Genotype-phenotype correlations are a challenge at this point. However, some studies have suggested that EPM2B patients have a slower disease progression (12
). Another correlation was suggested associating mutations in the first exon of EMP2A to an early onset of cognitive deficit (15
is located on chromosome 6q24. It consists of four exons coding for a 331 amino acid protein called laforin (10
). Laforin has two isoforms, A and B which localize to the ER and to the nucleus, respectively (16
). The isoforms differ in their C-termini, and mutations in the unique isoform A’s C-terminus suggests that this is the disease-relevant isoform (17
). To date, 40 different mutations and four polymorphisms were identified in this gene (18
). These include missense and nonsense mutations, frameshifts and deletions located in the coding region of the gene.
Laforin is a unique protein in that it contains a carbohydrate-binding domain (CBD) of the CBM20 type (19
) in its N-terminus and a dual-specificity protein tyrosine phosphatase (DSP) domain in its C-terminus (6
). Given the accumulation of polyglycosans in LD and the presence of a CBD, laforin is thought to play an important role in glycogen metabolism (either its synthesis or degradation) (6
). Importantly, self-dimerization appears to be necessary for laforin to be functional in vivo
Co-immunoprecipitation studies suggest that full-length laforin binds an uncharacterized protein termed EMP2AIP1 (for EPM2A interacting protein). This protein does not appear to be responsible for LD in those LD families with normal EPM2A and EPM2B genes (23
). HIRIP5 is another protein shown to interact with laforin. This protein contains a NifU-like domain and a putative MurD ligase domain. However the role of those domains in HIRIP5 function is not yet clear. Interestingly HIRIP5, like laforin, is ubiquitously expressed in subregions of the brain, but predominantly in the cerebellum and hippocampus. This protein also co-localizes with laforin at the subcellular level. Finally, laforin was able to dephosphorylate HIRIP5 on both tyrosine and serine/threonine residues, suggesting that HIRIP5 is a substrate for laforin (24
). A third protein shown to interact with laforin, called PTG, is a regulatory subunit of protein phosphatese-1 (PP1) that enhances glycogen accumulation (21
). It was shown that the G240S missense mutation identified in some LD patients disrupts the interaction between laforin and PTG (while glycogen binding and phosphatase activity remain preserved). This observation suggests that PTG is critical for laforin function and that laforin is part of a complex of proteins associated with glycogen and may have a role in regulating its metabolism. Studies using a mammalian two-hybrid system demonstrated that laforin interacts with glycogen synthase kinase-3 (GSK3). Furthermore, laforin reduces GSK3 Ser 9 phosphorylation (25
). GSK3 is a potent glycogen synthase (GS) inhibitor. The relationship between GSK3, GS, laforin and LBs is discussed below.
was identified through genome-wide linkage scan followed by haplotype analysis and homozygosity mapping performed in a cluster of French-Canadian families from Quebec (11
). To date, 40 mutations have been found in the EMP2B gene, including insertion, missense and nonsense changes, frameshifts and deletions in both compound heterozygous as well as homozygous states. The EMP2B gene product encodes a 395 amino acid protein named malin which contains a zinc finger of the RING type at the N-terminus and six NHL-repeat motifs at the C-terminus. NHL motifs are likely involved in protein-protein interactions, while the RING-finger motif of malin is typical of E3 ubiquitin ligases. Sub-celllular localization studies showed that MYC-tagged malin, similarly to laforin, also localizes to the cytoplasm at the ER and the nucleus (16
The E3 ubiquitin ligase activity of malin was confirmed in vitro (25
). At least two mutations associated with LD (Cys26Ser and Phe33Ser) result in inactivation of malin’s ubiquitinase function (13
). Ubiquitination can serve several purposes including targeting the ubiquitinated protein for destruction or actively regulating its function (30
). Recent studies demonstrated that laforin and malin interact and that this interaction occurs at the central regions of both proteins (25
). There is data suggesting that malin ubiquitinates laforin, targeting laforin for destruction, but this is presently difficult to understand, as destruction of laforin by malin would be expected to result in Lafora disease (29
). Finally, it was demonstrated through co-immunoprecipitation studies that malin and glycogen synthase (GS) interact, although the result of such interaction is not known.