Student Theses and Dissertations

Date of Award

2022

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

RU Laboratory

Tavazoie Laboratory

Abstract

Despite substantial progress in treatment, breast cancer remains a leading cause of cancer mortality among women. During malignant transformation, healthy mammary cells must bypass tumor suppressive checkpoints and activate pro-growth pathways. Enhanced protein translation is one such hallmark of tumor transformation and cancer progression; many oncogenes promote translation by driving synthesis and activity of translational machinery. In the first part of this thesis, we report the surprising finding that leucyl-tRNA synthetase (LARS), a tRNA synthetase responsible for ligating leucine to corresponding leucyl-tRNAs, becomes strongly repressed during mammary cell transformation and in breast cancer. Monoallelic genetic inactivation of LARS in mouse mammary glands enhanced tumor formation and proliferation. LARS repression reduced the abundance of specific leucyl-tRNA isoacceptors tRNA-LeuCAG, tRNA-LeuAAG and tRNA-LeuTAG. This repression resulted in decreased leucine codon-dependent translation and expression of key leucine-rich, growth suppressive genes. We uncovered two candidate tumor suppressor genes, EMP3 and GGT5, that act downstream of LARS. This work uncovers a tumor suppressive tRNA synthetase and reveals that its repression, along with its downstream tRNAs, can modulate translation to enhance breast tumorigenesis and proliferation in a codon-dependent manner. Modulation of translation by tRNAs and their associated factors can affect not only cancer transformation, but also normal cell physiology. In the second half of this thesis, we describe work on mammary epithelial cells which, in the context of oxidative stress, exhibited a stressinduced tRNA fragmentation response. Though tRNA fragmentation is classically not thought to occur at high enough levels to deplete mature tRNA pools, we demonstrated that tyrosyl-tRNA fragmentation depleted mature tRNA-TyrGUA following 24 hours of oxidative stress. tRNATyrGUA depletion was sufficient to reduce translation and expression of growth-promoting, tyrosine-enriched transcripts in a codon-dependent manner, contributing to the restriction of cell growth in response to stress. We further demonstrated that the tRNA fragment, tRF-TyrGUA, was generated in a DIS3L2 exoribonuclease-dependent manner and acted in trans with RNA binding proteins, including HNRNPA1, to inhibit HNRNPA1-mediated transcript destabilization. These findings uncover and characterize a second adaptive, codon-based regulatory response that can contribute to cellular stress responses in physiology and disease.

Comments

A Thesis Presented to the Faculty of The Rockefeller University in Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy

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