Impaired DNA Damage Repair in Severe Congenital Neutropenia Patients

Severe congenital neutropenia (CN) is a pre-malignant bone marrow failure syndrome with maturation arrest of granulopoiesis at the level of promyelocytes in the bone marrow. We hypothesized that increased genetic instability in hematopoietic stem and progenitor cells (HSPC) of CN patients caused by inherited mutations in ELANE (neutrophil elastase) or HAX1(mitochondrial HCLS1-associated protein X-1) may lead to high risk of malignant transformation. Treatment of CN patients with granulocyte-colony stimulating factor (G-CSF) overcomes maturation arrest by forcing unfit HSPC to proliferate and differentiate despite the presence of inherited mutations and thus increasing the risk of leukemogenic transformation.

We first investigated differences in DNA damage susceptibility of CD34⁺ and CD33⁺ bone marrow cells from CN-ELANE (n = 3) and CN-HAX1 (n = 3) patients, as compared to healthy donors using short-term treatment (5 minutes) with bleomycin to induce DNA double-strand breaks. To detect DNA lesions we used the LORD-Q method, a high-sensitivity long-run real-time PCR-based technique for DNA damage quantification (Lehle S. et al., Nucleic Acids Research, 2014). We found no differences in DNA damage induction between both groups of CN patients and healthy donors. Therefore, we hypothesized that not DNA damage but DNA repair mechanisms may be affected in these patients. Indeed, Gene Set Enrichment Analysis (GSEA) of microarray data revealed a marked inhibition of gene expression in pathways associated with DNA double-strand break (DSB) repair, mismatch repair as well as cell cycle regulation in HSPC from CN patients as compared to cells from healthy individuals. Validation by qRT-PCR confirmed severe downregulation of genes related to DSB repair (BRCA1 and RAD51), mismatch repair (MSH2 and PCNA) as well ascell cycle regulation (CHEK2 and CDKN2C) in CD33⁺ of both CN groups as compared to healthy individuals. Interestingly, CN-ELANE and CN-HAX1 groups behaved similarly with some exceptions showing decreased expression of CDC25B, RAD50 and ATR expression in the CN-HAX1 group only and of MRE11A in the CN-ELANEgroup only.

Taken together, disrupted DNA repair and impaired expression of cell cycle regulating genes resulting from inherited mutations in ELANE and HAX1 indicate that HSPC of CN patients are more susceptible to malignant transformation.