The Fanconi anemia pathway is downregulated upon macrophage differentiation through two distinct mechanisms

The Fanconi anaemia (FA) pathway is a DNA-damage inducible pathway critical for genomic stability. FA patients typically display high cancer susceptibility and hypersensitivity to DNA-damaging agents such as cross-linkers and ionizing radiation. A key step in the activation of the FA pathway is monoubiquitination of the FancD2 protein. Here we report that the FA pathway is downregulated by two distinct mechanisms upon differentiation of THP-1 and HL-60 leukaemia cells into macrophages. Firstly, qRT-PCR analysis revealed a transcriptional downregulation of most components of the FA complex, including FancD2. Secondly, DNA damage-induced monoubiquitination of the remaining FancD2 became deficient at various stages of differentiation depending on the type of damage. This was attributed to the differentiation-induced downregulation of Chk1, which phosphorylates FancD2 as a prelude to its ubiquitination. Although Western blotting revealed that levels of FancD2 were greatly reduced in terminally differentiated macrophages and that FancD2 ubiquitination was abolished, double-strand breaks were proficiently repaired, likely through an increase in non-homologous end joining (NHEJ). It has been suggested that the FA pathway promotes repair of double-strand breaks via homologous recombination rather than NHEJ. Its downregulation in macrophages may thus be required to avoid promoting a repair mechanism that is inefficient in post-mitotic cells.