Establishment of genotype-phenotype relationships is fundamental to elucidating pathogenic effects of human mutations. In addition to influencing susceptibility to ADPEAF, recent studies have also shown that LGI1
plays an important role in limbic encephalitis, a disorder in which autoimmunity against the Lgi1 protein is the underlying cause.31
These observations support the importance of LGI1
in a spectrum of neurologic functions.
We found that ADPEAF-causing mutations cluster significantly within the LRR domain of LGI1. This finding was primarily attributed to missense mutations. The most likely explanation for this finding is that mutations in the LRR domain confer a greater susceptibility to ADPEAF than those in the EPTP domain, although mutations in both domains are clearly pathogenic. In addition, given the very strong conservation of this gene, an alternative explanation is that mutations in the LRR domain are more compatible with life and hence are observed more frequently than those in the EPTP domain. Either or both possibilities could underlie the mutation clustering in the LRR domain.
We observed a significantly lower frequency of auditory symptoms among individuals with truncation mutations in the EPTP domain than among those with other mutation types/locations. This finding is difficult to interpret with our current state of understanding of the biological function of the LGI1 gene products. Analysis of disease severity (e.g., medication response or seizure frequency) might have been more revealing but was not possible with the data abstracted from the literature. One possible explanation for the lower frequency of auditory symptoms among patients with truncation mutations in the EPTP domain is that they had a milder form of epilepsy, with fewer seizures overall and less recognition of associated symptoms.
Further elucidation of the function of LGI1
and its products is essential for better understanding of the possible phenotypic consequences of particular mutations. The pathogenicity of LGI1
mutations may involve both haploinsufficiency and dominant-negative mechanisms. In Lgi1
mouse knockout models, homozygous ablation of the gene leads to spontaneous seizures and early death,32,33
whereas heterozygous Lgi1
+/− mice showed a lowered threshold for auditory stimuli-induced seizures.32
These findings are consistent with a mechanism involving haploinsufficiency, because the phenotype is more extreme in homozygotes than in heterozygotes. Although in many genes, truncation mutations lead to nonsense-mediated decay of the abnormal mRNA,34
evidence from animal and human studies indicates that this process does not always occur with truncation mutations in LGI1.35,36
For example, in transgenic mice with an extra copy of the LGI1
gene containing a truncation mutation in exon 6, maturation of glutamatergic synapses was arrested, clearly indicating a dominant-negative effect of the mutation.36
These results imply that the abnormal mRNA transcript resulting from the truncation mutation was not simply degraded by nonsense-mediated decay.
Protein-protein interactions involving Lgi1's EPTP domain have been studied more extensively than those involving the LRR domain. Several studies have used EPTP truncation mutations to investigate interactions of Lgi1 with Kv1 channels in presynaptic neurons35
or with ADAM proteins at transsynaptic sites.37,38
However, a recent model proposed that the formation of Lgi1 dimers through the LRR domain may influence pre- and postsynaptic transport or communication.39
These interactions may underlie the maturation and activity of glutamatergic synapses and thus modify the manifestation of ADPEAF.36,37
Given our finding of mutation clustering in the LRR domain, the reported dominant-negative effects of the EPTP truncation mutants, and the observation of naturally occurring Lgi1 isoforms that resemble EPTP truncation mutants,40
further investigation of protein-protein interactions or other molecular mechanisms involving the LRR domain that might influence epileptogenesis is warranted.
Our inclusion of all known LGI1 mutation-positive ADPEAF families in these genotype-phenotype association analyses provided an opportunity to gain insights into the structure-function relationships of LGI1 in a physiologic context. The sliding window analysis used here may be applicable to the analysis of mutation clustering in other disease genes.
Conversely, the lack of consistent reporting of phenotypic characteristics in the literature restricted our genotype-phenotype association analysis to penetrance, age at onset, and auditory symptoms. Statistical power was also limited by the relatively few previously reported ADPEAF families with LGI1 mutations. We are in the process of developing a more structured and standardized platform that can be used across different research centers for collection of clinical information on ADPEAF. Future analyses using a larger collection of ADPEAF families and more comprehensive phenotypic information will facilitate further elucidation of genotype-phenotype relationships of LGI1 mutations.