New Bone Marrow Failure Genes: DNAJC21 and ERCC6L2

A significant number of cases with bone marrow failure present with one or more extra-hematopoietic abnormality. This suggests a constitutional or genetic basis, and yet many of them remain uncharacterized. Through exome sequencing, we have recently identified two sub groups of these cases, one defined by germline biallelic mutations in DNAJC21 (DNAJ homolog subfamily C member 21) and the other in ERCC6L2 (excision repair cross complementing 6 like-2).

Patients with DNAJC21 mutations are characterized by global bone marrow failure in early childhood. They can also have a variable number of extra-hematopoietic abnormalities such as short stature and retinal dystrophy. The encoded protein associates with ribosomal RNA (rRNA) and plays a highly conserved role in the maturation of the 60S ribosomal subunit. Lymphoblastoid patient cells exhibit increased sensitivity to the transcriptional inhibitor actinomycin D and reduced levels of rRNA. Characterisation of mutations has revealed impairment in interactions with cofactors (PA2G4, HSPA8 and ZNF622) involved in 60S maturation. DNAJC21 deficiency results in cytoplasmic accumulation of the 60S nuclear export factor PA2G4, aberrant ribosome profiles and increased cell death. Collectively these findings demonstrate that biallelic mutations in DNAJC21 cause disease due to defects in early nuclear rRNA biogenesis and late cytoplasmic maturation of the 60S subunit.

Patients harbouring biallelic ERCC6L 2 mutations are characterized by bone marrow failure (in childhood or early adulthood) and one or more extra-hematopoietic abnormality such as microcephaly. Knockdown of ERCC6L2 in human cells significantly reduces their viability upon exposure to the DNA damaging agent irofulven but not etoposide and camptothecin suggesting a role in nucleotide excision repair. ERCC6L2 knockdown cells and patient cells harbouring biallelic ERCC6L2 mutations also display H2AX phosphorylation that significantly increases upon genotoxic stress, suggesting an early DNA damage response. ERCC6L2 is seen to translocate to mitochondria as well as the nucleus in response to DNA damage and its knockdown induces intracellular reactive oxygen species (ROS). Treatment with the ROS scavenger, N-acetyl-cysteine, attenuates the irofulven-induced cytotoxicity in ERCC6L2 knockdown cells and abolishes its traffic to mitochondria and nucleus in response to this DNA damaging agent. Collectively, these observations suggest that ERCC6L2has a pivotal rolein DNA repair and mitochondrial function.

In conclusion, ERCC6L2 and DNAJC21 have an important role in maintaining genomic stability and ribosome biogenesis, respectively. They bring into focus new biological connections/pathways whose constitutional disruption is associated with defective hematopoiesis since patients harbouring germline biallelic mutations in these genes uniformly have bone marrow failure.