FANCD2 Localizes to Cross-Linked Induced Nuclear Foci That Are Distinct From Those to Which αaII Spectrin and the Cross-Link Repair Protein, XPF, Co-Localize, Indicating An Involvement of These Proteins in Different Steps of the DNA Interstrand Cross-Link Repair Process.
Authors: Zhang Pan , Sridharan Deepa , Acosta Michael , Lambert Muriel 
Published: October 2019

Abstract
Abstract 3196
Poster Board III-133
The hereditary bone marrow failure disorder, Fanconi anemia (FA), is characterized by a markedly increased incidence of acute myelogenous leukemia, diverse congenital abnormalities and a defect in ability to repair DNA interstrand cross-links. We have previously shown that in FA cells there is a deficiency in the structural protein nonerythroid a spectrin (aSpII), which is involved in repair of DNA interstrand cross-links and binds to cross-linked DNA. aSpII co-localizes in nuclear foci with FANCA and the cross-link repair protein, XPF, after normal human cells are damaged with a DNA interstrand cross-linking agent. One of the FA proteins which is thought to play an important role in the repair of DNA interstrand cross-links is FANCD2, which is known to form nuclear foci after cross-link damage. The present study was undertaken in order to get a better understanding of the relationship between aSpII and FANCD2, whether they interact with each other during the DNA repair process and co-localize in damage-induced nuclear foci. Immunofluorescence microscopy was carried out to determine whether these proteins co-localized in nuclear foci after cells were damaged with a DNA interstrand cross-linking agent, 8-methylpsoralen plus UVA light (8-MOP) or mitomycin C (MMC). Time course measurements showed that FANCD2 foci were first visible at 2 hours after damage and increased up to 16 hours and were still present at 72 hours after damage. This time course of foci formation correlated with levels of monoubiquitination of FANCD2. Measurement of gH2AX foci formation showed that the time course of foci formation was similar to that of FANCD2 measured up to 72 hours post damage. In contrast, aSpII foci were first visible between 8-10 hours after damage. The number of these foci peaked at 16 hours and by 24 hours foci were no longer observed. Co-localization studies showed that there was little co-localization of the FANCD2 and aSpII foci over this time course. This indicates that these two proteins may be involved in different steps in the DNA interstrand cross-link repair process. Based on models that have been proposed for the role of FANCD2 in the repair of DNA interstrand cross-links, we propose that, after DNA damage, FANCD2 localizes at DNA replication forks stalled at sites of interstrand cross-links and aids in the assembly of proteins at this site. This is followed by localization of aSpII and XPF and other proteins involved in the initial incision steps in DNA interstrand cross-link repair where they play a role in the unhooking of the cross-link. FANCD2 is then involved in subsequent steps in the repair process, which involve homologous recombination. Thus two proteins, FANCD2 and aSpII, both of which have been shown to be critical for the DNA interstrand cross-link repair process may be involved in different or distinct steps in this repair process. Deficiencies in these proteins would impact on DNA interstrand cross-link repair and, as we have shown for aIISp, would have an adverse effect on the genomic stability of FA cells. .

Disclosures
No relevant conflicts of interest to declare.