Modulation of functional properties of laforin phosphatase by alternative splicing reveals a novel mechanism for the EPM2A gene in Lafora progressive myoclonus epilepsy

Abstract

The EPM₂A gene, encoding the dual-phosphatase laforin, is mutated in a fatal form of progressive myoclonus epilepsy known as Lafora disease (LD). The EPM₂A gene, by differential splicing of its transcripts, is known to encode two laforin isoforms having distinct carboxyl termini; a major isoform localized in the cytoplasm (laf331), and a minor isoform that is targeted to the nucleus as well (laf317). We show here that the two laforin isoforms interact with each other and form homo and heterodimers. The homodimer of laf331 display robust phosphatase activity, whereas the laf317 homodimer and the laf331–laf317 heterodimer lack phosphatase activity. Laf331 binds to glycogen only as a monomeric form. Laf317, on the other hand, was unable to bind to glycogen as a homodimer or as a heterodimer. Similar to laf331, laf317 interacts with and functions as a substrate for the malin ubiquitin ligase—a product of another gene defective in LD. Malin, however, shows higher affinity towards laf331 when compared with laf317. We have also tested the effect of LD-associated mutations, whose effects are restricted to the laf331 isoform, on laf331–laf317 interaction. Two such mutations are known and both abolish the interactions between laf317 and laf331 and their heterodimerization, but not the homodimerization property of laf331. Thus, laf317 could function as a dominant-negative regulator of laf331, and laf331-specific mutations might affect laf317 functions as well. Thus, our findings reveal a novel mechanism for the EPM₂A gene function, regulated by alternative splicing, in normal as well as disease conditions.