Elevated FANCA Expression Marks a Worse Prognosis in Chronic Lymphocytic Leukemia. Evidence of a New Role of FANCA in the Stability of p53 By a Neddylation-Related Mechanism

Background: Chronic lymphocytic leukemia (CLL) is the most common hematological malignancy in western countries. It is characterized by a failure in the mechanisms of apoptosis that leads to an accumulation of mature B cells in peripheral blood, bone marrow and lymphoid organs. With exceptions, CLL is considered incurable and some patients show a worse prognosis related to the expression of certain cytogenetic or molecular markers such as p53 dysfunction (17p13 deletion or TP53 inactivation), 11q23 deletion, unmutated IgVH and the presence of a complex karyotype. FANC proteins have been related to chromosomal instability and alterations in the mechanisms of p53 activation, control of cell cycle and apoptosis. Germline mutations in any of the 20 FANC genes known so far generates Fanconi Anemia, a syndrome characterized by a extraordinary proneness to cell apoptosis leading to a progressive bone marrow aplasia and pancytopenia. Some FANCA proteins aggregate in response to DNA damage forming the FANCcore complex that mediates the monoubiquitination of FANCD2 and FANCI, thus activating the mechanism to repair stalled replication forks. In addition, individual FANC proteins have been involved in functions out of the FANCcore. This is the case of FANCA, that has recently been involved in the neddylation of CXCR5 and beta-2-microglobulin, processes reported to be altered in CLL.

Hypothesis: Given the fragmentary information connecting FANC proteins with cellular processes altered in CLL, like apoptosis, cell cycle or neddylation, we hypothesize a role of these proteins in the diagnosis or prognosis of CLL.

Methods: We analyzed the expression of 5 FANC genes in a cohort of 160 patients of CLL by quantitative RT-PCR. Statistical analysis were carried out to establish relations between FANC genes deregulation and clinical manifestations. We also investigated the role of the FANCA gene in primary circulating B-lymphocytes from CLL patients by either gain- or loss-of-function approaches.

Results: Our data identified a group of CLL patients with high expression of FANCA in peripheral B-CLL cells, and we stablished its relationship with the deletion of 11q23 and a worse prognosis. When we investigated the molecular mechanisms of this bad prognosis, we observed a reduction in the mRNA expression of two p53 target genes, p21 and ∆Np73, in CLL primary cells transfected with FANCA. Luciferase studies demonstrated an impairment of p53 function by FANCA. Moreover, we obtained evidence of a cooperation between FANCA and the NEDD8-interacting protein NUB1L in the destabilization of p53.

Conclusion: These results point to FANCA as a bad prognosis marker in CLL and unveils a new role of this protein aside from its role in the FA-BRCA DNA repair pathway.