p53 Deficiency Rescues Neuronal Apoptosis but Not Differentiation in DNA Polymerase β-Deficient Mice

Abstract

In mammalian cells, DNA polymerase ␤ (Pol␤) functions in base excision repair. We have previously shown that Pol␤-deficient mice exhibit extensive neuronal cell death (apoptosis) in the developing nervous system and that the mice die immediately after birth. Here, we studied potential roles in the phenotype for p53, which has been implicated in DNA damage sensing, cell cycle arrest, and apoptosis. We generated Pol␤ ؊/؊ p53 ؊/؊ double-mutant mice and found that p53 deficiency dramatically rescued neuronal apoptosis associated with Pol␤ deficiency, indicating that p53 mediates the apoptotic process in the nervous system. Importantly, proliferation and early differentiation of neuronal progenitors in Pol␤ ؊/؊ p53 ؊/؊ mice appeared normal, but their brains obviously displayed cytoarchitectural abnormalities; moreover, the mice, like Pol␤mice, failed to survive after birth. Thus, we strongly suggest a crucial role for Pol␤ in the differentiation of specific neuronal cell types.Repair of DNA damage is essential for maintaining the integrity of the genetic information necessary for normal development and physiological consequences (28). DNA polymerase ␤ (Pol␤) is a 39-kDa protein of a single polypeptide, consisting of two catalytic, functional domains. The N-terminal 8-kDa domain carries a 5Ј-deoxyribose phosphate lyase activity, whereas the C-terminal 31-kDa domain carries a polymerase activity that fills a short gap with a 5Ј-phosphate (26,40). Pol␤ is a critical component of the base excision repair (BER) pathway. The BER pathway repairs DNA damage, such as apurinic/apyrimidinic (AP) sites and base modifications, which spontaneously occur or are induced by a variety of endogenous and exogenous agents, including reactive oxygen species and DNA alkylating agents. Biochemical studies have identified two types of BER in mammalian cells: a short-patch pathway involving replacement of one nucleotide and a long-patch pathway involving gap-filling of several nucleotides (46). The BER pathway is generally initiated by a specific DNA glycosylase that recognizes and removes a damaged base to generate an AP site in DNA, followed by incision of the site by an AP endonuclease. In the short-patch BER pathway, Pol␤ removes the 5Ј-deoxyribose phosphate and fills the single nucleotide gap, and finally DNA ligase I or a complex of XRCC1 and DNA ligase III ligates the nick. On the other hand, the long-

p53 deficiency rescues apoptosis and differentiation of multiple cell types in zebrafish flathead mutants deficient for zygotic DNA polymerase δ1

“…Thus far, implications of p53 deficiency on the rescue of apoptosis during vertebrate development have been mainly studied in the context of DNA repair, such as in the case of Polb-deficient mice 61 and of Fancd-morphant zebrafish. 39 In the context of DNA replication stress, similar studies have been largely restricted to cell culture systems.…”

Genetic interaction between DNA polymerase β and DNA-PKcs in embryogenesis and neurogenesis

“…45 However, since the phenotypes of our Polb-null mice considerably differ from those of the NHEJ-deficient mice, we favor the view that unrepaired SSBs cause neuronal apoptosis and lethality associated with Polb deficiency. 7,40 Double-mutant mice deficient in Polb and DNA-PKcs showed more severe phenotypes than Polb À/À mice as well as DNA-PKcs scid/scid mice (Figures 1-4). What is the reason for this difference?…”

“…These results indicate that p53 is a key factor for neuronal apoptosis responsive to DNA DSBs. In contrast, p53 deficiency cannot rescue lethality of Polb-deficient mice, 40 suggesting that Polb is a critical factor for neurogenesis. To examine whether p53 deficiency rescued apoptosis and lethality observed in Polb À/À DNA-PKcs scid/scid double-mutant mice, we generated Polb À/À DNA-PKcs scid/scid p53 À/À triple-mutant mice.…”

“…38,40,42 Furthermore, p53 deficiency is able to rescue lethality of the LigIV À/À and XRCC4 À/À mice, concomitant with the disappearance of apoptosis. These results indicate that p53 is a key factor for neuronal apoptosis responsive to DNA DSBs.…”

“…In this study, however, we found that in Polb We observed that apoptosis associated with Polb deficiency is mediated by the p53-dependent apoptosis pathway in the nervous system. 40 In SCID mice, there is no report on whether p53 deficiency rescues apoptosis in the nervous system. It is controversial whether DNA-PKcs is an upstream mediator of the p53 response to DNA damage.…”