2042 – THE LANDSCAPE OF SOMATIC RETROTRANSPOSON MUTATIONS IN HSCS, AND A NOVEL JAK2 SIGNALING PATHWAY TO HNRNPA1 THAT PROTECTS THE HSC GENOME FROM RETROTRANSPOSONS

Abstract

Retrotransposon (RT) expression requires tight control, particularly in long-lived multipotent stem cells, to prevent deleterious consequences including insertional mutations. Several mechanisms repress RT transcription during development and are thought to persist thereafter. However, many RT-derived sequences have acquired advantageous host functions. Little is known about RT expression and mobility in adult stem cells or about how these processes might be dynamically regulated.

We developed PEAR-TREE, a new tool to detect somatic RT mutations in whole genome sequences from single- hematopoietic stem cell (HSC)-derived colonies. Using PEAR-TREE, we describe the landscape of somatic retrotransposition mutations in HSCs. We provide the first evidence that RTs are active in HSCs, observing 460 somatic mutations derived from retrotransposons within the human HSC population across a lifetime (95% confidence interval 126–1,456). The mouse HSC population acquires 4610 mutations over their shorter lifetime.

We also explored whether RTs are dynamically regulated during HSC cell division, and detected a diverse range of long interspersed nuclear elements (LINEs), short interspersed nuclear elements (SINEs), and endogenous retrovirus (ERV) mRNAs downregulated by thrombopoietin (THPO) treatment in human and mouse HSCs. We determined that the RNA-binding protein HNRNPA1 is phosphorylated by JAK2 downstream of THPO treatment, and analysis of HSCs from phosphomutant mice indicated that HNRNPA1 phosphorylation accounted for at least one-third of THPO/JAK2-mediated RT mRNA repression. HNRNPA1 phosphorylation also reduced mutations caused by a LINE-1 reporter, and by IAP ERVs.

This novel THPO/JAK2/HNRNPA1 pathway provides a mechanism that integrates RT repression with cytokine-induced proliferation, protecting the HSC genome although it is most vulnerable to RT-mediated mutations.

Retrotransposon (RT) expression requires tight control, particularly in long-lived multipotent stem cells, to prevent deleterious consequences including insertional mutations. Several mechanisms repress RT transcription during development and are thought to persist thereafter. However, many RT-derived sequences have acquired advantageous host functions. Little is known about RT expression and mobility in adult stem cells or about how these processes might be dynamically regulated.

We developed PEAR-TREE, a new tool to detect somatic RT mutations in whole genome sequences from single- hematopoietic stem cell (HSC)-derived colonies. Using PEAR-TREE, we describe the landscape of somatic retrotransposition mutations in HSCs. We provide the first evidence that RTs are active in HSCs, observing 460 somatic mutations derived from retrotransposons within the human HSC population across a lifetime (95% confidence interval 126–1,456). The mouse HSC population acquires 4610 mutations over their shorter lifetime.

We also explored whether RTs are dynamically regulated during HSC cell division, and detected a diverse range of long interspersed nuclear elements (LINEs), short interspersed nuclear elements (SINEs), and endogenous retrovirus (ERV) mRNAs downregulated by thrombopoietin (THPO) treatment in human and mouse HSCs. We determined that the RNA-binding protein HNRNPA1 is phosphorylated by JAK2 downstream of THPO treatment, and analysis of HSCs from phosphomutant mice indicated that HNRNPA1 phosphorylation accounted for at least one-third of THPO/JAK2-mediated RT mRNA repression. HNRNPA1 phosphorylation also reduced mutations caused by a LINE-1 reporter, and by IAP ERVs.

This novel THPO/JAK2/HNRNPA1 pathway provides a mechanism that integrates RT repression with cytokine-induced proliferation, protecting the HSC genome although it is most vulnerable to RT-mediated mutations.