John A. Damiano

Strikingly Different Clinicopathological Phenotypes Determined by Progranulin-Mutation Dosage

We performed hypothesis-free linkage analysis and exome sequencing in a family with two siblings who had neuronal ceroid lipofuscinosis (NCL). Two linkage peaks with maximum LOD scores of 3.07 and 2.97 were found on chromosomes 7 and 17, respectively. Unexpectedly, we found these siblings to be homozygous for a c.813_816del (p.Thr272Serfs∗10) mutation in the progranulin gene (GRN, granulin precursor) in the latter peak. Heterozygous mutations in GRN are a major cause of frontotemporal lobar degeneration with TDP-43 inclusions (FTLD-TDP), the second most common early-onset dementia. Reexamination of progranulin-deficient mice revealed rectilinear profiles typical of NCL. The age-at-onset and neuropathology of FTLD-TDP and NCL are markedly different. Our findings reveal an unanticipated link between a rare and a common neurological disorder and illustrate pleiotropic effects of a mutation in the heterozygous or homozygous states.

GRIN2A mutations cause epilepsy-aphasia spectrum disorders

Epilepsy-aphasia syndromes (EAS) are a group of rare, severe epileptic encephalopathies of unknown etiology with a characteristic electroencephalogram (EEG) pattern and developmental regression particularly affecting language. Rare pathogenic deletions that include GRIN2A have been implicated in neurodevelopmental disorders. We sought to delineate the pathogenic role of GRIN2A in 519 probands with epileptic encephalopathies with diverse epilepsy syndromes. We identified four probands with GRIN2A variants that segregated with the disorder in their families. Notably, all four families presented with EAS, accounting for 9% of epilepsy-aphasia cases. We did not detect pathogenic variants in GRIN2A in other epileptic encephalopathies (n = 475) nor in probands with benign childhood epilepsy with centrotemporal spikes (n = 81). We report the first monogenic cause, to our knowledge, for EAS. GRIN2A mutations are restricted to this group of cases, which has important ramifications for diagnostic testing and treatment and provides new insights into the pathogenesis of this debilitating group of conditions.

Kufs Disease, the Major Adult Form of Neuronal Ceroid Lipofuscinosis, Caused by Mutations in CLN6

The molecular basis of Kufs disease is unknown, whereas a series of genes accounting for most of the childhood-onset forms of neuronal ceroid lipofuscinosis (NCL) have been identified. Diagnosis of Kufs disease is difficult because the characteristic lipopigment is largely confined to neurons and can require a brain biopsy or autopsy for final diagnosis. We mapped four families with Kufs disease for whom there was good evidence of autosomal-recessive inheritance and found two peaks on chromosome 15. Three of the families were affected by Kufs type A disease and presented with progressive myoclonus epilepsy, and one was affected by type B (presenting with dementia and motor system dysfunction). Sequencing of a candidate gene in one peak shared by all four families identified no mutations, but sequencing of CLN6, found in the second peak and shared by only the three families affected by Kufs type A disease, revealed pathogenic mutations in all three families. We subsequently sequenced CLN6 in eight other families, three of which were affected by recessive Kufs type A disease. Mutations in both CLN6 alleles were found in the three type A cases and in one family affected by unclassified Kufs disease. Mutations in CLN6 are the major cause of recessive Kufs type A disease. The phenotypic differences between variant late-infantile NCL, previously found to be caused by CLN6, and Kufs type A disease are striking; there is a much later age at onset and lack of visual involvement in the latter. Sequencing of CLN6 will provide a simple diagnostic strategy in this disorder, in which definitive identification usually requires invasive biopsy.

Glucose Transporter 1 Deficiency as a Treatable Cause of Myoclonic Astatic Epilepsy

OBJECTIVE To determine if a significant proportion of patients with myoclonic-astatic epilepsy (MAE) have glucose transporter 1 (GLUT1) deficiency. DESIGN Genetic analysis. SETTING Ambulatory and hospitalized care. PATIENTS Eighty-four unrelated probands with MAE were phenotyped and SLC2A1 was sequenced and analyzed by multiplex ligation-dependent probe amplification. Any identified mutations were then screened in controls. MAIN OUTCOME MEASURE Any SLC2A1 mutations. RESULTS Four of 84 probands with MAE had a mutation of SLC2A1 on sequencing. Multiplex ligation-dependent probe amplification analysis did not reveal any genomic rearrangements in 75 of the remaining cases; 5 could not be tested. Two patients with MAE with SLC2A1 mutations also developed paroxysmal exertional dyskinesia in childhood. CONCLUSIONS Five percent of our patients with MAE had SLC2A1 mutations, suggesting that patients with MAE should be tested for GLUT1 deficiency. Diagnosis of GLUT1 deficiency is a strong indication for early use of the ketogenic diet, which may substantially improve outcome of this severe disorder.

Glucose transporter 1 deficiency in the idiopathic generalized epilepsies

We examined whether glucose transporter 1 (GLUT1) deficiency causes common idiopathic generalized epilepsies (IGEs).

Cathepsin F mutations cause Type B Kufs disease, an adult-onset neuronal ceroid lipofuscinosis

Kufs disease, an adult-onset neuronal ceroid lipofuscinosis, is challenging to diagnose and genetically heterogeneous. Mutations in CLN6 were recently identified in recessive Kufs disease presenting as progressive myoclonus epilepsy (Type A), whereas the molecular basis of cases presenting with dementia and motor features (Type B) is unknown. We performed genome-wide linkage mapping of two families with recessive Type B Kufs disease and identified a single region on chromosome 11 to which both families showed linkage. Exome sequencing of five samples from the two families identified homozygous and compound heterozygous missense mutations in CTSF within this linkage region. We subsequently sequenced CTSF in 22 unrelated individuals with suspected recessive Kufs disease, and identified an additional patient with compound heterozygous mutations. CTSF encodes cathepsin F, a lysosomal cysteine protease, dysfunction of which is a highly plausible candidate mechanism for a storage disorder like ceroid lipofuscinosis. In silico modeling suggested the missense mutations would alter protein structure and function. Moreover, re-examination of a previously published mouse knockout of Ctsf shows that it recapitulates the light and electron-microscopic pathological features of Kufs disease. Although CTSF mutations account for a minority of cases of type B Kufs, CTSF screening should be considered in cases with early-onset dementia and may avoid the need for invasive biopsies.

Recent advances in the molecular genetics of epilepsy

Recent advances in molecular genetics have translated into the increasing utilisation of genetic testing in the routine clinical practice of neurologists. There has been a steady, incremental increase in understanding the genetic variation associated with epilepsies. Genetic testing in the epilepsies is not yet widely practiced, but the advent of new screening technologies promises to exponentially expand both knowledge and clinical utility. To maximise the value of this new genetic insight we need to rapidly extrapolate genetic findings to inform patients of their diagnosis, prognosis, recurrence risk and the clinical management options available for their specific genetic condition. Comprehensive, highly specific and sensitive genetic test results improve the management of patients by neurologists and clinical geneticists. Here we discuss the latest developments in clinical genetic testing for epilepsy and describe new molecular genetics platforms that will transform both genetic screening and novel gene discovery.

Early onset absence epilepsy: 1 in 10 cases is caused by GLUT1 deficiency.

Glucose transporter 1 (GLUT1) deficiency caused by mutations of SLC2A1 is an increasingly recognized cause of genetic generalized epilepsy. We previously reported that >10% (4 of 34) of a cohort with early onset absence epilepsy (EOAE) had GLUT1 deficiency. This study uses a new cohort of 55 patients with EOAE to confirm that finding. Patients with typical absence seizures beginning before 4 years of age were screened for solute carrier family 2 (facilitated glucose transporter), member 1 (SLC2A1) mutations or deletions. All had generalized spike‐waves on electroencephalography (EEG). Those with tonic and/or atonic seizures were excluded. Mutations were found in 7 (13%) of 55 cases, including five missense mutations, an in‐frame deletion leading to loss of a single amino acid, and a deletion spanning two exons. Over both studies, 11 (12%) of 89 probands with EOAE have GLUT1 deficiency. Given the major treatment and genetic counseling implications, this study confirms that SLC2A1 mutational analysis should be strongly considered in EOAE.

Recurrence risk of epilepsy and mental retardation in females due to parental mosaicism of PCDH19 mutations

Objective: Two unrelated families were ascertained in which sisters had infantile onset of epilepsy and developmental delay. Mutations in the protocadherin 19 (PCDH19) gene cause epilepsy and mental retardation limited to females (EFMR). Despite both sister pairs having a PCDH19 mutation, neither parent in each family was a heterozygous carrier of the mutation. The possibility of parental mosaicism of PCDH19 mutations was investigated. Methods: Genomic DNA from peripheral blood was obtained and sequenced for PCDH19 mutations. Parentage was confirmed by markers. Results: Both sister pairs have a mutation in PCDH19. Sister pair 1 has a missense mutation, c.74T>C, L25P, while sequence analysis indicates both of their parents are negative for the mutation. Diagnostic restriction enzyme analysis detected low-level mosaicism of the mutation in their mother. Sister pair 2 are half-sisters who share a mother and each has the missense PCDH19 mutation c.1019 A>G, N340S. The sequence chromatograph of their mother shows reduced signal for the same mutation. These data indicate maternal somatic and gonadal mosaicism of the PCDH19 mutation in both sister pairs. Phenotyping is suggestive of, and PCDH19 mutation detection is diagnostic for, the disorder EFMR in the affected girls. Conclusions: We show that gonadal mosaicism of a PCDH19 mutation in a parent is an important molecular mechanism associated with the inheritance of EFMR. This should be considered when providing genetic counseling for couples who have one affected daughter as they may risk recurrence of affected daughters and having sons at risk of transmitting EFMR.

Recurrent mutations in DNAJC5 cause autosomal dominant Kufs disease

We sought to identify the molecular basis of the autosomal dominant form of Kufs disease, an adult onset form of neuronal ceroid lipofuscinosis. We used a combination of classic linkage analysis and Next Generation Sequencing to map and identify mutations in DNAJC5 in a total of three families. We analyzed the clinical manifestations in 20 individuals with mutation in DNAJC5. We report here the mapping and the identification of a p.L116del mutation in DNAJC5 segregating with the disease in two distinct American families, as well as a p.L115R mutation in an additional family. The age of onset and clinical manifestations were very homogeneous among mutation positive individuals, including generalized tonic–clonic seizures, myoclonus, ataxia, speech deterioration, dementia, and premature death. A few individuals also exhibited parkinsonism. DNAJC5, which encodes the cysteine string protein (CSPα), a presynaptic protein implicated in neurodegeneration, causes autosomal dominant Kufs disease. The leucine residues at positions 115 and 116 are hotspots for mutations and result in a homogeneous phenotype of progressive myoclonus epilepsy with onset around 30 years old.