Variants in the genes DCTN2, DNAH10, LRIG3, and MYO1A are associated with intermediate Charcot-Marie-Tooth disease in a Norwegian family.

INTRODUCTION: Charcot-Marie-Tooth disease (CMT) is a heterogeneous inherited neuropathy. The number of known CMT genes is rapidly increasing mainly due to next-generation sequencing technology, at present more than 70 CMT-associated genes are known. We investigated whether variants in the DCTN2 could cause CMT. MATERIAL AND METHODS: Fifty-nine Norwegian CMT families from the general population with unknown genotype were tested by targeted next-generation sequencing (NGS) for variants in DCTN2 along with 32 CMT genes and 19 other genes causing other inherited neuropathies or neuronopathies, due to phenotypic overlap. In the family with the DCTN2 variant, exome sequencing was then carried out on all available eight family members to rule out the presence of more potential variants. RESULTS: Targeted NGS identified in one family a variant of DCTN2, c.337C>T, segregating with the phenotype in five affected members, while it was not present in the three unaffected members. The DCTN2 variant c.337C>T; p.(His113Tyr) was neither found in in-house controls nor in SNP databases. Exome sequencing revealed a singular heterozygous shared haplotype containing four genes, DCTN2, DNAH10, LRIG3, and MYO1A, with novel sequence variants. The haplotype was shared by all the affected members, while the unaffected members did not have it. CONCLUSIONS: This is the first time a haplotype on chromosome 12 containing sequence variants in the genes DCTN2, DNAH10, LRIG3, and MYO1A has been linked to an inherited neuropathy in humans.

Genetic epidemiology of Charcot-Marie-Tooth disease.

BACKGROUND: Charcot-Marie-Tooth disease (CMT) is the most common inherited disorder of the peripheral nervous system. The frequency of different CMT genotypes has been estimated in clinic populations, but prevalence data from the general population is lacking. Point mutations in the mitofusin 2 (MFN2) gene has been identified exclusively in Charcot-Marie-Tooth disease type 2 (CMT2), and in a single family with intermediate CMT. MFN2 point mutations are probably the most common cause of CMT2. The CMT phenotype caused by mutation in the myelin protein zero (MPZ) gene varies considerably, from early onset and severe forms to late onset and milder forms. The mechanism is not well understood. The myelin protein zero (P(0) ) mediates adhesion in the spiral wraps of the Schwann cell's myelin sheath. X-linked Charcot-Marie Tooth disease (CMTX) is caused by mutations in the connexin32 (cx32) gene that encodes a polypeptide which is arranged in hexameric array and form gap junctions. AIMS: Estimate prevalence of CMT. Estimate frequency of Peripheral Myelin Protein 22 (PMP22) duplication and point mutations, insertions and deletions in Cx32, Early growth response 2 (EGR2), MFN2, MPZ, PMP22 and Small integral membrane protein of lysosome/late endosome (SIMPLE) genes. Description of novel mutations in Cx32, MFN2 and MPZ. Description of de novo mutations in MFN2. MATERIAL AND METHODS: Our population based genetic epidemiological survey included persons with CMT residing in eastern Akershus County, Norway. The participants were interviewed and examined by one geneticist/neurologist, and classified clinically, neurophysiologically and genetically. Two-hundred and thirty-two consecutive unselected and unrelated CMT families with available DNA from all regions in Norway were included in the MFN2 study. We screened for point mutations in the MFN2 gene. We describe four novel mutations, two in the connexin32 gene and two in the MPZ gene. RESULTS: A total of 245 affected from 116 CMT families from the general population of eastern Akershus county were included in the genetic epidemiological survey. In the general population 1 per 1214 persons (95% CI 1062-1366) has CMT. Charcot-Marie-Tooth disease type 1 (CMT1), CMT2 and intermediate CMT were found in 48.2%, 49.4% and 2.4% of the families, respectively. A mutation in the investigated genes was found in 27.2% of the CMT families and in 28.6% of the affected. The prevalence of the PMP22 duplication and mutations in the Cx32, MPZ and MFN2 genes was found in 13.6%, 6.2%, 1.2%, 6.2% of the families, and in 19.6%, 4.8%, 1.1%, 3.2% of the affected, respectively. None of the families had point mutations, insertions or deletions in the EGR2, PMP22 or SIMPLE genes. Four known and three novel mitofusin 2 (MFN2) point mutations in 8 unrelated Norwegian CMT families were identified. The novel point mutations were not found in 100 healthy controls. This corresponds to 3.4% (8/232) of CMT families having point mutations in MFN2. The phenotypes were compatible with CMT1 in two families, CMT2 in four families, intermediate CMT in one family and distal hereditary motor neuronopathy (dHMN) in one family. A point mutation in the MFN2 gene was found in 2.3% of CMT1, 5.5% of CMT2, 12.5% of intermediate CMT and 6.7% of dHMN families. Two novel missense mutations in the MPZ gene were identified. Family 1 had a c.368G>A (Gly123Asp) transition while family 2 and 3 had a c.103G>A (Asp35Asn) transition. The affected in family 1 had early onset and severe symptoms compatible with Dejerine-Sottas syndrome (DSS), while affected in family 2 and 3 had late onset, milder symptoms and axonal neuropathy compatible with CMT2. Two novel connexin32 mutations that cause early onset X-linked CMT were identified. Family 1 had a deletion c.225delG (R75fsX83) which causes a frameshift and premature stop codon at position 247 while family 2 had a c.536G>A (Cys179Tyr) transition which causes a change of the highly conserved cysteine residue, i.e. disruption of at least one of three disulfide bridges. The mean age at onset was in the first decade and the nerve conduction velocities were in the intermediate range. DISCUSSION: Charcot-Marie-Tooth disease is the most common inherited neuropathy. At present 47 hereditary neuropathy genes are known, and an examination of all known genes would probably only identify mutations in approximately 50% of those with CMT. Thus, it is likely that at least 30-50 CMT genes are yet to be identified. The identified known and novel point mutations in the MFN2 gene expand the clinical spectrum from CMT2 and intermediate CMT to also include possibly CMT1 and the dHMN phenotypes. Thus, genetic analyses of the MFN2 gene should not be restricted to persons with CMT2. The phenotypic variation caused by different missense mutations in the MPZ gene is likely caused by different conformational changes of the MPZ protein which affects the functional tetramers. Severe changes of the MPZ protein cause dysfunctional tetramers and predominantly uncompacted myelin, i.e. the severe phenotypes congenital hypomyelinating neuropathy and DSS, while milder changes cause the phenotypes CMT1 and CMT2. The two novel mutations in the connexin32 gene are more severe than the majority of previously described mutations possibly due to the severe structural change of the gap junction they encode. CONCLUSION: Charcot-Marie-Tooth disease is the most common inherited disorder of the peripheral nervous system with an estimated prevalence of 1 in 1214. CMT1 and CMT2 are equally frequent in the general population. The prevalence of PMP22 duplication and of mutations in Cx32, MPZ and MFN2 is 19.6%, 4.8%, 1.1% and 3.2%, respectively. The ratio of probable de novo mutations in CMT families was estimated to be 22.7%. Genotype- phenotype correlations for seven novel mutations in the genes Cx32 (2), MFN2 (3) and MPZ (2) are described. Two novel phenotypes were ascribed to the MFN2 gene, however further studies are needed to confirm that MFN2 mutations can cause CMT1 and dHMN.

Genetic epidemiology of Charcot-Marie-Tooth in the general population.

BACKGROUND AND PURPOSE: the frequency of different Charcot-Marie-Tooth (CMT) genotypes has been estimated in clinic populations, but prevalence data from the general population are lacking. METHODS: our population-based genetic epidemiological survey included persons with CMT residing in eastern Akershus County, Norway. The participants were interviewed and examined by one geneticist/neurologist and classified clinically, neurophysiologically and genetically. RESULTS: two hundred and forty-five persons from 116 families had CMT. This corresponds to 1 per 1214 persons (95% CI 1062-1366) have CMT in the general population. CMT1 (motor conduction velocity (MCV) <38 m/s), CMT2 (MCV >38 m/s) and CMT intermediate (MCV 25-45 m/s) were found in 48.2%, 49.4% and 2.4% of the families. A total of 27.2% of the families and 28.6% of the affected had a mutation in the investigated CMT genes. The prevalence of the peripheral myelin protein 22 (PMP22) duplication and point mutation in the connexin32 (Cx32), myelin protein zero (MPZ) and mitofusin2 (MFN2) genes was found in 13.6%, 6.2%, 1.2%, 6.2% of the families, and in 19.6%, 4.8%, 1.1%, 3.2% of the affected, respectively. None of the families had point mutations in the early growth response 2 (EGR2), PMP22 or small integral membrane protein of lysosome/late endosome (SIMPLE) genes. CONCLUSIONS: CMT is the most common inherited neuropathy. At present, 43 CMT genes are known, and an examination of all known genes would probably only identify mutations in approximately 50% of those with CMT. Thus, it is probable that at least 30-50 CMT genes are yet to be identified.

Symptomatic Charcot-Marie-Tooth? A pair of concordant monozygotic twins.

BACKGROUND: A pair of monozygotic twin brothers were referred due to hereditary peripheral neuropathy resembling late onset Charcot-Marie-Tooth (CMT). AIM OF THE STUDY: Diagnostic classification of the twin pair. METHOD: Clinical, neurological, genetical and neurophysiological examination, and molecular genetic testing. RESULTS: The clinic and neurophysiology was compatible with CMT disease with late onset. Molecular genetic analysis excluded mutations in PMP22, connexin32, MPZ, LITAF and MFNZ genes, as well as duplication and deletion of PMP22. CONCLUSIONS: The twins were employed in PVC production and developed symptoms after 14 years of massive exposure. We think that the heavy exposure to various neurotoxic compounds caused symptoms that mimic late-onset CMT. However, the twins had distal dysesthesia which is unusual in inherited neuropathies. This illustrates the importance of an occupational history even in the molecular genetic era.

[Diagnostic DNA testing for Huntington disease]. / Diagnostisk DNA-testing for Huntingtons sykdom.

Direct DNA testing for the autosomal dominant neurodegenerative disorder, Huntington's disease, may be performed in a diagnostic setting, as a presymptomatic procedure or prenatally. This paper is intended for physicians practising outside departments of medical genetics who are considering diagnostic testing of patients presenting with symptoms or signs compatible with Huntington's disease. It offers a brief overview of practically relevant clinical, epidemiological, molecular and legal aspects of diagnostic genetic testing for Huntington's disease in Norway. We stress the need for adequate information before sampling and after the test has been performed, and for close contact with the genetics centre which offers the test for Huntington's disease and provides genetic counselling. As with other diagnostic tests, the treating physician is responsible for informing the patient about the result of the test and for ensuring adequate follow-up. The physician will often need the assistance of an expert in clinical genetics. A positive DNA test for Huntington's disease in a patient may have a profound impact on family members, who should be offered genetic counselling and support. Since asymptomatic at-risk family members may ask for a presymptomatic test in the future, diagnostic confirmation at the DNA level is warranted in any person examined because of clinical signs of Huntington's disease, even when the clinical diagnosis is considered unquestionable.

[Genetic counseling in presymptomatic testing for Huntington disease]. / Genetisk veiledning ved presymptomatisk testing for Huntingtons sykdom.

Norwegian law and international guidelines require genetic counselling before, during and after presymptomatic testing for Huntington's disease. The genetic counselling of at-risk persons who considers taking tests, includes explanation of the possible implications of a test result for both participant and relatives. The test is performed only when explicitly requested by the participant and after informed consent. The participant decides if and when the test should be conducted. The participant also has major influence on the timing of the consecutive phases of the testing procedure, in compliance with medical and ethical recommendations. This paper reviews main issues raised during genetic counselling and the preparation period preceding the test and communication of the test result. We illustrate different individual situations and backgrounds for considering presymptomatic testing for Huntington's disease by describing three anonymized cases and associated pedigrees.