The Fanconi Anaemia Protein D2 has an Essential Role in Telomere Maintenance in Cells that Utilize the Alternative Lengthening of Telomeres Pathway

Fanconi anaemia (FA) is an inherited disorder characterized by bone marrow failure, cancer predisposition and congenital abnormalities. The 12 known FA genes have been implicated in homologous recombination (HR), a process involved in telomere maintenance. A complex of at least 7 FA proteins promotes FANCD2 monoubiquitination and nuclear foci formation. FANCD2 colocalizes and interacts with HR proteins, however the role of FANCD2 in HR is unclear. Telomeres in dividing human somatic cells shorten until they reach a critical length, triggering most cells to undergo senescence or apoptosis. Rare immortal cells escape this crisis by expressing telomerase, or activating the Alternative Lengthening of Telomeres (ALT) pathway, which involves HR. FA core complex proteins and FANCD2 colocalize with telomeric foci in ALT, but not telomerase positive cells. Localization of FANCD2 to ALT telomeric foci requires monoubiquitination by the FA core complex, but is independent of ATM and ATR. FANCD2 primarily colocalizes with ALT telomeric DNA within ALT-associated PML bodies (APBs). Electron spectroscopic imaging and FISH experiments show that APBs contain extra-chromosomal telomeric repeat (ECTR) DNA that is non-nucleosomal. Depletion of FANCD2 causes marked increases in ECTR in ALT, but not telomerase positive cells. Overexpression of BLM, the helicase mutated in Bloom syndrome, also causes an ALT-specific increase in ECTR DNA. FANCD2 coimmunoprecipitates with BLM in ALT cells, and FANCD2 localization to ALT telomeric foci requires BLM expression. FANCD2-depleted ALT cells have reduced viability, signs of mitotic catastrophe, and multiple types of telomeric abnormalities, including increases in telomeric recombination, entanglements, colocalization with DNA repair proteins, and expression of fragile site characteristics. SiRNA depletion of FANCD2 does not cause overexpression of BLM, however codepletion of BLM with FANCD2 suppresses the telomere phenotypes caused by FANCD2 knockdown. Together this suggests that FANCD2 regulates BLM-dependent recombination and amplification of telomeric DNA within ALT cells.Ph