Pathogenic mutations and sequence variants within mitofusin 2 gene in Polish patients with different hereditary motor-sensory neuropathies.

At the time of its first description in 2004, MFN2 was considered the most frequently mutated gene in hereditary motor and sensory neuropathy type 2 (HMSN 2). However recent studies have shown that the frequency of MFN2 gene mutations in HMSN II patients is surprisingly low. To date, no systematic studies devoted to HMSN IIa in Poland have been carried out. In this study, we searched for MFN2 gene mutations in Polish patients representing the population of nearly 40 million. We decided to include a wide spectrum of clinical phenotypes in the study, proving able to detect, in a group of 67 affected patients: 1) 3 pathogenic mutations; 2) 3 sequence variants of unknown pathogenic status; 3) 9 rare MFN2 gene sequence variants; 4) 6 common polymorphisms. The frequency of MFN2 gene mutations in the whole group of patients is 4.5%. Due to the high frequency of MFN2 gene sequence variants within single patients we could not definitely exclude the cumulative effect of these contributing to the HMSN II phenotype. The MFN2 gene should therefore be considered in Polish HMSN II patients, though it is still not possible to determine its position in HMSN II molecular diagnostics.

Emery-Dreifuss muscular dystrophy type 2 associated (?) with mild peripheral polyneuropathy.

In recent years numerous mutations in the LMNA gene encoding lamin A/C were shown to segregate with a wide spectrum of phenotypes. A recurrent p.R377H mutation in the LMNA gene was reported in patients with Emery-Dreifuss dystrophy (EDMD2) with various ethnic backgrounds. We present a patient with EDMD2 caused by a p.R377H mutation, associated with mild peripheral polyneuropathy. The analysis of peripheral myelin protein 22 (PMP22), ganglioside induced differentiation-associated protein 1 (GDAP1), gap junction ß-1 protein (GJB1), and myelin protein zero (MPZ) genes did not reveal mutations; however, we identified a new sequence intronic variant in the mitofusin 2 (MFN2) gene of unknown pathogenic significance. A complex phenotype in the presented patient might depend either on single mutation in the LMNA gene or on bigenic defect; therefore, a wide genetic investigation is needed to elucidate the molecular background of EDMD2/polyneuropathy in this case.

A severe recessive and a mild dominant form of Charcot-Marie-Tooth disease associated with a newly identified Glu222Lys GDAP1 gene mutation.

Charcot-Marie-Tooth (CMT) disease caused by mutations in the GDAP1 gene has been shown to be inherited via traits that may be either autosomal recessive (in the majority of cases) [CMT4A] or autosomal dominant [CMT2K]. CMT4A disease is characterized by an early onset, and a severe clinical course often leading to a loss of ambulation, whereas CMT2K is characterized by a mild clinical course of benign axonal neuropathy beginning even in the 6th decade of life. Clinical data from a GDAP1 mutated patient suggests that the presence of a particular mutation is associated with a certain trait of inheritance. The association of a particular GDAP1 gene mutation and a dominant or recessive trait of inheritance is of special importance for genetic counseling and the prenatal diagnostics as regards severe forms of CMT. In the present study we report on two CMT families in which a newly identified Glu222Lys mutation within the GDAP1 gene segregates both in autosomal dominant and recessive traits. Our study shows that at least some GDAP1 gene mutations may segregate with the CMT phenotype as both dominant and recessive traits. Thus, genetic counseling for CMT4A/CMT2K families requires more extensive data on GDAP1 phenotype-genotype correlations.

A late-onset and mild form of Charcot-Marie-Tooth disease type 2 caused by a novel splice-site mutation within the Mitofusin-2 gene.

Charcot-Marie-Tooth type 2A disease (CMT2A) caused by mutations in the Mitofusin 2 gene (Mfn2) has been shown to be an early-onset axonal neuropathy with severe clinical course in the majority of the patients. In this study we present a unique phenotype of CMT2A disease characterized by late-onset polyneuropathy with a very mild clinical course. This rare form of CMT2A disease is caused by a new splice-site (c.311+1G>T) mutation within the MFN2 gene. Due to disturbance of the MFN2 splicing process, this mutation generates a short transcript which encodes a very short fragment of MFN2 protein. The c.311+1G>T mutation within the MFN2 gene results in the late -onset CMT2 disease.

Two pathogenic mutations located within the 5'-regulatory sequence of the GJB1 gene affecting initiation of transcription and translation.

In contrast to mutations in the coding sequences of a genes involved in the pathogenesis of Charcot-Marie-Tooth disease (CMT), little is known about CMT phenotypes resulting from a DNA variants located in regulatory sequences of a given " CMT gene". Charcot-Marie-Tooth type X1 disease (CMTX1) is caused by mutations in the GJB1 gene coding for an ion channel known as connexin, with a molecular mass of 32 kDa (Cx32). Only 0.01% of the GJB1 gene mutations have been reported in its 5' regulatory sequence. Pathogenic mutations occured in the internal ribosome entry site (IRES) are extremely rarely reported in human genetic disorders. To the best of our knowledge, in this study we report for the first time in an Eastern European population, two CMTX1 families in which two pathogenic mutations in the 5' regulatory sequence of the GJB1 gene (c.-529T>C and -459C>T) have been found. The two mutations identified in our study disturb the 5' UTR sequence in two different ways, by affecting the transcription factor SOX10 binding site (c.-529T>C) and by the disrupting IRES element of GJB1 gene (c.-459C>T). These regions are responsible for transcription (SOX10) and initiation of translation (IRES), respectively. On the basis of our findings that, in contrast to the most DNA sequence variants reported in untranslated regulatory regions of genes, the c.-459C>T and c.-529T>C mutations remain pathogenic in the context of different ethnic background.

[Mitofusin 2 as a crucial peripheral nervous system protein and a common regulator of cell metabolism]. / Mitofuzyna 2--wazne bialko obwodowego ukladu nerwowego i powszechnie wystepujacy regulator metabolizmu.

Some of the metabolic disorders are manifested by a predominantly expressed symptoms from the single organ, however, they display discrete symptoms from other tissues. Charcot Marie Tooth disease (CMT) divided into demyelinating (CMT1) and axonal (CMT2) subtypes is characterized by a slowly progressive wasting of distal muscles. CMT2A form diagnosis requires identification of mutation in a gene coding for mitofusin 2 (MFN2). Mitofusin 2 is a protein of an outer mitochondrial membrane encoded in the nuclear genome and characterized by numerous biochemical functions. Mfn2 is involved mainly in the fusion of mitochondria and the cooperation between endoplasmic reticulum and mitochondria. It seems probably that Mfn2 possesses also some regulatory functions and takes part in a regulation of respiratory chain activity, transcription of several proteins and in intracellular signals transduction. Mfn2-linked pathology is also observed in diabetes and heart diseases. Here, we aim to show that mitofusin 2 is a protein crucial not only for peripheral nerve disorders but is a one of the common regulator of cell metabolism.

[Molecular pathogenesis of hereditary motor and sensory neuropathy]. / Patogeneza molekularna choroby Charcot-Marie-Tooth 2.

Charcot-Marie-Tooth disease 2 is an inherited axonal motor and sensory neuropathy. It is very heterogenous, both clinically and genetically. Till present, 15 types of CMT2, 14 loci and 13 genes are known to be causative of CMT2. Studying mechanisms of molecular pathogenesis is very important for finding a therapy for patients but the diversity of proteins involved in pathogenesis makes this very difficult. Proteins involved in molecular pathogenesis are e.g. proteins of the mitochondrial outer membrane with opposite functions (mitofusin 2 and GDAP1) responsible for fusion and fission of the mitochondrial network. Mutations also occur in genes encoding tRNA-synthetases, neuronal cytoskeletal protein, cation channel protein and molecular chaperones. This review presents knowledge of CMT2 and possible pathogenetic mechanisms responsible for the disease.