Clinical and molecular genetic findings in COLQ-mutant congenital myasthenic syndromes

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SLC25A22 is a novel gene for migrating partial seizures in infancy

ObjectiveTo identify a genetic cause for migrating partial seizures in infancy (MPSI).MethodsWe characterized a consanguineous pedigree with MPSI and obtained DNA from affected and unaffected family members. We analyzed single nucleotide polymorphism 500K data to identify regions with evidence of linkage. We performed whole exome sequencing and analyzed homozygous variants in regions of linkage to identify a candidate gene and performed functional studies of the candidate gene SLC25A22.ResultsIn a consanguineous pedigree with 2 individuals with MPSI, we identified 2 regions of linkage, chromosome 4p16.1-p16.3 and chromosome 11p15.4-pter. Using whole exome sequencing, we identified 8 novel homozygous variants in genes in these regions. Only 1 variant, SLC25A22 c.G328C, results in a change of a highly conserved amino acid (p.G110R) and was not present in control samples. SLC25A22 encodes a glutamate transporter with strong expression in the developing brain. We show that the specific G110R mutation, located in a transmembrane domain of the protein, disrupts mitochondrial glutamate transport.InterpretationWe have shown that MPSI can be inherited and have identified a novel homozygous mutation in SLC25A22 in the affected individuals. Our data strongly suggest that SLC25A22 is responsible for MPSI, a severe condition with few known etiologies. We have demonstrated that a combination of linkage analysis and whole exome sequencing can be used for disease gene discovery. Finally, as SLC25A22 had been implicated in the distinct syndrome of neonatal epilepsy with suppression bursts on electroencephalogram, we have expanded the phenotypic spectrum associated with SLC25A22

Patient F2 (P1) MRI, age 3 years 3 months (A-D).

<p>Coronal T2-weighted (A), axial T2-weighted (B), axial diffusion (C), axial FLAIR (D) showing cerebellar atrophy (arrows) with widening of cerebellar folia (A), T2 normal signal intensity (arrows) of <i>globus </i><i>pallidus</i> (B), highlighted subtle iron deposition at globus pallidi (arrows) as reduction in signal intensity (C), and high signal intensity (arrows) at cerebral white matter (D). Patient F2 (P2) MRI, age 1 year 3 months (E-H)). Coronal T2-weighted (E), axialT2-weighted (F), axial diffusion (G), axial FLAIR (H) showing mild cerebellar atrophy (arrows) with mild widening of cerebellar folia (E), normal signal intensity (arrows) of <i>globus </i><i>pallidus</i> (F and G), and high signal intensity (arrows) at cerebral white matter (H).</p

Genotype-phenotype correlation.

<p>Graphic representation of the evolution exponential tendency curves of the functional disability (red curve and red squares) and of the age at onset of ataxia (blue curve and blue squares) which both seem to depend on the nature of the mutation. Each number in X axis corresponds to one patient. Below this axis are indicated the codes of patients (from Families 1 to 6) and the corresponding mutation. Y axis corresponds to the age per years. Two groups are identified (dashed ellipses) depending on the age at onset of ataxia. The first one encompasses the patients with ataxia manifesting at or before 15 months of age, and the second one, patients with an onset between 3 and 6 years. The age when becoming wheelchair-bound (red squares) and the disease duration (green triangles) are most prominent clusters in the second group. Abbreviations: wb = wheelchair-bound. Expon = exponential.</p

Patient F5 (P1) MRI, age 4 years 2 months (A-D), and 7 years 6 months (E-H).

<p>Sagittal T1-weighted (A and E), coronal T2-weighted (B and F), axial T2-weighted (C and G) and axial diffusion (D and H) sequences showing mild cerebellar cortical atrophy with mildly prominent folia (arrows in C). There is also simple corpus callosum (arrows in E). (F) Coronal T2-weighted reveals progressive cerebellar cortical atrophy and gliosis, with widening of folia and increased signal in the residual cerebellar cortex (arrow). Axial T2-weighted (G), and axial diffusion (H) sequences highlight iron deposition as reduction in signal intensity in the globus pallidi (arrows).</p

Genetics findings.

<p>Pedigrees (Ped) of the six Saudi Arabian families (F1-6) are represented in the same order as in the tables and in Figure 7. Patients were also numbered according to the Tables. Haplotypes were reconstructed manually (Family F4 was not subjected to genotyping) and chromatograms of each identified mutation are shown, except Family F4 which had direct sequencing in a private company. The segregation of the mutation, when possible, was shown for each pedigree with the corresponding symbols (“+”= wild type and “-“= mutated).</p

Patient F6 (P3) MRI, age 17 years 5 months.

<p>Axial T2-weighted (A), axial gradient (B and C), sagittal T1-weighted (D) and coronal fluid-attenuated inversion recovery (FLAIR, E) sequences highlight iron deposition as reduction in signal intensity in the globus pallidi (arrows in A and B) and <i>substantia </i><i>nigra</i> (arrow in C). There is also cerebellar cortical atrophy with increased CSF spaces around the cerebellum (arrows in D and E).</p