Adaptive genetic mechanisms in mammalian Parp1 locus

Abstract

PARP1 is a highly conserved nuclear protein in multicellular organisms that by modulating chromatin opening facilitates gene expression during development. All reported Parp1 null knockout mice strains are viable with no developmental anomalies. It was believed that functional redundancy with other PARP family members, mainly PARP2, explain such a controversy. However, while PARP2 has similar catalytic domain to PARP1, it lacks other domains, making the absence of developmental problems in Parp1 mice knockouts unlikely. Contrary to prior assumptions, in our analysis of the best-investigated Parp1 knockout mice strain we identified persistent mRNA expression, albeit at reduced levels. Transcript analysis revealed an alternatively spliced Parp1 variant lacking exon two. Subsequent protein analysis confirmed the existence of a truncated PARP1 protein in knockout mice. The decreased level of pADPr was detected in Parp1 knockout ES cells with Western Blotting analysis, but immunofluorescence staining did not detect any difference in distribution or level of pADPr in nuclei of knockout ES cells. pADPr level in double Parp1 Parg mutant ES cells greatly exceeded its amount in normal and even in hypomorph Parg mutant ES cells, suggesting the presence of functionally active PARP1. Therefore, our findings challenge the conventional understanding of PARP1 depletion effects.