RUNX1 as a recombinase cofactor

Interactions between the DNA Damage Response and the Telomere Complex in Carcinogenesis: A Hypothesis

Contrary to what was once thought, direct cancer originating from normal stem cells seems to be extremely rare. This is consistent with a preneoplastic period of telomere length reduction/damage in committed cells that becomes stabilized in transformation. Multiple observations suggest that telomere damage is an obligatory step preceding its stabilization. During tissue turnover, the telomeres of cells undergoing differentiation can be damaged as a consequence of defective DNA repair caused by endogenous or exogenous agents. This may result in the emergence of new mechanism of telomere maintenance which is the final outcome of DNA damage and the initial signal that triggers malignant transformation. Instead, transformation of stem cells is directly induced by primary derangement of telomere maintenance mechanisms. The newly modified telomere complex may promote survival of cancer stem cells, independently of telomere maintenance. An inherent resistance of stem cells to transformation may be linked to specific, robust mechanisms that help maintain telomere integrity.

Evidence for a role of RUNX1 as recombinase cofactor for TCRβ rearrangements and pathological deletions in ETV6-RUNX1 ALL

T-cell receptor gene beta (TCRβ) gene rearrangement represents a complex, tightly regulated molecular mechanism involving excision, deletion and recombination of DNA during T-cell development. RUNX1, a well-known transcription factor for T-cell differentiation, has recently been described to act in addition as a recombinase cofactor for TCRδ gene rearrangements. In this work we employed a RUNX1 knock-out mouse model and demonstrate by deep TCRβ sequencing, immunostaining and chromatin immunoprecipitation that RUNX1 binds to the initiation site of TCRβ rearrangement and its homozygous inactivation induces severe structural changes of the rearranged TCRβ gene, whereas heterozygous inactivation has almost no impact. To compare the mouse model results to the situation in Acute Lymphoblastic Leukemia (ALL) we analyzed TCRβ gene rearrangements in T-ALL samples harboring heterozygous Runx1 mutations. Comparable to the Runx1+/- mouse model, heterozygous Runx1 mutations in T-ALL patients displayed no detectable impact on TCRβ rearrangements. Furthermore, we reanalyzed published sequence data from recurrent deletion borders of ALL patients carrying an ETV6-RUNX1 translocation. RUNX1 motifs were significantly overrepresented at the deletion ends arguing for a role of RUNX1 in the deletion mechanism. Collectively, our data imply a role of RUNX1 as recombinase cofactor for both physiological and aberrant deletions.