Genome instability and DNA repair defect in patients' cells. (A) Representative picture of normal chromosomes, radial chromosomes, and chromosome breaks found in metaphase. (B-C) Quantitative analysis of radial chromosomes (B) and chromosome breaks (C) in untreated and MMC-treated cells. Cells from a Fanconi (FANC-G deficient) patient are used as a sensitive control. Percentage of events per chromosomes is indicated (2 independent experiments); Counted chromosomes: Ctrl -MMC: n = 583; Ctl +MMC: n = 929; P1 -MMC: n = 789; P1 +MMC: n = 620; P2 -MMC: n = 710; P2 +MMC: n = 674; Fanconi -MMC: n = 811; Fanconi +MMC: n = 965). (D) MMC sensitivity of SV40-transformed fibroblasts from P1 and P2, a Fanconi patient, and control. Mean and standard deviation of triplicates are represented. Result representative of 4 independent experiments. A 2-tailed standard t-test was used. (E) Aphidicolin sensitivity of SV40-transformed fibroblasts from P1 and P2, control, and control with ATR inhibitor (ATRi, 10µM). Mean and standard deviation of triplicates are represented. Result representative of 3 independent experiments. A 2-tailed standard t-test was used. (F) Phleomycin sensitivity of SV40-transformed fibroblasts from P1 and P2, a Cernunnos-deficient patient,⁵⁹ and a healthy control. Mean and standard deviation of triplicates are represented. Result representative of 5 independent experiments. A 2-tailed standard t-test was used. (G) Schematic representation of the correction of 1 Apollo variant in P2's cells via CRISPR/Cas9 generating a heterozygous Apollo-mutated P2 cell line (noted P2^L142S/WT). (H) Phleomycin sensitivity of hTERT SV40-fibroblasts from P2, P2^L142S/WT and a healthy control. Mean and standard deviation of triplicates are represented. Result representative of 3 independent experiments. A 2-tailed standard t-test was used. *P < .05, **P < .01, ***P < .001, ****P < .0001.