Hereditary motor and sensory neuropathies. Present status of types I, II and III

PMP22 related neuropathies: Charcot-Marie-Tooth disease type 1A and Hereditary Neuropathy with liability to Pressure Palsies

PMP22 related neuropathies comprise (1) PMP22 duplications leading to Charcot-Marie-Tooth disease type 1A (CMT1A), (2) PMP22 deletions, leading to Hereditary Neuropathy with liability to Pressure Palsies (HNPP), and (3) PMP22 point mutations, causing both phenotypes. Overall prevalence of CMT is usually reported as 1:2,500, epidemiological studies show that 20-64% of CMT patients carry the PMP22 duplication. The prevalence of HNPP is not well known. CMT1A usually presents in the first two decades with difficulty walking or running. Distal symmetrical muscle weakness and wasting and sensory loss is present, legs more frequently and more severely affected than arms. HNPP typically leads to episodic, painless, recurrent, focal motor and sensory peripheral neuropathy, preceded by minor compression on the affected nerve. Electrophysiological evaluation is needed to determine whether the polyneuropathy is demyelinating. Sonography of the nerves can be useful. Diagnosis is confirmed by finding respectively a PMP22 duplication, deletion or point mutation. Differential diagnosis includes other inherited neuropathies, and acquired polyneuropathies. The mode of inheritance is autosomal dominant and de novo mutations occur. Offspring of patients have a chance of 50% to inherit the mutation from their affected parent. Prenatal testing is possible; requests for prenatal testing are not common. Treatment is currently symptomatic and may include management by a rehabilitation physician, physiotherapist, occupational therapist and orthopaedic surgeon. Adult CMT1A patients show slow clinical progression of disease, which seems to reflect a process of normal ageing. Life expectancy is normal.

Hand strength and fatigue in patients with hereditary motor and sensory neuropathy (types I and II)

OBJECTIVES

To compare maximal isometric hand strength and fatigue between subjects with hereditary motor and sensory neuropathy (HMSN) and healthy controls and to test the reproducibility of handgrip strength (peak force of handgrip [PFgrip]) and fatigue.

DESIGN

PFgrip and the decline in PFgrip during 3 sets of 15 contractions were compared.

SETTING

University hospital in The Netherlands.

PARTICIPANTS

Twenty subjects with HMSN and 20 age- and sex-matched healthy controls; 15 healthy subjects for the reproducibility part of the study.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

PFgrip and the decline in PFgrip were compared by using a digital handgrip dynamometer. Two-point and lateral pinch measurements of subjects with HMSN were standardized against reference values. Reproducibility measurements were performed on 15 healthy subjects on 2 separate occasions within a 1-week interval.

RESULTS

PFgrip was significantly lower in the HMSN subjects compared with controls (P<.05). Pinch measurements also showed a large variance from average normal performance. No significant difference was found in the decline in percentage of PFgrip. Reproducibility was excellent for PFgrip (intraclass correlation coefficient [ICC]=.98; 95% confidence interval [CI],.95-.99) but poor for fatigue (ICC=.62; 95% CI,.20-.85).

CONCLUSION

PFgrip and 2-point and lateral pinch in HMSN subjects were significantly reduced compared with healthy controls. Our findings indicated that the rate of decline of PFgrip during effort does not vary between groups.

Molecular genetic analysis of Charcot-Marie-Tooth 1A duplication in Norwegian patients by quantitative photostimulated luminescence imaging

Around 70% of Charcot-Marie-Tooth 1 (CMT1) cases are caused by a dominantly inherited 1.5-Mb duplication at 17p11.2-12 (CMT1A). Using photostimulated luminescence (PSL) imaging of MspI Southern blots, hybridization signals of the probe pVAW409R3a in relation to cohybridized probe SF85a, were densitometrically quantified and an RFLP allele-band ratio determined. A total of 55 Norwegian CMT patients and 16 asymptomatic family members from 26 separate families, clinically and neurophysiologically classified as CMT1 (n=46) and CMT2 (n=9), were studied. Thirty-two of 46 CMT1 cases (69.6%), all heterozygous but one homozygous for the pVAW409R3a MspI polymorphism, from 12 of 21 families (57.1%) were positive for the CMT1A duplication. In autosomal dominant familial cases (n=30), 26 of 30 cases (86.7%), all heterozygous, from six of seven families (85.7%) were positive for duplication. None of the CMT2 patients, asymptomatic family members or healthy controls were positive for duplication. The CMT1A frequency of duplication in Norwegian CMT1 patients is in general agreement with those reported in other European countries and the present results show that quantitative densitometric PSL imaging is a highly reliable test in diagnosing CMT1A duplication.

A locus for an axonal form of autosomal recessive Charcot-Marie-Tooth disease maps to chromosome 1q21.2-q21.3

Charcot-Marie-Tooth disease (CMT) is a heterogeneous group of disorders that affect the peripheral nervous system. Three loci are known for the autosomal dominant forms of axonal CMT (CMT2), but none have yet been identified for autosomal recessive axonal CMT (ARCMT2). We have studied a large consanguineous Moroccan ARCMT2 family with nine affected sibs. The onset of CMT was in the 2d decade in all affected individuals who presented with a severe motor and sensory neuropathy, with proximal muscle involvement occurring in some patients. After exclusion of known loci for CMT2 and for demyelinating ARCMT2, a genomewide search was performed. Evidence for linkage was found with markers on chromosome 1q. The maximum pairwise LOD score was above the threshold value of 3.00, for markers D1S514, D1S2715, D1S2777, and D1S2721, and it reached 6.10 at the loci D1S2777, D1S2721, and D1S2624, according to multipoint LOD-score analysis. These markers defined a region of homozygosity that placed the gene in a 4.4-cM interval. Moreover, a recombination event detected in an unaffected 48-year-old individual excludes the D1S506 marker, thereby reducing the interval to 1.7 cM. In addition, the P0 gene, an attractive candidate because of both its location on chromosome 1q and its role in myelin structure, was excluded by physical mapping and direct sequencing.

Impaired intracellular trafficking is a common disease mechanism of PMP22 point mutations in peripheral neuropathies

The most common forms of hereditary motor and sensory neuropathies (HMSN) or Charcot-Marie-Tooth disease (CMT) are associated with mutations affecting myelin genes in the peripheral nervous system. A minor subgroup of CMT type 1A (CMT1A) is caused by point mutations in the gene encoding the peripheral myelin protein 22 (PMP22). To study the mechanisms by which these mutations cause the CMT pathology, we transiently transfected COS7 and Schwann cells with wild-type and PMP22 expression constructs carrying six representative dominant or de novo point mutations and one putative recessive point mutation. All but one of the first group of mutant PMP22 proteins failed to be incorporated into the plasma membrane and were retained in intracellular compartments of transfected cells. Surprisingly, the recessive PMP22 mutation produced a protein that was also mildly impaired in trafficking. Thus, our results suggest a common disease mechanism underlying the pathology of CMT1A due to PMP22 point mutations.

Many facets of the peripheral myelin protein PMP22 in myelination and disease

Peripheral myelin protein 22 (PMP22) is a small, hydrophobic glycoprotein, which is most prominently expressed by Schwann cells as a component of compact myelin of the peripheral nervous system (PNS). Recent progress in molecular genetics revealed that mutations affecting the PMP22 gene including duplications, deletions, and point mutations are responsible for the most common forms of hereditary peripheral neuropathies including Charcot-Marie-Tooth disease type 1A (CMT1A), hereditary neuropathy with liability to pressure palsies (HNPP), and a subtype of Dejerine-Sottas Syndrome (DSS). Functionally, PMP22 is involved in correct myelination during development of peripheral nerves, the stability of myelin, and the maintenance of axons. While most of these functions relate to a role of PMP22 as a structural component of myelin, PMP22 has also been proposed as a regulator of Schwann cell proliferation and differentiation. In this review, we will discuss our current knowledge of PMP22 and its related proteins in the normal organism as well as in disease. In particular, we will focus on how the function of PMP22 and its regulation may be relevant to particular disease mechanisms.

Decreased electrical excitability of peripheral nerves in demyelinating polyneuropathies

Not recognising the presence of decreased excitability may give rise to a seemingly low compound muscle action potential, which may lead erroneously to the conclusion of conduction block. To quantify decreased electrical excitability, stimulation-response curves and the current needed to achieve 90% of the maximal compound muscle action potential amplitude, i90, were obtained in 17 healthy controls, eight patients with Guillain-Barre syndrome, 14 with chronic inflammatory demyelinating polyneuropathy, and 10 with hereditary motor sensory neuropathy type I. Decreased electrical excitability was found in patients with chronic inflammatory demyelinating polyneuropathy and hereditary motor sensory neuropathy type I, by contrast with patients with Guillain-Barré syndrome. Recognising decreased excitability prevents the false assertion of conduction block and has electrodiagnostic importance for the differential diagnosis of demyelinating polyneuropathies.

Molecular basis of common hereditary motor and sensory neuropathies in humans and in mouse models

The Hereditary Motor and Sensory Neuropathies (HMSNs) are well known to be clinically, morphologically, and genetically heterogeneous. Yet, recent advances in the cellular and molecular biology of the peripheral nervous system coupled with remarkable progress in human and mouse genetics have provided a framework that has profoundly changed our understanding of the pathogenesis of these diseases. It now appears that most of the HMSNs are related to mutations affecting genes encoding Schwann cell proteins, specifically the Peripheral Myelin Protein PMP22, Myelin Protein Zero, and one of the gap junction proteins, connexin-32. Accordingly, these findings are discussed in the context of the clinical and pathologic features of the human HMSNs, but are interpreted in the context of basic research findings on the cellular and molecular biology of the peripheral nervous system derived from in vivo and in vitro studies in spontaneously-occurring and genetically engineered animal models for the HMSNs.

Amyloid and peripheral nervous system disease

The peripheral nervous system can be involved in the following amyloid deposition diseases. (1) Amyloid deposition composed of beta 2-microglobulin in patients on long term hemodialysis causing a carpal tunnel syndrome; (2) deposition of light chain immunoglobulin derived amyloid leading to polyneuropathy, carpal tunnel syndrome and autonomic nervous system involvement in patients with primary amyloidosis, or amyloidosis secondary to or associated with multiple myeloma, Waldenström's macroglobulinemia, non-Hodgkin's lymphoma, and solid neoplasms like hypernephroma; and (3) several types of heredofamilial amyloid polyneuropathies, which are mainly caused by a point-mutation in the transthyretin gene on chromosome 18. The clinical and biochemical features of these three groups will be discussed, with special attention for recently developed therapy. In clinical practice, amyloid polyneuropathy should be considered in case of familial occurrence of a polyneuropathy and when a patient presents with a polyneuropathy and a monoclonal gammopathy.