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In contrast to Charcot-Marie-Tooth type 1 disease (CMT1), which is most commonly caused by 17p11.2-p12 duplication (in 70% of CMT1 cases), the axonal form of hereditary motor and sensory neuropathy (CMT2) seemed to be a genetically heterogeneous disease group, with no single gene playing a major pathogenetic role. In 2004, 10 mutations were identified in CMT2A families in the MFN2 gene coding for the mitochondrial protein mitofusin-2, previously mapped to the 1p35-36 locus. In the last two years, MFN2 gene mutations were shown to be the most common cause of autosomal dominant hereditary axonopathy. In addition, MFN2 gene mutations were also identified in CMT type 6 (axonal neuropathy with optic nerve atrophy). Recent reports indicate that some MFN2 gene mutations may by inherited as autosomal recessive traits. As MFN2 gene mutations are the most common cause of autosomal dominant CMT2 disease (33% of cases), MFN2 gene testing may be considered a diagnostic test for CMT2.

In the past, each nucleotide change causing amino acid substitution in a gene in which other mutations responsible for a neurodegenerative disorder had been found was considered as a causative mutation. However, in recent years, mainly due to the progress of the Human Genome Project (HGP), numerous DNA variants have been identified in many neurodegenerative disorders. Some of them likely belong to the class of pathogenic (causative) mutations, whereas others, which may occasionally coexist with the disease phenotype, should be classified as non-pathogenic DNA polymorphisms. How to differentiate between a pathogenic mutation and a harmless DNA polymorphism nowadays, i.e. in the post-genomic era? The question still remains open. Erroneously assumed pathogenicity of a mutation may result in a misdiagnosis of the disease and in consequence lead to inappropriate genetic counselling. The aim of this short review is to present a set of mutations with no clear pathogenic effect that have been identified in some neurodegenerative disorders.

Both intra- and interfamiliar variability of the clinical course of Charcot-Marie-Tooth disorders (CMT) were reported in the descriptions of the CMT disease in the 19th century. In the 1950s, it was shown that CMT disorders may be classified into two main groups i.e. CMT 1 and 2 on the basis of motor nerve conduction velocity in the motor fibers of the median nerve (MNCV=38 m/s). With the neuropathological studies in the 1960s, especially electron microscopy, a further enrichment of CMT classification was possible (hypomyelinating neuropathy, CMT with focally folded myelin). Thus, CMT classification based on EMG and neuropathological studies created an enormous opportunity to develop molecular genetics studies in CMT. Since early 1980s up to date, molecular genetic studies in CMT disorders resulted in a discovery of 30 genes. The new forms have been added to CMT classification on the basis of molecular DNA studies. In the "DNA era" in CMT genetic counseling in this group of disorders may be offered to the patients. Due to a possibility of delineating CMT patients on the basis of "genetic background", myological semiology and diagnostics may be improved. The access, costs and range of molecular DNA studies are still limited in CMT disorders. It seems possible that in the near future microarray DNA technology may revolutionize CMT diagnostics. The question whether and when gene therapy will be available in CMT remains to be answered, similar to other disorders with a genetic background.

The classification of amino-acid substitutions into pathogenic mutations and harmless polymorphisms should be revised. In the recent years it was shown that some amino-acid substitutions considered as pathogenic mutations were polymorphisms. Similarly, some 'harmless' polymorphisms have been shown to be pathogenic mutations. Functional analysis considered as a good method to estimate the pathogenic nature of mutations is also limited. The selection of DNA samples for the control group is also difficult. Due to the molecular mechanism mediated by recently discovered exonic splicing enhancers and silencers (ESE and ESS) it is hard to predict a pathogenic effect of some mutations. In addition, the phenotype variability observed between unrelated patients harboring the same mutation may reflect the effects of modifying genes as well as the lack of association between mutation and "its" phenotype. The aim of this study is to describe the problem of the pathogenic effect of mutations.

Initially the Myelin Protein Zero gene was shown to be mutated in the demyelinating form of Charcot-Marie-Tooth disease (CMT1). The vast majority of the mutations in the Myelin Protein Zero gene have been detected in the Charcot-Marie-Tooth (1B) disease, however, some of them were found in patients suffering from congenital hypomyelinating neuropathy and axonal type Charcot-Marie-Tooth disease. In this study, a Charcot-Marie-Tooth disease phenotype diversity associated with different mutations in the MPZ gene, is described.

Neurofilaments (NFs) have been shown to be involved in the molecular pathology of numerous neurode-generative human disorders. Recently a set of mutations in the neurofilament light gene (NF-L) was reported in patients suffering from axonal and demyelinating forms of Charcot-Marie-Tooth disease (CMT1 and CMT2). Although a few of the NEFL gene sequence variants have been shown to be rather pathogenous mutations than harmless polymorphisms, the status of some of these variants remains unclear. The aim of this study was to analyse a potential pathogenous effect of the mutations in the NEFL gene identified in CMT affected patients.

Charcot-Marie-Tooth type X1 disease (CMTX1) is an X-dominant peripheral neuropathy caused by mutations in the GJB1 gene. Molecular genetic analysis of the GJB1 gene is crucial for CMTX1 diagnosis and for genetic counselling. To date, molecular genetic analysis of the GJB1 gene revealed 264 mutations in the GJB1 gene. In spite of the rising number of GJB1 gene mutations, family history was documented in only a few CMTX1 cases. The aim of this study was a molecular genetic analysis of the GJB1 gene in 7 families, performed in 19 CMTX1-affected patients and 13 healthy family members. Moreover, we attempted to report evidence of effects of 6 amino-acid substitutions described in this study. To the best of our knowledge, the G110D, V152D and K167E mutations are novel substitutions, which have not been reported so far.

To date, 12 cases of heterozygous Ser72Leu mutations in the peripheral myelin protein 22 have been reported in patients suffering from severe demyelinating form of Charcot-Marie-Tooth disease (CMT1) and congenital hypomyelinating neuropathy (CHN) [MIM# 605253]. In the present study we report two cases of de novo S72L mutations in the PMP22 gene detected in patients of Polish origin suffering from CMT1 disease.

Charcot-Marie-Tooth type X disease (CMTX) is the second most frequent inherited neuropathy, after CMT1A type associated with 17p11.2-p12 duplication. CMTX is inherited as X dominant trait and is caused by point mutations in Cx32 gene. In the study the first Polish group of 11 patients with CMTX from 4 families is presented. The following four mutations were found in Cx32 gene: Gly110Asp, Val 152 Asp, Arg 183 His and Glu 208 Gly. CMTX is characterized by X dominant trait of inheritance, a mild clinical course in affected females and slowly progressive atrophy and weakness of distal limb muscles. Both electrophysiological and sural nerve biopsy studies show axonal changes with secondary demyelination.

Results of clinical, electrophysiological and morphological examination, were presented in 19 patients from 8 families with hereditary motor-sensory neuropathy type I (HMSN type I) with 17p11.2-12 duplication (i.e. CMT IA). The course of the disease was rather mild, slowly progressive. Generalized demyelinating lesion of peripheral nerves was found on EMG examination, with median nerve conduction velocity between 10-20 m/s and very prolonged F wave latency. Sural nerve biopsy was characteristic of chronic demyelinating process. Phenotypic characteristics of our HMSN type I patients shows clinical, electrophysiological and morphological homogeneity, however there are some data from literature indicating possibility of intrafamilial and interfamilial variability.

Hereditary motor-sensory neuropathies (HMSN) are a heterogeneous group of disorders of peripheral nervous system. Four genes in HMSN have been characterized so far i.e.: PMP22, MPZ, Cx32 and EGR-2. The advent of molecular genetic techniques over the past few years has provided identification of molecular defects in a few forms of HMSN. The present study describes the application of modern molecular genetic methods, which are used in the studies of HMSN. Southern blot hybridisation, Fluorescence in situ hybridisation (FISH), Short Tandem Repeat analysis (STR), Semiquantitative PCR analysis (SQ-PCR), Single Strand Conformation Polymorphism method (SSCP), Heteroduplex analysis (HD) and finally DNA automated sequencing are described in the present paper. In the conclusions the advantages and limits of mentioned methods of DNA analysis in HMSN have been described.

A 16-year-old girl with a typical features of hereditary neuropathy with liability to pressure palsies (HNPP) and deletion on chromosome 17p11.2 was described. In the mother who was asymptomatic the same genetic defect was found. In a sural nerve biopsy obtained from the girl myelin thickenings characteristic for this disease and de- and remyelination in nerve fibers were found. Special attention was paid to the occurrence of uncompacted myelin, which was present in diffuse and focal forms. It is concluded that high amount of uncompacted myelin is characteristic for HNPP and it is probably related to the under-expression of peripheral myelin protein 22.