Loss of PICH Results in Chromosomal Instability, p53 Activation, and Embryonic Lethality

Abstract

PICH is a DNA translocase necessary for the resolution of ultrafine anaphase DNA bridges and to ensure the fidelity of chromosomal segregation. Here, we report the generation of an animal model deficient for PICH that allowed us to investigate its physiological relevance. Pich KO mice lose viability during embryonic development due to a global accumulation of DNA damage. However, despite the presence of chromosomal instability, extensive p53 activation, and increased apoptosis throughout the embryo, Pich KO embryos survive until day 12.5 of embryonic development. The absence of p53 failed to improve the viability of the Pich KO embryos, suggesting that the observed developmental defects are not solely due to p53-induced apoptosis. Moreover, Pich-deficient mouse embryonic fibroblasts exhibit chromosomal instability and are resistant to RAS^V12/E1A-induced transformation. Overall, our data indicate that PICH is essential to preserve chromosomal integrity in rapidly proliferating cells and is therefore critical during embryonic development and tumorigenesis.

Keywords: DNA damage; Ercc6l; Pich; UFBs; X chromosome inactivation; genomic instability; ultrafine anaphase DNA bridges.

Graphical abstract

Figure 1

Pich KO Leads to Embryonic Lethality in Mice (A) Gene-targeting strategy for the generation of Pich KO mice (see Experimental Procedures for details). ORF, open reading frame. (B) Morphology of WT and Pich KO embryos at 10.5–13.5 days post-coitum (DPC). Pich KO embryos are smaller than WT embryos and die at approximately 12.5 DPC. (C) Quantification of embryo length. Error bars indicate SD; data from at least three embryos per genotype. (D) Immunohistochemical staining for PICH in WT, Pich heterozygous (Het), and Pich KO embryos at 10.5 DPC confirms the absence of PICH protein in Pich KO embryos. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001, ^∗∗∗∗p < 0.0001. See also Figure S1.

Figure 2

Pich KO Embryos Show Increased DNA Damage and Apoptosis as Compared to WT and Heterozygous Embryos (A–C) Immunohistochemical staining of WT, Pich heterozygous (Het), and Pich KO embryos for γH2AX (A), p53 (B), and cleaved caspase-3 (C) at 10.5 DPC. (D) Quantification of the number of positive cells for γH2AX, p53, and cleaved caspase-3 in the brains of WT, Pich Het, and Pich KO 10.5 DPC embryos. Data shown correspond to four to six fields per embryo, counting at least 200 cells per field. For γH2AX, one WT embryo, two Pich Het, and two Pich KO embryos were assessed. For p53 and cleaved caspase-3, two WT, three Pich Het, and three Pich KO embryos were assessed. Error bars indicate SD. See also Figures S1, S2, and S3.

Figure 3

Characterization of Pich KO MEFs (A) Western blotting confirming the absence of PICH protein in extracts from two Pich KO MEF cell lines as compared to WT MEF cell lines. Vinculin was used as the loading control. (B) Cell proliferation curve of WT, Pich Het, and Pich KO primary MEFs derived from 11.5 DPC littermate embryos. Pich KO MEFs proliferated at a lower rate than WT and Pich Het MEFs from passage one and stopped proliferating after four passages. Data represent means and SDs from two different cell lines per genotype, two replicates per cell line. (C) Senescence-associated beta-galactosidase (SA-β-gal) staining of WT, Pich Het, and Pich KO MEFs. (D) Quantification of the number of SA-β-gal⁺ cells. Means and SDs were calculated from at least five fields per cell line and two cell lines per genotype. (E) DNA replication rate and cell-cycle profile of WT and Pich KO MEFs determined by high-content microscopy. S-phase cells are shown in red, G2 cells in blue, and polyploid cells in green. (F) Quantification of DNA replication rate (percentages of EdU incorporating cells) and percentages of G2 cells and polyploid cells from the cellular profiles shown in (E). Data shown correspond to means and SDs from two WT and two Pich KO MEF lines analyzed in technical triplicates. ^∗∗p ≤ 0.01; ^∗∗∗∗p ≤ 0.0001. See also Figure S4.

Figure 4

Genomic Instability and RAS^V12/E1A Transformation in Pich KO MEFs (A) Representative images of immunofluorescence staining for 53BP1 in WT and Pich KO MEFs at early passages. Arrows indicate micronuclei, the number of which is increased in Pich KO MEFs. (B) Quantification of cells with 53BP1 foci by high-content microscopy. Data correspond to three WT, one Pich Het, and two Pich KO MEF cell lines. Three wells were counted per cell line, with >1,000 cells for each three wells combined. Means and SDs are indicated. (C) Quantification of cells with micronuclei. Images were acquired by an automated microscope and micronuclei were manually scored by two independent researchers. Data correspond to technical triplicates of three WT, one Pich Het, and two Pich KO MEF cell lines, counting three wells per cell line and at least 500 cells per sample. Means and SDs are indicated. (D) RAS^V12/E1A transformation assay of WT, Pich Het, and Pich KO MEF cell lines with pBabe-Ras/E1A vector (left) and empty pBabe vector as control (right). Colony formation after seeding infected cells at low density and staining 2 weeks later. (E) Quantification of the number of colonies in (D) and Figure S4D. Means and SDs were calculated from three (Ras/E1A) or two (pBabe) technical replicates and two MEF lines per genotype. ^∗p ≤ 0.05; ^∗∗p ≤ 0.01; ^∗∗∗p ≤ 0.001; ^∗∗∗∗p ≤ 0.0001. See also Figure S4.