We have described a large family suffering from autosomal dominant, cortical myoclonus. Cortical myoclonus is dominantly inherited in epileptic disorders such as FAME3
. However, the clinical presentation of FAME is distinctly different from that observed in our family where myoclonus was slowly progressive and became disabling late in the course of the illness, and overt seizures were not observed. Moreover, linkage to all known FAME loci4–6
was ruled out. The presentation in our family is not consistent with a progressive myoclonic epilepsy syndrome because there were neither seizures nor progressive dementia. The phenotype would be more consistent with the syndrome of progressive myoclonic ataxia defined by infrequent seizures and little or no cognitive dysfunction23
, although ataxia was mild and occurred late in the illness, and we could not detect even infrequent seizures. Another possible diagnosis is hereditary hyperekplexia, an autosomal dominant disorder characterized by an excessive startle reaction. However, hereditary hyperekplexia is clinically distinct and neurophysiologically characterized by hyperexcitability of brain stem origin: giant SSEP cortical responses are not observed24
The presentation in our family is also distinct from Myoclonus Dystonia Syndrome (MDS), which is, to our knowledge, the sole hereditary essential myoclonus syndrome hitherto described. MDS is characterized by juvenile-onset essential myoclonus, dystonia, and psychiatric comorbidity8,9
, whereas in this family myoclonus was adult-onset and not accompanied by dystonia or psychiatric comorbidity. Furthermore, in MDS the symptoms are clearly ameliorated by alcohol, which was not the case in this family. Additionally, in MDS the myoclonus is of subcortical origin and is not stimulus-sensitive, and giant SSEPs are never observed10,11
. Finally, in this family we excluded linkage to known MDS chromosomal regions7,8
, and we sequenced but found no mutations in the SCGE
Hence, by exclusion we conclude that this family represents the first report of a novel clinical syndrome characterized by autosomal dominant, adult-onset, cortical myoclonus without associated seizures. We propose for this disorder the term Familial Cortical Myoclonus (FCM).
We used SNP mapping, linkage, and targeted massively parallel sequencing to identify a mutation in NOL3. Although to date we have been unable to identify additional FCM kindreds with mutations in NOL3, we have presented substantial evidence that NOL3 is the disease gene underlying FCM. First, we sequenced all known or predicted splice junctions and coding sequence within the critical region at 634x average coverage and detected only a single novel variant, the E21Q mutation in NOL3, which co-segregated with the phenotype. Second, the E21Q mutation was absent in over 10,000 control chromosomes; given the deluge of sequencing data recently deposited in public databases, the absence of the E21Q mutation in these databases constitutes additional evidence that it is extremely rare and therefore likely causative of this family’s phenotype. Finally, we found bioinformatic and in vitro evidence suggesting that the E21Q mutation has functional effects on NOL3 protein.
Nevertheless, we do concede the possibility that the E21Q variant in NOL3 is a private variant that is linked to the actual disease mutation but is not itself causative. Some plausible explanations to explain this scenario are the following: (1) the causative mutation resides in coding sequence within the critical region that is currently not known or predicted; (2) the causative mutation resides in non-coding sequence within the critical region, e.g. a UTR or promoter mutation; (3) the causative mutation was not detected due to systematic sequencing error. However remote these possibilities, we are pursuing two parallel lines of investigation in order to definitively demonstrate that NOL3 is the causative gene underlying FCM. The first is characterization of a knock-in mouse model harboring an E21Q missense mutation. The second is identification of independent NOL3 alleles that cause FCM, or FCM-like phenotypes such as FAME, SCGE mutation-negative MDS, and even idiopathic generalized epilepsy. We invite collaborations to study patients suffering from either familial or sporadic undiagnosed cortical myoclonus or a related phenotype.
The molecular mechanism by which NOL3
mutation may cause FCM remains an open question. One possibility is that the E21Q
mutation alters NOL3 phosphorylation, because NOL3 has 7 known phospho-residues25
(), one of which, T149, has been extensively characterized26
(). T149 is dephosphorylated by the phosphatase calcineurin27,28
, and it is interesting to speculate whether the diverse neurotoxicities of calcineurin inhibitors, which include seizures, may be related to alterations in phospho-NOL3. Since the mutation occurs in a protein-protein interaction motif and is predicted to alter the electrostatic surface potential (), one plausible hypothesis is that the E21Q
mutation alters binding of NOL3 to its kinase and/or phosphatase, thereby increasing the level of phospho-NOL3 in cells. However, at this time we cannot rule out other NOL3 post-translational modifications.
The absence of an excitability phenotype in Nol3−/−
mice, in concert with the observation that one neurologically normal human control subject harbors a heterozygous deletion spanning NOL3ref 29
, strongly argues against a haploinsufficiency or dominant negative mechanism causing this autosomal dominant disorder. Instead, it seems more plausible that NOL3
mutation causes FCM by a gain-of-function or neomorphic mechanism, although at this time the exact mechanistic link between NOL3 and neuronal hyperexcitability remains speculative.
In conclusion, we identified a family suffering from a novel movement disorder for which we propose the term Familial Cortical Myoclonus (FCM). FCM is characterized by autosomal dominant, adult-onset, slowly progressive, multifocal, cortical myoclonus. We utilized unbiased, genome-wide approaches to identify a NOL3 mutation that likely causes FCM. We anticipate that this work will enhance diagnosis and genetic counseling in patients with myoclonus. Furthermore, as has proved the case for many other Mendelian diseases, further investigation to understand the role of NOL3 in FCM pathophysiology may provide insight into mechanisms of membrane hyperexcitability relevant to other forms of myoclonus and epilepsy.