A human-specific micro-rna encoded by notch-1 modulates mitotic checkpoint machinery

Abstract

An expanded and sophisticated genome has provided the eukaryotes with a highly versatile genetic toolbox to orchestrate complex interaction networks necessary for development and homeostasis of multi-organ systems. The expansion of genome could not happen without significant contribution of mobile genetic elements. It is estimated that up to 50% of human genome is composed of transposable elements. Yet, the absolute dependence of metazoans on transposable elements for genomic expansion and concomitant evolutionary sophistication poses a radical dilemma. Any Expansion phase that is not followed by periods of genomic stability offsets the evolutionary advantages offered by genomic alterations. Following speciation, periods of genomic stability are required to allow for viability of evolved species and accumulation of further changes in transiently stable species. Transient stability of genome in extant species is contingent upon functionality of a complex genomic surveillance cascade that scans and repairs alterations of genomic landscape. In addition to germ line cells, somatic cells depend on alteration of highly plastic genomic content to generate heterogeneity amongst population of equipotent cells. Neurons are prime examples of somatic cells that demonstrate a labile and plastic genome. Arguably presence of putative controlled bypass mechanisms to evade genome surveillance is necessary to generate somatic diversity in neuronal populations. To date, such mechanisms in Homo Sapiens -with absolute dependence on neuronal heterogeneity- remain unknown. Here the evidence is provided regarding the presence and activity of a hominin-specific genome surveillance evasion mechanism in neuronal precursor cells that is contingent upon harmonized activity of notch-1 signaling cascade. Intronic miR-4673 is embedded in the intron 4 of notch-1 partially contributed by splicing acceptor site of the intron. MiR-4673 is expressed concomitant with expression of Notch-1 and targets X-Ray Repair Cross-Complementing Protein 3 (XRCC3) by hybridizing into a sequence embedded in the immediate upstream proximal enhancer element of the gene and suppressing the transcription. Suppression of transcription occurs only in proliferating cells when miRNA can access nuclear content during mitosis. Differentiating cell with no mitotic activity are immune from the suppressive activity of miRNA. In addition to XRCC3, strong suppression of Breast Cancer 2, Early Onset (BRCA2) and RAD51 Recombinase (Rad51) is observed in pericytes transfected with a plasmid expressing miR-4673. Transfected mir-4673high cells also demonstrated significantly lower levels of P53 and P21 proteins. Vascular endothelial growth factor-A (VEGF)-A was analyzed to detect genomic rearrangement and transpositional events after transfection of pericytes with miR-4673. The results demonstrated remarkable alteration of the structure of VEGF-A towards the 3’ end in the miRNAhigh pericytes. Further, miRNAhigh/XRCC-/P53low pericytes exhibited significant resistance to ionizing radiation (UV-C) 24 hours after transfection while control pericytes demonstrated nearly complete apoptosis. These results provides novel evidence regarding the existence and activity of a hominin-specific genome surveillance evasion mechanism in proliferating neural pericytes that is inactive in differentiating cells and hence may generate diversity only in proliferating neural precursors. In proliferating neural precursors notch1-dependent activity of miR4673 links proliferation to generation of heterogeneity which is a hallmark of human central nervous system.