We describe here mutations in the NHLRC1
gene in Lafora's disease families primarily from European populations. Given that only three mutations detected in the present study have been reported previously, it is likely that the majority of the Lafora's disease mutations arise as a single event and that only a very small proportion of mutant alleles can be predicted in certain populations. The C26S missense mutation, which is yet to be detected in other populations, was previously identified in four independent families which were exclusively of French Canadian descent, and which originated from the same region of east Quebec.8,18
The affected individuals of the four families reported by Chan and colleagues8,18
share the same haplotype around the NHLRC1
locus, suggesting a founder effect for the mutation.3
Our identification of a yet another French Canadian Lafora's disease family with the same mutation suggests that this mutation could be the cause for the prevalence of Lafora's disease in this ethnic community. Thus there is a strong rationale to screen for this mutation in any patient, from the French Canadian isolate, who is suspected of having Lafora's disease. The diagnostic test described in our report using genomic DNA offers a rapid and accurate method.
The missense mutation C26S is the third most frequent mutation observed for NHLRC1
and targets the RING domain (fig 1A). Biochemical studies have shown that mutation C26S indeed affects the ubiquitin ligase activity of malin.9
The functional implications of the four other missense mutations identified in the present study are unknown. It is of interest to note that missense mutations D195N and P218S target the second and third NHL domains of malin, respectively (fig 1A), and mutations affecting the NHL domain are proven to affect the ubiquitin ligase activity of malin.9
As the assay systems for ubiquitin ligase activities have already been established, it would be of interest now to check the effects of these mutations on malin's ubiquitin ligase activity.
The identification of disease causing mutations in a large panel of Lafora's disease families provided us with a good basis for studying phenotype–genotype relations within and between families for both EPM2A
mutations. As of now, we are unable to assign any specific phenotypic variation that relates to specific mutations in the NHLRC1
gene. However, the present study clearly indicates that disease progression is longer in patients with NHLRC1
mutations as against those with EPM2A
mutations. While our studies were being prepared for publication, similar observations were reported by Gomez‐Abad and colleagues,14
who noted that patients with NHLRC1
mutations had a slightly milder clinical course. The recent demonstration that laforin is a substrate for malin E3 ubiquitin ligase implies that the malin could probably act upstream of laforin in the cellular cascade.9,10
Considering the difference in the rate of disease progression in patients with NHLRC1
gene defects (in the present study and that of Gomez‐Abad et al14
), it is tempting to speculate that at least some functions of laforin are regulated by multiple factors and that malin could be only one of them. Thus, while loss of laforin may result in rapid progression of the disease, the effect of malin defects would be restricted to a few and not all functions of laforin, leading to a slower clinical course. This suggestion also supports a role for the protein product of a third Lafora's disease gene, as well as other factors that could regulate the function of laforin.3
Further studies exploring the biochemical pathways that link the regulators of laforin to glycogen metabolism might therefore unravel the molecular mechanisms that lead to polyglucosan accumulation and cell death in Lafora's disease.