Fungal Ku prevents permanent cell cycle arrest by suppressing DNA damage signaling at telomeres

Abstract

The Ku heterodimer serves in the initial step in repairing DNA double-strand breaks by the non-homologous end-joining pathway. Besides this key function, Ku also plays a role in other cellular processes including telomere maintenance. Inactivation of Ku can lead to DNA repair defects and telomere aberrations. In model organisms where Ku has been studied, inactivation can lead to DNA repair defects and telomere aberrations. In general Ku deficient mutants are viable, but a notable exception to this is human where Ku has been found to be essential. Here we report that similar to the situation in human Ku is required for cell proliferation in the fungus Ustilago maydis. Using conditional strains for Ku expression, we found that cells arrest permanently in G2 phase when Ku expression is turned off. Arrest results from cell cycle checkpoint activation due to persistent signaling via the DNA damage response (DDR). Our results point to the telomeres as the most likely source of the DNA damage signal. Inactivation of the DDR makes the Ku complex dispensable for proliferation in this organism. Our findings suggest that in U. maydis, unprotected telomeres arising from Ku depletion are the source of the signal that activates the DDR leading to cell cycle arrest.

Contributions of recombination and repair proteins to telomere maintenance in telomerase‐positive and negative Ustilago maydis

“…Two of our recent observations highlight the striking resemblance of the U. maydis telomere regulatory mechanisms to those in humans. First, just like the human KU complex (but unlike any other system that has been studied to date), the U. maydis Ku complex is essential for cell viability and the loss of Ku expression causes massive telomere aberrations (de Sena-Tomas et al, 2015;Yu et al, 2015). Second, the two core recombination proteins in U. maydis (i.e., Rad51 and Brh2) and the Blm helicase are all required for normal telomere maintenance in telomerase-positive cells (Badie et al, 2010;Yu et al, 2013;; deleting each gene causes a significant reduction in telomere lengths in the mutants.…”

Telomere recombination pathways: tales of several unhappy marriages

“…It had been shown earlier that human KU86 (the human Ku80 gene) is essential and that KU86 -null cancer cells rapidly lose viability due to massive loss of telomere repeats from chromosome ends (Wang, et al 2009). Remarkably, these observations were recently shown to hold true for the U. maydis ku70 and ku80 genes (even though other vertebrate and mammalian KU mutants are known to be viable) (de Sena-Tomas, et al 2015, Yu, et al 2015). The non-viability of the Ustilago Ku70- and Ku80-deficient cells can be suppressed by atr1 or chk1 deletion, and these ku/checkpoint double mutants exhibit all the hallmark telomere aberrations of ALT cancer cells, including telomere length heterogeneity and high levels of ECTRs.…”

“…While these fungal mutants do not recapitulate all the telomere phenotypes of ALT cells, they have provided general insights on the variety of recombination factors and pathways that can be triggered at telomeres. More recently, a fungal model that exhibits greater similarity to ALT than previous models was reported in Ustilago maydis (de Sena-Tomas, et al 2015, Yu, et al 2015). U. maydis is a basidiomycete that is distantly related to budding and fission yeasts, and features of the U. maydis ALT model provide insights on the reasons that may underlie mechanistic distinctions and resemblances between different telomere recombination pathways in different organisms.…”

MRN- and 9-1-1-Independent Activation of the ATR-Chk1 Pathway during the Induction of the Virulence Program in the Phytopathogen Ustilago maydis

“…Components from 9-1-1 complex, like Rec1 (the Rad1 ortholog), and from MRN complex, like Mre11, have been previously described for U . maydis [ 38 , 39 ]. We queried the NCBI and Broad Institute databases for U .…”