P. De Jonghe

Duplication in chromosome 17p11.2 in Charcot-Marie-Tooth neuropathy type 1a (CMT 1a)

Hereditary motor and sensory neuropathy type I (HMSN I) or Charcot-Marie-Tooth disease type 1 (CMT 1) is an autosomal dominant disorder of the peripheral nervous system characterized by progressive weakness and atrophy of distal limb muscles. In the majority of HMSN I families, linkage studies localized the gene (CMT 1a) to the pericentromeric region of chromosome 17. We have detected with probe pVAW409R3 (D17S122) localized in 17p11.2 a duplication, co-segregating with the disease in 12 HMSN I families. In these families the duplication was present in all 128 patients but absent in the 84 unaffected and 44 married-in individuals (lod score of 58.44 at zero recombination). Further, on one HMSN I family the disease newly appeared simultaneously with a de novo duplication originating from an unequal crossing-over event at meiosis. Since different allelic combinations were found segregating with the duplication in different families linkage disequilibrium was not a significant factor. These findings led us to propose that the duplication in 17p11.2 itself is the disease causing mutation in all the HMSN I families analyzed.

Heterozygous missense mutations in BSCL2 are associated with distal hereditary motor neuropathy and Silver syndrome

Distal hereditary motor neuropathy (dHMN) or distal spinal muscular atrophy (OMIM #182960) is a heterogeneous group of disorders characterized by an almost exclusive degeneration of motor nerve fibers, predominantly in the distal part of the limbs. Silver syndrome (OMIM #270685) is a rare form of hereditary spastic paraparesis mapped to chromosome 11q12–q14 (SPG17) in which spasticity of the legs is accompanied by amyotrophy of the hands and occasionally also the lower limbs. Silver syndrome and most forms of dHMN are autosomal dominantly inherited with incomplete penetrance and a broad variability in clinical expression. A genome-wide scan in an Austrian family with dHMN-V (ref. 4) showed linkage to the locus SPG17, which was confirmed in 16 additional families with a phenotype characteristic of dHMN or Silver syndrome. After refining the critical region to 1 Mb, we sequenced the gene Berardinelli-Seip congenital lipodystrophy (BSCL2) and identified two heterozygous missense mutations resulting in the amino acid substitutions N88S and S90L. Null mutations in BSCL2, which encodes the protein seipin, were previously shown to be associated with autosomal recessive Berardinelli-Seip congenital lipodystrophy (OMIM #269700). We show that seipin is an integral membrane protein of the endoplasmic reticulum (ER). The amino acid substitutions N88S and S90L affect glycosylation of seipin and result in aggregate formation leading to neurodegeneration.

POLG mutations in neurodegenerative disorders with ataxia but no muscle involvement

Objective: To identify POLG mutations in patients with sensory ataxia and CNS features. Methods: The authors characterized clinical, laboratory, and molecular genetic features in eight patients from five European families. The authors conducted sequencing of coding exons of POLG, C10orf2 (Twinkle), and ANT1 and analyzed muscle mitochondrial DNA (mtDNA), including Southern blot analysis and long-range PCR. Results: Ataxia occurred in combination with various CNS features, including myoclonus, epilepsy, cognitive decline, nystagmus, dysarthria, thalamic and cerebellar white matter lesions on MRI, and neuronal loss in discrete gray nuclei on autopsy. Gastrointestinal dysmotility, weight loss, cardiomyopathy, and valproate-induced hepatotoxicity occurred less frequently. Two patients died without preceding signs of progressive external ophthalmoplegia. In muscle, typical findings of mitochondrial disease, such as ragged red fibers and Southern blot mtDNA abnormalities, were absent. POLG mutations were present in eight patients, including two isolated cases, and one Finnish and two unrelated Belgian families contained in total six patients. All POLG mutations were recessive, occurring in a homozygous state in seven patients and in a compound heterozygous state in one patient. The novel W748S mutation was identified in five patients from three unrelated families. Conclusions: The clinical spectrum of recessive POLG mutations is expanded by sensory ataxic neuropathy, combined with variable features of involvement of CNS and other organs. Progressive external ophthalmoplegia, myopathy, ragged red fibers, and Southern blot abnormalities of muscle mitochondrial DNA also are not mandatory features associated with POLG mutations.

Recessive POLG mutations presenting with sensory and ataxic neuropathy in compound heterozygote patients with progressive external ophthalmoplegia

Autosomal recessive progressive external ophthalmoplegia is a mitochondrial disease characterized by accumulation of multiple large-scale deletions of mitochondrial DNA. We previously reported missense mutations in POLG, the gene encoding the mitochondrial DNA polymerase gamma in two nuclear families compatible with autosomal recessive progressive external ophthalmoplegia. Here, we report a novel POLG missense mutation (R627W) in a sporadic patient and we provide genetic support that all these POLG mutations are actually causal and recessive. The novel patient presented with sensory ataxic neuropathy and has the clinical triad of sensory ataxic neuropathy, dysarthria and ophthalmoparesis (SANDO). This is the first finding of a genetic cause of Sensory Ataxic Neuropathy, Dysarthria and Ophthalmoparesis and it implies that this disorder may actually be a variant of autosomal recessive progressive external ophthalmoplegia. Sensory neuropathy is the initial feature in Belgian compound heterozygote autosomal recessive progressive external ophthalmoplegia patients, all carrying the POLG A467T mutation, which occurs at a frequency of 0.6% in the Belgian population.

A deletion in SCN1B is associated with febrile seizures and early-onset absence epilepsy

Generalized epilepsy with febrile seizures plus (GEFS+) is a clinically and genetically heterogeneous syndrome with childhood onset, characterized by febrile seizures (FS) and a variety of afebrile epileptic seizure types. The authors performed a mutational analysis of SCN1B on 74 unrelated probands with GEFS+, FS, or FS plus (FS+). In a family with FS+ and early-onset absence epilepsy, a mutation was identified that predicts a deletion of five amino acids in the extracellular immunoglobulin-like domain of SCN1B and potential loss of function. SCN1B mutations are associated with GEFS+ and may have a role in the elicitation of absence seizures.

Novel missense mutation in the early growth response 2 gene associated with Dejerine–Sottas syndrome phenotype

Background: Mutations in the early growth response 2 (EGR2) gene have recently been found in patients with congenital hypomyelinating neuropathy and Charcot-Marie-Tooth type 1 (CMT1) disease. Objective: To determine the frequency of EGR2 mutations in patients with a diagnosis of CMT1, Dejerine–Sottas syndrome (DSS), or unspecified peripheral neuropathies. Methods: Fifty patients and 70 normal control subjects were screened. Results: A de novo missense mutation (Arg359Trp) in the α-helix of the first zinc-finger domain of the EGR2 transcription factor was identified in a patient diagnosed with a clinical phenotype consistent with DSS. This patient had a motor median nerve conduction velocity of 8 m/s. A sural nerve biopsy showed a severe loss of myelinated and unmyelinated fibers, evidence for demyelination, numerous classic onion bulbs, and focally folded myelin sheaths. DSS is a severe, childhood-onset demyelinating peripheral neuropathy initially thought to be inherited as an autosomal recessive trait. However, several dominant heterozygous mutations in the peripheral myelin protein 22 (PMP22) gene and dominant mutations in the peripheral myelin protein zero (MPZ) gene, both in the heterozygous and homozygous state, have been reported in patients with DSS. Conclusions: Hereditary peripheral neuropathies represent a spectrum of disorders due to underlying defects in myelin structure or formation.

Mutations in GDAP1 - Autosomal recessive CMT with demyelination and axonopathy

Background: Mutations in the ganglioside-induced differentiation-associated protein 1 gene (GDAP1) were recently shown to be responsible for autosomal recessive (AR) demyelinating Charcot–Marie–Tooth disease (CMT) type 4A (CMT4A) as well as AR axonal CMT with vocal cord paralysis. Methods: The coding region of GDAP1 was screened for the presence of mutations in seven families with AR CMT in which the patients were homozygous for markers of the CMT4A locus at chromosome 8q21.1. Results: A nonsense mutation was detected in exon 5 (c.581C>G, S194X), a 1-bp deletion in exon 6 (c.786delG, G262fsX284), and a missense mutation in exon 6 (c.844C>T, R282C). Conclusions: Mutations in GDAP1 are a frequent cause of AR CMT. They result in an early-onset, severe clinical phenotype. The range of nerve conduction velocities (NCV) is variable. Some patients have normal or near normal NCV, suggesting an axonal neuropathy, whereas others have severely slowed NCV compatible with demyelination. The peripheral nerve biopsy findings are equally variable and show features of demyelination and axonal degeneration.

Progranulin genetic variability contributes to amyotrophic lateral sclerosis.

Objectives: Null mutations in progranulin (PGRN) cause ubiquitin-positive frontotemporal dementia (FTD) linked to chromosome 17q21 (FTDU-17). Here we examined PGRN genetic variability in amyotrophic lateral sclerosis (ALS), a neurodegenerative motor neuron disease that overlaps with FTD at a clinical, pathologic, and epidemiologic level. Methods: We sequenced all exons, exon-intron boundaries, and 5′ and 3′ regulatory regions of PGRN in a Belgian sample of 230 patients with ALS. The frequency of observed genetic variants was determined in 436 healthy control individuals. The contribution of eight frequent polymorphisms to ALS risk, onset age, and survival was assessed in an association study in the Belgian sample and a replication series of 308 Dutch patients with ALS and 345 Dutch controls. Results: In patients with ALS we identified 11 mutations, 5 of which were predicted to affect PGRN protein sequence or levels (four missense mutations and one 5′ regulatory variant). Moreover, common variants (rs9897526, rs34424835, and rs850713) and haplotypes were significantly associated with a reduction in age at onset and a shorter survival after onset of ALS in both the Belgian and the Dutch studies. Conclusion: PGRN acts as a modifier of the course of disease in patients with amyotrophic lateral sclerosis, through earlier onset and shorter survival.

Absence epilepsies with widely variable onset are a key feature of familial GLUT1 deficiency.

Background: Familial glucose transporter type 1 (GLUT1) deficiency due to autosomal dominant inheritance of SLC2A1 mutations is associated with paroxysmal exertional dyskinesia; epilepsy and intellectual disability occur in some family members. We recently demonstrated that GLUT1 deficiency occurs in over 10% of patients with early-onset absence epilepsy. Methods: This family study analyses the phenotypes in 2 kindreds segregating SLC2A1 mutations identified through probands with early-onset absence epilepsy. One comprised 9 individuals with mutations over 3 generations; the other had 6 individuals over 2 generations. Results: Of 15 subjects with SLC2A1 mutations, epilepsy occurred in 12. Absence seizures were the most prevalent seizure type (10/12), with onset from 3 to 34 years of age. Epilepsy phenotypes varied widely, including idiopathic generalized epilepsies (IGE) with absence (8/12), myoclonic-astatic epilepsy (2/12), and focal epilepsy (2/12). Paroxysmal exertional dyskinesia occurred in 7, and was subtle and universally undiagnosed prior to molecular diagnosis. There were 2 unaffected mutation carriers. Conclusions: GLUT1 deficiency is an important monogenic cause of absence epilepsies with onset from early childhood to adult life. Individual cases may be phenotypically indistinguishable from common forms of IGE. Although subtle paroxysmal exertional dyskinesia is a helpful diagnostic clue, it is far from universal. The phenotypic spectrum of GLUT1 deficiency is considerably greater than previously recognized. Diagnosis of GLUT1 deficiency has important treatment and genetic counseling implications.

A novel GABRG2 mutation associated with febrile seizures

Mutations in the gene encoding the γ2 subunit of the γ-aminobutyric acid type A receptor (GABRG2) have been reported to cause childhood absence epilepsy (CAE), febrile seizures (FS), and generalized epilepsy with FS plus (GEFS+). The authors analyzed GABRG2 in 47 unrelated patients with CAE, FS, and GEFS+ and identified a novel mutation that cosegregated with FS. Electrophysiologic studies demonstrated altered current desensitization and reduced benzodiazepine enhancement in mutant receptors.