LOSS OF ADAR1 BY MICRORNAS IN METASTATIC MELANOMA PROMOTES MALIGNANCY

Yael Wolff-Nemlich ^1,6 Eyal Greenberg ^1,7 Rona Ortenberg ^1,7 Michal J. Besser ^1,7 Iris Barshack ² Jasmine Jacob-Hirsch ³ Elad Jacoby ^3,4 Eran Eyal ³ Ludmila Rivkin ¹² Victor G. Prieto ⁸ Nitin Chakravarti ⁸ Lyn M. Duncan ¹⁰ David M Kallenberg ¹¹ Eitan Galun ¹² Dorothy C. Bennett ¹¹ Ninette Amariglio ³ Menashe Bar-Eli ⁹ Jacob Schachter ¹ Gideon Rechavi ^3,6 Gal Markel ^1,5,7

¹Ella Institute of Melanoma, Sheba Medical center, Ramat- Gan
²Institute of Pathology, Sheba Medical center, Ramat-Gan
³Cancer Research Center, Sheba Medical center, Ramat-Gan
⁴Pediatric Hemato-oncology, Sheba Medical center, Ramat-Gan
⁵Talpiot Medical Leadership Program, Sheba Medical center, Ramat-Gan
⁶Human Genetics and Biochemistry, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv
⁷Clinical Microbiology and Immunology, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv
⁸Pathology, MD Anderson Cancer Center, Houston, Texas
⁹Cancer Biology, MD Anderson Cancer Center, Houston, Texas
¹⁰Dermatopathology, Massachusetts General Hospital, Boston, Massachusetts
¹¹Biomedical Sciences Research Centre, St. George's, University of London, London
¹²Goldyne Savad Institute of Gene Therapy, Hadassah Hebrew University Hospital, Jerusalem

We show that the main RNA-editing enzyme, Adenosine Deaminase Acting on RNA-1 (ADAR1) is silenced in many metastatic melanoma cultures. In-depth studies on melanoma samples and progression tissue microarrays point on substantial ADAR1 downregulation during the metastatic transition. Accordingly, ADAR1 suppresses several cancer features, as its downregulation alters cell morphology, facilitates cell-cycle and proliferation in-vitro, and dramatically enhances the tumorigenicity in-vivo. ADAR1 controls the expression of >100 microRNAs, which regulate hundreds of genes that account for the observed phenotype. Importantly, ADAR1 fundamentally regulates the course of miRNA processing in the cell in an RNA-binding dependent yet RNA-editing-independent manner. ADAR1 regulates the expression of Dicer at the level of translation via let-7. In addition, it creates a complex with DGCR8, which is mutually exclusive with the DGCR8-Drosha complex that processes pri-miRNAs in the nucleus. Cancer cells silence ADAR1 by overexpressing miR-17 and miR-432, both of which directly target ADAR1 and operate additively. Both of their genomic sequences are frequently amplified to increase expression, but uniquely, an aberrant hypomethylation of the imprinted DLK1-DIO3 region in chromosome 14 accounts for the miR-432 overexpression.