DIS3 isoforms vary in their endoribonuclease activity and are differentially expressed within haematological cancers

DIS3 is the catalytic subunit of the exosome, a protein complex involved in the 3’ to 5’ degradation of RNAs. DIS3 is a highly conserved exoribonuclease, also known as Rrp44. Global sequencing studies have identified DIS3 as being mutated in a range of cancers, with a considerable incidence in multiple myeloma. In this work, we have identified two proteincoding isoforms of DIS3. Both isoforms are functionally relevant and result from alternative splicing. They differ from each other in the size of their N-terminal PIN domain, which has been shown to have endoribonuclease activity and tether DIS3 to the exosome. Isoform 1 encodes a full-length PIN domain, whereas the PIN domain of isoform 2 is shorter and is missing a segment with conserved amino-acids. We have carried out biochemical activity assays on both isoforms of full-length DIS3 as well as the isolated PIN domains. We find that isoform 2, despite missing part of the PIN domain, has greater endonuclease activity compared to isoform 1. Examination of the available structural information allows us to provide a hypothesis to explain this altered behaviour. Our results also show that multiple myeloma patient cells and all cancer cell lines tested have higher levels of isoform 1 compared to isoform 2 whereas Acute Myeloid Leukemia (AML) and chronic myelomonocytic leukaemia (CMML) patient cells and samples from healthy donors have similar levels of isoforms 1 and 2. Together, our data indicate that significant changes in the ratios of the two isoforms could be symptomatic of haematological cancers.