Fanconi anemia genes are highly expressed in primitive CD34+ hematopoietic cells.

BACKGROUND: Fanconi anemia (FA) is a complex recessive genetic disease characterized by progressive bone marrow failure (BM) and a predisposition to cancer. We have previously shown using the Fancc mouse model that the progressive BM failure results from a hematopoietic stem cell defect suggesting that function of the FA genes may reside in primitive hematopoietic stem cells. METHODS: Since genes involved in stem cell differentiation and/or maintenance are usually regulated at the transcription level, we used a semiquantitative RT-PCR method to evaluate FA gene transcript levels in purified hematopoietic stem cells. RESULTS: We show that most FA genes are highly expressed in primitive CD34-positive and negative cells compared to lower levels in more differentiated cells. However, in CD34- stem cells the Fancc gene was found to be expressed at low levels while Fancg was undetectable in this population. Furthermore, Fancg expression is significantly decreased in Fancc -/- stem cells as compared to wild-type cells while the cancer susceptibility genes Brca1 and Fancd1/Brac2 are upregulated in Fancc-/- hematopoietic cells. CONCLUSIONS: These results suggest that FA genes are regulated at the mRNA level, that increased Fancc expression in LTS-CD34+ cells correlates with a role at the CD34+ differentiation stage and that lack of Fancc affects the expression of other FA gene, more specifically Fancg and Fancd1/Brca2, through an unknown mechanism.

Interplay between human DNA repair proteins at a unique double-strand break in vivo.

DNA repair by homologous recombination is essential for preserving genomic integrity. The RAD51 paralogs (RAD51B, RAD51C, RAD51D, XRCC2 and XRCC3) play important roles in this process. In this study, we show that human RAD51 interacts with RAD51C-XRCC3 or RAD51B-C-D-XRCC2. In addition to being critical for RAD51 focus formation, RAD51C localizes to DNA damage sites. Inhibition of RAD51C results in a decrease in cellular proliferation consistent with a role in repairing double-strand breaks (DSBs) that occur naturally. To monitor a single DNA repair event, we developed immunofluorescence and chromatin immunoprecipitation (ChIP) methods on human cells where a unique DSB can be created in vivo. Using this system, we observed a single focus of RAD51C, RAD51 and 53BP1, which colocalized with gamma-H2AX. ChIPs revealed that endogenous human RAD51, RAD51C, RAD51D, XRCC2, XRCC3 and MRE11 proteins are recruited in the S-G2 phase of the cell cycle, while Ku80 is recruited during G1. We propose that RAD51C ensures a tight regulation of RAD51 assembly during DSB repair and plays a direct role in repairing DSBs in vivo.

Hematopoietic stem cells from fancc(-/-) mice have lower growth and differentiation potential in response to growth factors.

Fanconi anemia (FA) is a complex recessive genetic disease characterized by progressive bone marrow (BM) failure. We have previously shown that stem cells from the FA group C mouse model have lower long-term primary and secondary reconstitution ability, and that bone marrow of Fancc(-/-) mice contained fewer lineage-negative (Lin(-))Thy1.2(low)Sca-1(+)c-kit(+) CD34(+) cells but normal levels of Lin(-)Thy1.2(low)Sca-1(+)c-kit(+)CD34(-) primitive cells. These data suggest that CD34(+) primitive cells have either a lower growth or differentiation potential, or that these cells have greater apoptosis levels. To investigate the role Fancc might have on the growth and differentiation potentials of primitive hematopoietic stem cells, we used a single-cell culture system and monitored cell viability, doubling potential, and apoptosis levels of Fancc(-/-) primitive Lin(-)Thy1.2(-)Sca-1(+) (LTS)-CD34(+) and LTS-CD34(-) stem cells. Results showed that Fancc(-/-) LTS-CD34(-) and LTS-CD34(+) cells had altered growth and apoptosis responses to combinations of stimulatory cytokines, most dramatically in response to a combination of factors that included interleukin-3 (IL-3) and IL-6. In addition, Fancc(-/-) LTS-CD34(-) and LTS-CD34(+) cells showed a lower differentiation potential than Fancc(+/+) cells. These results support a role for Fancc in the growth and differentiation of primitive hematopoietic cells and suggest that an altered response to stimulatory cytokines may contribute to BM aplasia in FA patients.