Stable inheritance of the CMT1A DNA duplication in two patients with CMT1 and NF1

Severe Phenotype in Patients with Large Deletions of NF1

Complete deletion of the NF1 gene is identified in 5-10% of patients with neurofibromatosis type 1 (NF1). Several studies have previously described particularly severe forms of the disease in NF1 patients with deletion of the NF1 locus, but comprehensive descriptions of large cohorts are still missing to fully characterize this contiguous gene syndrome. NF1-deleted patients were enrolled and phenotypically characterized with a standardized questionnaire between 2005 and 2020 from a large French NF1 cohort. Statistical analyses for main NF1-associated symptoms were performed versus an NF1 reference population. A deletion of the NF1 gene was detected in 4% (139/3479) of molecularly confirmed NF1 index cases. The median age of the group at clinical investigations was 21 years old. A comprehensive clinical assessment showed that 93% (116/126) of NF1-deleted patients fulfilled the NIH criteria for NF1. More than half had café-au-lait spots, skinfold freckling, Lisch nodules, neurofibromas, neurological abnormalities, and cognitive impairment or learning disabilities. Comparison with previously described "classic" NF1 cohorts showed a significantly higher proportion of symptomatic spinal neurofibromas, dysmorphism, learning disabilities, malignancies, and skeletal and cardiovascular abnormalities in the NF1-deleted group. We described the largest NF1-deleted cohort to date and clarified the more severe phenotype observed in these patients.

Integrative Analysis Identifies Candidate Tumor Microenvironment and Intracellular Signaling Pathways that Define Tumor Heterogeneity in NF1

Neurofibromatosis type 1 (NF1) is a monogenic syndrome that gives rise to numerous symptoms including cognitive impairment, skeletal abnormalities, and growth of benign nerve sheath tumors. Nearly all NF1 patients develop cutaneous neurofibromas (cNFs), which occur on the skin surface, whereas 40-60% of patients develop plexiform neurofibromas (pNFs), which are deeply embedded in the peripheral nerves. Patients with pNFs have a ~10% lifetime chance of these tumors becoming malignant peripheral nerve sheath tumors (MPNSTs). These tumors have a severe prognosis and few treatment options other than surgery. Given the lack of therapeutic options available to patients with these tumors, identification of druggable pathways or other key molecular features could aid ongoing therapeutic discovery studies. In this work, we used statistical and machine learning methods to analyze 77 NF1 tumors with genomic data to characterize key signaling pathways that distinguish these tumors and identify candidates for drug development. We identified subsets of latent gene expression variables that may be important in the identification and etiology of cNFs, pNFs, other neurofibromas, and MPNSTs. Furthermore, we characterized the association between these latent variables and genetic variants, immune deconvolution predictions, and protein activity predictions.

Charcot-Marie-Tooth syndrome and neurofibromatosis type 1 with multiple neurofibromas of the entire spinal nerve roots

The coexistence of polyneuropathy which has the definite clinical and electromyographical findings consistent with Charcot-Marie-Tooth (CMT) syndrome and neurofibromatosis type 1 (NF1) has infrequently been reported. We describe a patient with both CMT and NF1, who had multiple neurofibromas involving the entire spinal neural axis. In addition, he had multiple neurofibromas distributed within the ileopsoas and gluteus muscles and subcutaneous tissues. These lesions were detected readily by MRI and the patient underwent successful surgical resection of the largest tumours compressing bilateral C2 nerve roots. To our knowledge, this is the first reported case of CMT syndrome coexisting with NF1 in which multiple neurofibromas involved the entire spinal nerve roots. We discuss the diagnostic and therapeutic challenges, emphasising the role of MRI and electrophysiology in such cases and provide a literature review.

Novel mutations in the PRX and the MTMR2 genes are responsible for unusual Charcot-Marie-Tooth disease phenotypes

Autosomal recessive Charcot-Marie-Tooth diseases, relatively common in Algeria due to high prevalence of consanguineous marriages, are clinically and genetically heterogeneous. We report on two consanguineous families with demyelinating autosomal recessive Charcot-Marie-Tooth disease (CMT4) associated with novel homozygous mutations in the MTMR2 gene, c.331dupA (p.Arg111LysfsX24) and PRX gene, c.1090C>T (p.Arg364X) respectively, and peculiar clinical phenotypes. The three patients with MTMR2 mutations (CMT4B1 family) had a typical phenotype of severe early onset motor and sensory neuropathy with typical focally folded myelin on nerve biopsy. Associated clinical features included vocal cord paresis, prominent chest deformities and claw hands. Contrasting with the classical presentation of CMT4F (early-onset Dejerine-Sottas phenotype), the four patients with PRX mutations (CMT4F family) had essentially a late age of onset and a protracted and relatively benign evolution, although they presented marked spine deformities. These observations broaden the spectrum of clinical phenotypes associated with these two CMT4 forms.

A patient with neurofibromatosis type 1 and Charcot-Marie-Tooth disease type 1B

We describe a patient with both neurofibromatosis type 1 and Charcot-Marie-Tooth disease type 1B. Although one might expect an overwhelming tumor burden due to the combination of these two disorders, the two mutations did not appear to interact.

Neurofibromatosis type 1 associated with Charcot-Marie-Tooth type 1A

Neurofibromatosis and Charcot-Marie-Tooth are genetic disorders of the nervous system affecting the development and growth of nerve cells and demyelination of peripheral neurons, respectively. We report a 22-year-old man who presented clinical manifestations of both neurofibromatosis type 1 (NF1) and Charcot-Marie-Tooth type 1A. The simultaneous occurrence of neurofibromatosis and Charcot-Marie-Tooth disease has rarely been reported. More extensive reports and further investigations of this combination will certainly provide a better understanding of this linkage in the near future.

A LARGE FAMILY WITH CHARCOT-MARIE-TOOTH TYPE 1A AND TYPE 2 DIABETES MELLITUS

Charcot-Marie-Tooth (CMT) disease is a hereditary demyelinating peripheral neuropathy, and CMT Type 1A is the most common form. In most cases, CMT1A is usually caused by duplication at chromosome 17p11.2-12. Type 2 diabetes mellitus (Type 2 DM) is a common metabolic disorder, characterized by chronic hyperglycemia that can be associated with micro- and/or macrovascular complications. Only a few studies reported CMT1A duplication in association with Type 2 DM. This article explores the characteristics of a large family of 69 members with respect to CMT1A and Type 2 DM. CMT1A was detected in 28 of them. Molecular genetic study was performed in 22, and duplication was detected in all of them. Six of the 22 members with CMT1A also had Type 2 DM based on the American Diabetes Association diagnostic criteria. Association of these two conditions may be coincidental; however, the occurrence of these two diseases in this large family may also suggest a genetic basis. More extensive reports and further investigations of such families having this combination will certainly provide a better understanding of this link.

Achromatopsia: the CNGB3 p.T383fsX mutation results from a founder effect and is responsible for the visual phenotype in the original report of uniparental disomy 14

Achromatopsia (ACHM) or rod monochromacy is an autosomal recessive and genetically heterogeneous retinal disorder. It is characterized by a lack of color discrimination, poor visual acuity, photodysphoria, pendular infantile nystagmus, and abnormal photopic electroretinographic (ERG) recordings with preservation of rod-mediated function. Mutations in three known genes are causative; including genes for the alpha and beta subunits of the cyclic nucleotide-gated cation channel (CNGA3 and CNGB3, respectively) and cone photoreceptor transducin--GNAT2. We investigated the prevalence of mutations in achromatopsia-causing genes in a cohort of 16 families with both clinical and electrophysiologic evidence consistent with autosomal recessive transmission, including one subject with achromatopsia and maternal isodisomy for chromosome 14. The most frequent mutation, p.T383fsX in CNGB3, accounted for 75% (18/24) of disease-associated alleles; intragenic SNPs in unrelated patients revealed transmission of a common haplotype consistent with a founder effect. Homozygous p.T383fsX mutation in CNGB3 that maps to chromosome 8 was detected in a patient with achromatopsia and systemic features associated with uniparental disomy (UPD) of chromosome 14. Two novel variants, p.R223G and p.A621E were found in CNGA3. We conclude that CNGA3 and CNGB3 mutations are responsible for the substantial majority of achromatopsia. Furthermore, the CNGB3 mutation p.T383fsX is a predominant mutation, results from a founder effect, and is responsible for the ACHM in the original clinical report of UPD 14.

[Polyneuropathy associated with neurofibromatosis]

INTRODUCTION

We review the literature on the spectrum of polyneuropathies associated with neurofibromatosis 1 (NF1) and 2 (NF2).

BACKGROUND

Symptomatic neuropathies in NF1 are rarer than in NF2, but constitute a potentially severe complication associated with frequent morbidity related to the neuropathy itself, spinal cord compression or peripheral nerve sheath tumor malignant degeneration. Neuropathies are typically observed in young men with subcutaneous neurofibromas (NF1) or cutaneous schwannomas (NF2) and are characterized by a chronic slowly worsening sensorimotor polyneuropathy of the lower limbs. In NF1, demyelinating neuropathy may occur alone or in association with axonal features, whereas in NF2, axonal neuropathies are reported. Large multinodular roots and nerves, which can be easily detected by limb MRI, are characteristic features of NF1-associated polyneuropathies.

PERSPECTIVES AND CONCLUSION

Although an associated pathogenic factor may reveal an asymptomatic neuropathy, patients should be monitored carefully because of the increased morbidity and mortality related to the significant proportion of malignant nerve-sheath tumors in these patients with NF1.

Stable inheritance of an 85-kb triplication in C57BL/WldS mice

The slow Wallerian degeneration mouse, C57BL/Wld(S), carries a dominant mutation that delays Wallerian degeneration in the distal stump of an injured axon. A highly unusual mutation, an 85-kb tandem triplication in the Wld(S) mouse was identified. Since two duplication cases have been identified before, pulsed field gel electrophoresis (PFGE) can be used to look for the instability of triplication at the chromosomal level. One hundred and eighty chromosomes of Wld(S) from three divergent breeding colonies have been examined and all found to carry the triplication. Thus, the triplication mutation is stable during both mitosis and meiosis, and the previously observed duplication is likely to have been surviving alleles of the original mutation rather than a partial reversion. The triplication has now been shown to be the causative mutation, acting through an Ube4b/Nmnat chimeric gene, indicating the possibility of Wld(S) preventing axon degeneration in diverse pathologies and altering the symptoms. The fact that triplication is stable rules out instability as a source of phenotypic variation. Thus, this result is essential for accurate interpretation of studies the effect of Wld(S) on neurodegenerative phenotypes.

Molecular pathogenesis of hereditary motor, sensory and autonomic neuropathies

The hereditary motor, sensory and autonomic neuropathies are a heterogeneous group of neurological diseases. The classification of such is presently in a state of change. The original classification system was based on clinical findings whose limitations are being unfurled with increasing insights into the molecular basis of these disorders. In particular, much progress has been achieved in understanding the demyelinating forms of Charcot-Marie-Tooth (type 1), for which at least a dozen loci have been delineated and six genes identified. As anticipated, these genes play predominant roles in myelin biology. Four separate loci for the axonal Charcot-Marie-Tooth neuropathies (type 2) have been identified and only now are researchers beginning to tease out the responsible genes and the underlying molecular mechanisms. Similarly, progress is being made with the pure hereditary motor neuropathies. This review presents an updated list of genes responsible for inherited peripheral neuropathies and explores the underlying molecular mechanisms actively being investigated.

Charcot-Marie-Tooth disease and related inherited neuropathies

Charcot-Marie-Tooth disease (CMT) was initially described more than 100 years ago by Charcot, Marie, and Tooth. It was only recently, however, that molecular genetic studies of CMT have uncovered the underlying causes of most forms of the diseases. Most cases of CMT1 are associated with a 1.5-Mb tandem duplication in 17p11.2-p12 that encompasses the PMP22 gene. Although many genes may exist in this large duplicated region, PMP22 appears to be the major dosage-sensitive gene. CMT1A is the first autosomal dominant disease associated with a gene dosage effect due to an inherited DNA rearrangement. There is no mutant gene, but instead the disease phenotype results from having 3 copies of a normal gene. Furthermore, these findings suggest that therapeutic intervention in CMT1A duplication patients may be possible by normalizing the amount of PMP22 mRNA levels. Alternatively, CMT1A can be caused by mutations in the PMP22 gene. Other forms of CMT are associated with mutations in the MPZ (CMT1B) and Cx32 (CMTX) genes. Thus, mutations in different genes can cause similar CMT phenotypes. The related but more severe neuropathy, Dejerine-Sottas syndrome (DSS), can also be caused by mutations in the PMP22 and MPZ genes. All 3 genes thus far identified by CMT researchers appear to play an important role in the myelin formation or maintenance of peripheral nerves. CMT1A, CMT1B, CMTX, hereditary neuropathy with liability to pressure palsies (HNPP), and DSS have been called myelin disorders or "myelino-pathies." Other demyelinating forms, CMT1C and CMT-AR, may be caused by mutations of not yet identified myelin genes expressed in Schwann cells. The clinically distinct disease HNPP is caused by a 1.5-Mb deletion in 17p11.2-p12, which spans the same region duplicated in most CMT1A patients. Underexpression of the PMP22 gene causes HNPP just as overexpression of PMP22 causes CMT1A. Thus, 2 different phenotypes can be caused by dosage variations of the same gene. It is apparent that the CMT1A duplication and HNPP deletion are the reciprocal products of a recombination event during meiosis mediated through the CMT1A-REPs. CMT1A and HNPP could be thought of as a "genomic disease" more than single gene disorders. Other genetic disorders may also prove to arise from recombination events mediated by specific chromosomal structural features of the human genome (102). Further studies on the recombination mechanism of CMT and HNPP might reveal the causes of site specific homologous recombination in the human genome. The discovery of the PMP22 gene in the 1.5-Mb CMT1A duplication/HNPP deletion critical region also suggests that the clinical phenotype of chromosome aneuploid syndromes may result from the effect of a small subset of dosage-sensitive genes mapping within the region of aneuploidy. The understanding of the molecular basis of CMT1 and related disorders has allowed accurate DNA diagnosis and genetic counseling of inherited peripheral neuropathies and will make it possible to develop rational strategies for therapy. As several loci for CMT2 have been identified, the genes responsible for CMT2 will most likely be disclosed using positional cloning and candidate gene approaches in the near future.

A recombination hotspot responsible for two inherited peripheral neuropathies is located near a mariner transposon-like element

The Charcot-Marie Tooth disease type 1A (CMT1A) duplication and hereditary neuropathy with liability to pressure palsies (HNPP) deletion are reciprocal products of an unequal crossing-over event between misaligned flanking CMT1A-REP repeats. The molecular aetiology of this apparently homologous recombination event was examined by sequencing the crossover region. Through the detection of novel junction fragments from the recombinant CMT1A-REPs in both CMT1A and HNPP patients, a 1.7-kb recombination hotspot within the approximately 30-kb CMT1A-REPs was identified. This hotspot is 98% identical between CMT1A-REPs indicating that sequence identity is not likely the sole factor involved in promoting crossover events. Sequence analysis revealed a mariner transposon-like element (MITE) near the hotspot which we hypothesize could mediate strand exchange events via cleavage by a transposase at or near the 3' end of the element.

Charcot-Marie-Tooth disease: a new paradigm for the mechanism of inherited disease

Recent work has identified the genes and mutational mechanisms that underlie several inherited diseases of the peripheral nervous system and has provided both the first genetic rationale for classification of these disorders and an insight into their biological basis. These studies have yielded some surprising findings, including the discovery that two very different mutational mechanisms (duplication and point mutation) can result in a similar clinical phenotype in Charcot-Marie-Tooth disease type 1A, and that mutations involving the same gene can give rise to different clinical phenotypes.

Molecular basis of Charcot-Marie-Tooth disease type 1A: gene dosage as a novel mechanism for a common autosomal dominant condition

Charcot-Marie-Tooth disease (CMT) comprises a clinically and genetically heterogeneous group of polyneuropathies. Two major types can be distinguished based on electrophysiologic phenotypes: CMT type 1 (CMT1) displays uniformly decreased nerve conduction velocity associated with a demyelinating hypertrophic neuropathy, and CMT type 2 (CMT2) displays normal or near-normal nerve conduction velocity associated with a neuronal defect. Charcot-Marie-Tooth disease type 1A (CMT1A) is the most common form, exhibiting autosomal dominant inheritance and linkage to chromosome 17p11.2p12. This review will focus on the underlying molecular mechanisms leading to CMT1A. DNA duplication of a 1.5-Mb region is associated with CMT1A in the majority of cases. A defined segmental DNA duplication that cosegregates with a disease in a dominant Mendelian pattern had been unprecedented. A candidate gene for CMT1A, PMP22, which maps within the duplication and encodes a myelin-specific protein, was identified from studies on the trembler and tremblerJ mouse models for CMT. Point mutations in PMP22 have since been identified in cases of familial, non-duplication CMT1A. The genetic data presents two alternative molecular mechanisms involving the PMP22 gene that result in the same clinical and electrophysiologic phenotype of CMT1A. The impact of the underlying molecular mechanisms on the prospects for therapeutic development are discussed.

Charcot-Marie-Tooth disease type 1A. Association with a spontaneous point mutation in the PMP22 gene

BACKGROUND

Charcot-Marie-Tooth disease (CMT) is the most common inherited peripheral neuropathy. CMT type 1A is associated with a 1.5-megabase DNA duplication in region p11.2-p12 of chromosome 17 in most patients. An increased dosage of a gene within the duplicated segment appears to cause the disease. The PMP22 gene, which encodes a myelin protein, has been mapped within the duplication and proposed as a candidate gene for CMT type 1A.

METHODS

We analyzed DNA samples from a cohort of 32 unrelated patients with CMT type 1 who did not have the 1.5-Mb tandem duplication in 17p11.2-p12 for mutations within the PMP22 coding region. Molecular techniques included the polymerase chain reaction (PCR), heteroduplex analysis to detect point mutations, and direct nucleotide-sequence determination of amplified PCR products.

RESULTS

A 10-year-old boy was identified with a point mutation in PMP22, which resulted in the substitution of cysteine for serine in a putative transmembrane domain of PMP22. Analysis of family members revealed that the PMP22 point mutation arose spontaneously and segregated with the CMT type 1 phenotype in an autosomal dominant pattern. The patients with the PMP22 point mutation had clinical and electrophysiologic phenotypes that were similar to those of patients with the 1.5-Mb duplication.

CONCLUSIONS

The PMP22 gene has a causative role in CMT type 1. Either a point mutation in PMP22 or a duplication of the region including the PMP22 gene can result in the disease phenotype.

Charcot-Marie-Tooth disease type 1A: mutational mechanisms and candidate gene

Charcot-Marie-Tooth disease type 1A, the most common inherited peripheral neuropathy, is associated with a submicroscopic DNA duplication of 1.5 Mb that can arise de novo, and which is flanked by a > 17 kb mosaic repeat. The PMP22 gene, encoding a peripheral myelin protein, maps within the duplication. In a subset of Charcot-Marie-Tooth patients, point mutations can occur within the gene. Thus, the alternative mechanisms of overexpression of PMP22 and structural alterations in the protein encoded can cause the disease phenotype.

Charcot-Marie-Tooth type 1A duplication appears to arise from recombination at repeat sequences flanking the 1.5 Mb monomer unit

We have constructed a 3.1 megabase (Mb) physical map of chromosome 17p11.2-p12, which contains a submicroscopic duplication in patients with Charcot-Marie-Tooth disease type 1A (CMT1A). We find that the CMT1A duplication is a tandem repeat of 1.5 Mb of DNA. A YAC contig encompassing the CMT1A duplication and spanning the endpoints was also developed. Several low copy repeats in 17p11.2-p12 were identified including the large (> 17 kb) CMT1A-REP unit which may be part of a mosaic repeat. CMT1A-REP flanks the 1.5 Mb CMT1A monomer unit on normal chromosome 17 and is present in an additional copy on the CMT1A duplicated chromosome. We propose that the de novo CMT1A duplication arises from unequal crossing over due to misalignment at these CMT1A-REP repeat sequences during meiosis.

Molecular genetics and neuropathology of Charcot-Marie-Tooth disease type 1A

Charcot-Marie-Tooth (CMT) syndrome describes a genetically and clinically heterogeneous group of polyneuropathies. Electrophysiologically, at least two types of CMT can be distinguished; CMT1 which has decreased nerve conduction velocities (NCV) and CMT2 which has normal or near normal NCV with decreased amplitudes. For CMT1, three gene loci (on chromosomes 1, 17 and the X chromosome) have been mapped. The locus on chromosome 17, CMT type 1A (CMT1A), is responsible for the most common form of CMT which has recently been shown to be associated with a large DNA duplication. Recent data demonstrates that the CMT1A phenotype results from an inherited DNA rearrangement and a gene dosage effect. The trembler (Tr) and allelic tremblerJ (TrJ) mice have been proposed as animal models for CMT. Tr has similar electrophysiological and neuropathological features to CMT1 patients and maps to mouse chromosome 11 in a region of conserved synteny with human chromosome 17p. Tr and TrJ have recently been shown to have different point mutations in regions encoding putative transmembrane domains of the myelin specific protein PMP-22. The human peripheral nerve-specific PMP-22 gene maps within the CMT1A duplication. PMP-22 is thus a candidate gene for CMT1A. This paper describes the molecular genetics of CMT1A and sural nerve pathology in CMT1A patients with the CMT1A duplication.