Prenatal diagnosis of Charcot-Marie-Tooth disease type 1A by interphase fluorescence in situ hybridization

Inherited focal, episodic neuropathies: hereditary neuropathy with liability to pressure palsies and hereditary neuralgic amyotrophy

Hereditary neuropathy with liability to pressure palsies (HNPP; also called tomaculous neuropathy) is an autosomal-dominant disorder that produces a painless episodic, recurrent, focal demyelinating neuropathy. HNPP generally develops during adolescence, and may cause attacks of numbness, muscular weakness, and atrophy. Peroneal palsies, carpal tunnel syndrome, and other entrapment neuropathies may be frequent manifestations of HNPP. Motor and sensory nerve conduction velocities may be reduced in clinically affected patients, as well as in asymptomatic gene carriers. The histopathological changes observed in peripheral nerves of HNPP patients include segmental demyelination and tomaculous or "sausage-like" formations. Mild overlap of clinical features with Charcot-Marie-Tooth (CMT) disease type 1 (CMT1) may lead patients with HNPP to be misdiagnosed as having CMT1. HNPP and CMT1 are both demyelinating neuropathies, however, their clinical, pathological, and electrophysiological features are quite distinct. HNPP is most frequently associated with a 1.4-Mb pair deletion on chromosome 17p12. A duplication of the identical region leads to CMT1A. Both HNPP and CMT1A result from a dosage effect of the PMP22 gene, which is contained within the deleted/duplicated region. This is reflected in reduced mRNA and protein levels in sural nerve biopsy samples from HNPP patients. Treatment for HNPP consists of preventative and symptom-easing measures. Hereditary neuralgic amyotrophy (HNA; also called familial brachial plexus neuropathy) is an autosomal-dominant disorder causing episodes of paralysis and muscle weakness initiated by severe pain. Individuals with HNA may suffer repeated episodes of intense pain, paralysis, and sensory disturbances in an affected limb. The onset of HNA is at birth or later in childhood with prognosis for recovery usually favorable; however, persons with HNA may have permanent residual neurological dysfunction following attack(s). Episodes are often triggered by infections, immunizations, the puerperium, and stress. Electrophysiological studies show normal or mildly prolonged motor nerve conduction velocities distal to the affected brachial plexus. Pathological studies have found axonal degeneration in nerves examined distal to the plexus abnormality. In some HNA pedigrees there are characteristic facial features, including hypotelorism. The prognosis for recovery of normal function of affected limbs in HNA is good, although recurrent episodes may cause residual deficits. HNA is genetically linked to chromosome 17q25, where mutations in the septin-9 (SEPT9) gene have been found.

Split-hand/split-foot malformation 3 (SHFM3) at 10q24, development of rapid diagnostic methods and gene expression from the region

Split-hand/split-foot malformation (SHFM, also called ectrodactyly) is a clinically variable and genetically heterogeneous group of limb malformations. Several SHFM loci have been mapped, including SHFM1 (7q21), SHFM2 (Xq26), SHFM3 (10q24), SHFM4 (3q27) and SHFM5 (2q31). To date, mutations in a gene (TP63) have only been identified for SHFM4. SHFM3 has been shown by pulsed-field gel electrophoresis to be caused by an approximately 500 kb DNA rearrangement at 10q24. This region contains a number of candidate genes for SHFM3, though which gene(s) is (are) involved in the pathogenesis of SHFM3 is not known. Our aim in this study was to improve the diagnosis of SHFM3, and to begin to understand which genes are involved in SHFM3. Here we show, using two different techniques, FISH and quantitative PCR that SHFM3 is caused by a minimal 325 kb duplication containing only two genes (BTRC and POLL). The data presented provide improved methods for diagnosis and begin to elucidate the pathogenic mechanism of SHFM3. Expression analysis of 13 candidate genes within and flanking the duplicated region shows that BTRC (present in three copies) and SUFU (present in two copies) are overexpressed in SHFM3 patients compared to controls. Our data suggest that SHFM3 may be caused by overexpression of BTRC and SUFU, both of which are involved in beta-catenin signalling.

A rapid and definitive test for Charcot-Marie-Tooth 1A and hereditary neuropathy with liability to pressure palsies using multiplexed real-time PCR

Alterations in gene copy number have been shown to cause disease in humans. Two of the most common inherited peripheral neuropathies, Charcot-Marie-Tooth 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP), are two such diseases resulting from alteration in gene copy number of the dosage sensitive peripheral myelin protein 22 (PMP22) gene. Many complicated and laborious diagnostic tests exist for the diagnosis of these diseases. The aim of our study was to develop the first quantitative multiplex real-time PCR assay for the diagnosis of CMT1A and HNPP. A total of 160 individuals who were known to have CMT1A, HNPP, or were normal from previous testing were assayed by our multiplex real-time PCR method. The results confirmed the previously determined gene copy number of all patient and control individuals tested. The range of ratio values between the disease and control groups were easily defined. The assay is accurate, simple, and cost effective and can detect a 50% change in gene copy number. This represents an ideal assay for any small diagnostic laboratory.

Rapid detection of 17p11.2 rearrangements by FISH without cell culture (direct FISH, DFISH): a prospective study of 130 patients with inherited peripheral neuropathies

Charcot-Marie-Tooth (CMT) disease and hereditary neuropathy with pressure palsies (HNPP) are two frequent hereditary motor and sensory neuropathies. CMT is characterized by slowly progressive weakness and atrophy, primarily in peroneal and distal leg muscles. The most frequent form, CMT1A, is due, in most cases, to the duplication of a 1.5 Mb region on chromosome 17p11.2 containing the peripheral myelin protein 22 gene (PMP22). The phenotype seems to result from dosage of the PMP22 gene. This hypothesis is reinforced by the existence of HNPP, which is clinically characterized by various recurrent truncular palsies or sensory loss precipitated by minor trauma, which is caused by deletion of the same 1.5 Mb region in 17p11.2. In clinical practice, the detection of the duplication or the deletion in 17p11.2, which permits a positive diagnosis, is still performed by time consuming methods (Southern blot or various combinations of molecular tools). We developed a method for the rapid detection of 17p11.2 rearrangements, using "direct FISH" and PRINS analyses, which does not require cell culture. In a prospective study of 92 patients with CMT and 38 with suspected HNPP, we compared this new technique to classical strategies like Southern blot. The results demonstrate the high sensitivity and specificity of the new FISH technique for the diagnosis of CMT1A and HNPP. Moreover, because of its simplicity and rapidity, this technique provides a useful alternative to the molecular approaches that have been used to diagnose segmental aneusomies, especially in the case of duplications that often go undetected.

Prenatal detection of the 17p11.2 duplication in Charcot-Marie-Tooth disease type 1A: necessity of a multidisciplinary approach for heterogeneous disorders

Charcot-Marie-Tooth (CMT) disease is a typical example of a clinically and genetically heterogeneous disorder and, in most cases, is dominantly inherited and caused by a 1.5 megabase duplication on chromosome 17p11.2 containing the PMP22 gene. This is a non-lethal disease with a wide spectrum of severity, from asymptomatism to severe motor and sensory disability. Unpredictable degree of disability is usually the reason why prenatal diagnosis is required and must be addressed. Molecular procedures such as the use of polymorphic non microsatellite STRs, allowing very fast and reliable results even when requiring a gene dosage interpretation are now available and have been recently validated in post-natal diagnosis. Our results indicate that this approach is also the best-adapted method in case of prenatal diagnosis. Nevertheless, ethical considerations raised by prenatal diagnosis in CMT and more generally in non-lethal disorders remain to be actively considered. Here, we present our experience in genetic counselling, and address the psychological issues for 7 CMT at risk pregnancies. In five cases, a CMT1A duplication was evidenced; pregnancy was terminated in four of these cases and the parents from one affected foetus decided to pursue the pregnancy.

Prenatal interphase FISH diagnosis of PLP1 duplication associated with Pelizaeus-Merzbacher disease

A submicroscopic genomic duplication in Xq22.2 that contains the entire proteolipid protein 1 gene (PLP1) is responsible for the majority of Pelizaeus-Merzbacher disease (PMD) patients. We previously developed an interphase FISH assay to screen for PLP1 duplications in PMD patients using peripheral blood and lymphoblastoid cell lines. This assay has been utilized as a clinical diagnostic test in our cytogenetics laboratory. To expand usage of the interphase FISH assay to prenatal diagnosis of PLP1 duplications, we examined three PMD families with PLP1 duplications utilizing aminiotic fluid samples. In two families the FISH assay revealed fetuses with PLP1 duplications, whereas the other fetus showed a normal copy number of PLP1. Haplotype analyses, as well as an additional FISH analysis using postnatal blood samples, confirmed the results of the prenatal analyses. Our study demonstrates utility of the interphase FISH assay in the prenatal diagnosis of PLP1 duplications in PMD.

New Polymorphic Short Tandem Repeats for PCR-based Charcot-Marie-Tooth Disease Type 1A Duplication Diagnosis

Background: Charcot-Marie-Tooth disease type 1A (CMT1A) accounts for 70–90% of cases of CMT1 and is most frequently caused by the tandem duplication of a 1.4-Mb genomic fragment on chromosome 17p12. Molecular diagnosis of CMT1A has been based primarily on pulsed-field electrophoresis, fluorescence in situ hybridization, polymorphic allele dosage analysis, and quantitative PCR. We sought to improve the fidelity and applicability of PCR-based diagnosis by developing a panel of novel, highly polymorphic short tandem repeats (STRs) from within the CMT1A duplicated region.

Methods: We used a recently available genomic sequence to identify potentially polymorphic simple repeats. We then amplified these sequences in a multiethnic cohort of unaffected individuals and assessed the heterozygosity and number of alleles for each STR. Highly informative markers were then tested in a set of previously diagnosed CMT1A duplication patients, and the ability to identify the genomic duplication through the presence of three bands was assessed.

Results: We identified 34 polymorphic markers, 15 of which were suitable for CMT1A diagnosis on the basis of high heterozygosity in different ethnic groups, peak uniformity, and a large number of alleles. On the basis of the fluorescent dye and allele range of each marker, we developed two panels, each of which could be analyzed concurrently. Panel 1, which comprised 10 markers, detected 37 of 39 duplications, whereas panel 2, which comprised the remaining 5 markers, identified 21 of 39 duplications. Through the combination of both panels, we identified 39 of 39 duplications in previously diagnosed CMT1A patients.

Conclusions: The newly developed 15-marker set has the capability of detecting >99% of duplications and thus is a powerful and versatile diagnostic tool.

The application of fluorescence in-situ hybridization to prenatal diagnosis

Fluorescence in-situ hybridization has become essential in prenatal diagnosis for identifying chromosome aberrations as well as in preimplantation genetic diagnosis and the analysis of fetal cells in maternal blood. Comparative genome hybridization, multicolor fluorescence in-situ hybridization and telomere probes provide technical approaches for the characterization of fetal chromosome anomalies not possible by conventional karyotyping.