Date of Award

Summer 8-14-2025

Embargo Period

8-14-2027

Document Type

Dissertation - MUSC Only

Degree Name

Doctor of Philosophy (PhD)

Department

Molecular and Cellular Biochemistry and Pathobiology

Additional Department

Regenerative Medicine and Cell Biology

College

College of Graduate Studies

First Advisor

Antonis Kourtidis

Second Advisor

Nancy DeMore

Third Advisor

Denis Guttridge

Fourth Advisor

Vamsi Gangaraju

Fifth Advisor

Joseph Delaney

Abstract

Transposons, or transposable elements (TEs), are mobile genomic elements that make up almost 45% of the human genome. Although TEs are mostly dormant, they can become re-activated and cause DNA damage, genomic instability, and gene mutations. To protect the genome from TE activity, cells target TEs for degradation through a mechanism that involves the PIWI ribonucleases and small RNAs called piRNAs (PIWI-interacting RNAs), which altogether form the piRNA induced silencing complex (piRISC). Although the function of the piRISC has been extensively studied in the germline, there is little understanding about its role in somatic tissues. Interestingly, recent evidence indicates that TE re-activation occurs in 50% of somatic tumors. However, the reasons for TE re-activation in somatic tissues are still poorly understood. Here, we demonstrate recruitment and regulation of piRISC by adherens junctions in differentiated epithelial cells. Specifically, we have found association of members of piRISC, including PIWIL2, the key catalytic component of piRISC, as well as of PIWIL4 and TDRD1, with the epithelial adherens junction components E-cadherin and PLEKHA7. Adherens junction disruption results in mis-localization of piRISC whereas PLEKHA7 or PIWIL2 depletion in colon epithelial cells result in downregulation of a set of noncanonical, piRNA-like, short RNAs targeting multiple classes of TEs, and in upregulation of TEs, especially of LINE-1, the only autonomous TE family in human tissues. As a result, PLEKHA7 and PIWIL2 depletion promote increased genomic insertional activity of TEs, as well as increased DNA damage. Examination of colon cancer cell lines, as well as of colon cancer patient tissue samples and bioinformatic analyses, indicate extensive disruption of the adherens junction - PIWI complex in colon cancer, as well as an overall association of piRISC defects with disease aggressiveness. Indeed, PIWIL2 knockout promotes pro-tumorigenic phenotypes of well-differentiated colon epithelial cells. Altogether, the data reveal that epithelial adherens junctions recruit piRISC to suppress transposon activity and maintain epithelial homeostasis. Since disruption of junctional integrity and increased transposon activity are both common events in cancer, this study unravels a novel mechanism that may shed light on the still obscure roles of piRISC and of TEs in somatic tissues.

Rights

Copyright is held by the author. All rights reserved.

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