK2 loss of function, we generated mice harboring genetraps that inhibit the expression of either gene. Surprisingly, mice lacking TLK1 were born normally, showed no overt pathology and aged normally over 18 months. Examination of developmental processes, such as lymphocyte maturation, revealed no abnormalities, and the DNA replication and cell cycle progression were normal, even following DNA damage. To determine if TLK2 provided redundant functions, we performed transient siRNA depletion of TLK2 in wild type and TLK1 null cell cultures. While this led to no defects in survival in WT cells, TLK1 mutants were profoundly sensitized to DNA damaging agents. We next generated mice harboring a genetrap allele to block the expression of TLK2. In contrast to TLK1, no liveborn homozygous mutants have been observed and embryos isolated for MEFs are severely runted, displaying heterogeneous defects including failure to close the neural tube and placental impairment. These data indicated that TLK1 and TLK2 do not play equivalent roles during development. Given that we see an acute response to DNA damage under conditions where total TLK activity is reduced and it is unlikely that cells lacking all TLK activity can support proliferation, we believe that the modulation of TLK activity represents a potentially valuable therapeutic approach. Collectively, the work I have presented represents a significant advance in our understanding of TLK function in cells and in mammalian development and supports the possibility that TLKs represent a potentially valuable target for the treatment of human disease