Paradoxical delay of senescence upon depletion of BRCA2 in telomerase‐deficient worms

Abstract

BRCA 2 is a multifunctional tumor suppressor involved in homologous recombination ( HR ), mitotic checkpoint regulation, and telomere homeostasis. Absence of Brca2 in mice results in progressive shortening of telomeres and senescence, yet cells are prone to neoplastic transformation with elongated telomeres, suggesting that BRCA 2 has positive and negative effects on telomere length regulation along the path to tumorigenesis. Using Caenorhabditis elegans as a model, we show here that depletion of BRC ‐2, an ortholog of BRCA 2, paradoxically delays senescence in telomerase‐deficient mutant worms. Telomerase‐deficient worms ( trt‐1 ) exhibit early replication senescence due to short telomeres. It should be noted that worms mutated in brc‐2 are not viable as well due to massive genotoxic insults. However, when BRC ‐2 is depleted by RNA interference in trt‐1 mutant worms, the number of generations is unexpectedly increased with telomere length maintained, compared to telomerase mutants. Interestingly, depletion of other HR genes such as rad‐51 and rad‐54 exhibited similar effects. In worms doubly deficient of telomerase and brc‐2 , rad‐51 , or rad‐54 , extra telomeric C‐circles were generated, suggesting that abrogation of HR induces an alteration in telomere environment favorable to illegitimate telomere maintenance when telomerase is absent. Collectively, absence of BRC ‐2 in telomerase‐deficient background first leads to telomere shortening, followed by an induction of an as‐yet‐unknown telomere maintenance pathway, resulting in delay of senescence. The results have implications in the understanding of dysfunctional BRCA 2‐associated tumorigenesis.

Brca2 abrogation engages with the alternative lengthening of telomeres via break‐induced replication

“…1C, II and III, +4-OHT). The result from the mouse pancreas organoid culture recapitulated the phenomena observed from telomerase mutant worms [13], suggesting that depletion of Brca2 in inducing ALT is evolutionarily conserved.…”

“…As the shortening of telomeres triggered by Brca2 deficiency is not related to telomerase expression and is incompatible with the fact that dysfunctional Brca2 leads to tumourigenesis, we had asked whether the paradoxical occurrence of neoplastic transformation is due to the induction of ALT. We showed in Caenorhabditis elegans that depletion of BRC-2, an orthologue of Brca2, results in a delay of senescence, accompanied by telomere length elongation [13]. Similarly, abrogation of Brh2, an orthologue of Brca2 in fungus, Ustilago maydis, led to an ALT-like feature [14].…”

Disruption of G-quadruplex dynamicity by BRCA2 abrogation instigates phase separation and break-induced replication at telomeres

“…Depletion of Brca2 in telomerase-negative cells has been shown to induce BIR at telomeres and to initiate ALT-like activity ( 6 , 7 ). One of the hallmarks observed in these Brca2 deficiency-induced ALT-like cells is the appearance of PML bodies at telomeres, known as ALT-associated promyelocytic leukemia bodies (APBs) ( 6 ).…”

“…Importantly, BRCA2 is involved in telomere replication homeostasis ( 4 , 5 ): lack of BRCA2 increases telomere shortening, particularly on the lagging strand ( 5 ). Paradoxically, the absence of Brca2 in telomerase-deficient mouse fibroblasts ( 6 ) or Caenorhabditis elegans ( 7 ) instigates BIR at telomeres. The absence of BRCA2 selectively chooses BIR over HR for the repair of telomere damage ( 8 ).…”