Variability of Presentation in Hereditary Neuropathy with Liability to Pressure Palsy Results in Underrecognition

New evidence for secondary axonal degeneration in demyelinating neuropathies

Development of peripheral nervous system (PNS) myelin involves a coordinated series of events between growing axons and the Schwann cell (SC) progenitors that will eventually ensheath them. Myelin sheaths have evolved out of necessity to maintain rapid impulse propagation while accounting for body space constraints. However, myelinating SCs perform additional critical functions that are required to preserve axonal integrity including mitigating energy consumption by establishing the nodal architecture, regulating axon caliber by organizing axonal cytoskeleton networks, providing trophic and potentially metabolic support, possibly supplying genetic translation materials and protecting axons from toxic insults. The intermediate steps between the loss of these functions and the initiation of axon degeneration are unknown but the importance of these processes provides insightful clues. Prevalent demyelinating diseases of the PNS include the inherited neuropathies Charcot-Marie-Tooth Disease, Type 1 (CMT1) and Hereditary Neuropathy with Liability to Pressure Palsies (HNPP) and the inflammatory diseases Acute Inflammatory Demyelinating Polyneuropathy (AIDP) and Chronic Inflammatory Demyelinating Polyneuropathy (CIDP). Secondary axon degeneration is a common feature of demyelinating neuropathies and this process is often correlated with clinical deficits and long-lasting disability in patients. There is abundant electrophysiological and histological evidence for secondary axon degeneration in patients and rodent models of PNS demyelinating diseases. Fully understanding the involvement of secondary axon degeneration in these diseases is essential for expanding our knowledge of disease pathogenesis and prognosis, which will be essential for developing novel therapeutic strategies.

Exome Sequence Analysis Suggests that Genetic Burden Contributes to Phenotypic Variability and Complex Neuropathy

Charcot-Marie-Tooth (CMT) disease is a clinically and genetically heterogeneous distal symmetric polyneuropathy. Whole-exome sequencing (WES) of 40 individuals from 37 unrelated families with CMT-like peripheral neuropathy refractory to molecular diagnosis identified apparent causal mutations in ∼ 45% (17/37) of families. Three candidate disease genes are proposed, supported by a combination of genetic and in vivo studies. Aggregate analysis of mutation data revealed a significantly increased number of rare variants across 58 neuropathy-associated genes in subjects versus controls, confirmed in a second ethnically discrete neuropathy cohort, suggesting that mutation burden potentially contributes to phenotypic variability. Neuropathy genes shown to have highly penetrant Mendelizing variants (HPMVs) and implicated by burden in families were shown to interact genetically in a zebrafish assay exacerbating the phenotype established by the suppression of single genes. Our findings suggest that the combinatorial effect of rare variants contributes to disease burden and variable expressivity.

Sonography of the median nerve in CMT1A, CMT2A, CMTX, and HNPP

INTRODUCTION

In this study we compare the ultrasound features in the median nerve in patients with different types of Charcot-Marie-Tooth (CMT) disease and hereditary neuropathies with liability to pressure palsies (HNPP) as a typical entrapment neuropathy.

METHODS

Median nerve ultrasound and conduction studies were performed in patients with CMT1A (n = 12), MFN2-associated CMT2A (n = 7), CMTX (n = 5), and HNPP (n = 5), and in controls (n = 28).

RESULTS

Median nerve cross-sectional area (CSA) was significantly increased in CMT1A, whereas, in axonal CMT2A, fascicle diameter (FD) was enlarged. CSA correlated with nerve conduction slowing in CMT1A and with axonal loss, as shown by motor and sensory nerve amplitudes in both CMT1A and CMT2A. A relatively low wrist-to-forearm-ratio (WFR <0.8) or a relatively high WFR (>1.8) appeared to be unlikely in MFN2 and Cx32 mutations of CMT2A and CMTX, respectively.

CONCLUSION

Differences in CSA, FD, and WFR of the median nerve can be helpful in defining subtypes of hereditary neuropathies.

Compound heterozygous deletions of PMP22 causing severe Charcot-Marie-Tooth disease of the Dejerine-Sottas disease phenotype

Dejerine-Sottas disease (DSD) is a particular phenotype of the Charcot-Marie-Tooth (CMT) disease spectrum that is genetically heterogeneous. It represents a severe form of hypertrophic axonal and demyelinating neuropathy. Although it is predominantly inherited as an autosomal recessive condition, autosomal dominant inheritance has also been described. To date, the autosomal recessive forms of DSD are classified into several CMT type 4 (CMT4) subclasses based on allelic heterogeneity. We present a 7-year-old boy with a severe form of CMT disease consistent with the autosomal recessive phenotype of DSD. He was found to be a compound heterozygote for mutations in the PMP22 gene resulting in homozygous deletion of exons 2 and 3. The maternally inherited allele was the typical 1.5 Mb deletion involving PMP22 seen with hereditary neuropathy with liability to pressure palsy (HNPP). The paternally inherited allele was a deletion of exons 2 and 3. Both parents presented with a typical clinical picture of HNPP. To our knowledge, this is the first patient reported with large deletions involving both PMP22 alleles. Our patient has also developed severe gastroesophageal reflux disease (GERD), a clinical feature not previously reported with CMT or DSD. The correlation of the phenotype and the molecular defects observed in this patient may set a new subcategory in the classification of DSD.

Germline rates of de novo meiotic deletions and duplications causing several genomic disorders

Meiotic recombination between highly similar duplicated sequences (nonallelic homologous recombination, NAHR) generates deletions, duplications, inversions and translocations, and it is responsible for genetic diseases known as 'genomic disorders', most of which are caused by altered copy number of dosage-sensitive genes. NAHR hot spots have been identified within some duplicated sequences. We have developed sperm-based assays to measure the de novo rate of reciprocal deletions and duplications at four NAHR hot spots. We used these assays to dissect the relative rates of NAHR between different pairs of duplicated sequences. We show that (i) these NAHR hot spots are specific to meiosis, (ii) deletions are generated at a higher rate than their reciprocal duplications in the male germline and (iii) some of these genomic disorders are likely to have been underascertained clinically, most notably that resulting from the duplication of 7q11, the reciprocal of the deletion causing Williams-Beuren syndrome.

[Hereditary neuropathy with liability to pressure palsies (HNPP) in hand surgery: reminds and warn against a usually unrecognised disease]

Tomacula is a rare hereditary disease due to a deletion on chromosome 17. Clinical presentation varies but patients usually complain of recurrent paraesthesiaes and palsies related to compression or trauma of a peripheral nerve. Diagnosis is based on electrophysiological studies, nerve biopsies and genetic tests. Implications for the patient and family members are a genetic counselling and some simple preventive measures. Although there is no curative treatment for this neuropathy, surgery can be useful for decompression of nerves and neurolysis. However, the surgical act increases the risk of nerve damage. Knowing about the diagnosis can help the patient and the surgical team avoid causing lesions.

Neuropathic scapuloperoneal syndrome (Davidenkow's syndrome) with chromosome 17p11.2 deletion

The nosologic boundary of neuropathic scapuloperoneal syndrome (Davidenkow's syndrome) remains ill defined and its genetic basis is unknown. A case of Davidenkow's syndrome with the monochromosomic 17p11.2 deletion that often is associated with hereditary neuropathy with liability to pressure palsies (HNPP) is described. The other allele at chromosome 17p11.2 locus was of normal length, and direct sequencing of the coding region of the peripheral nerve protein-22 gene in this allele revealed no additional mutation. The deleted allele in the proband was inherited from the paternal line in which the affected members had a late onset Charcot-Marie-Tooth type 1 clinical phenotype. This observation suggests that the rare Davidenkow's syndrome is clinically related to HNPP and its genotype could be a chromosome 17p11.2 deletion.

The prevalence of hereditary neuropathy with liability to pressure palsies in patients with multiple surgically treated entrapment neuropathies

PURPOSE

Hereditary neuropathy with liability to pressure palsies (HNPP) is an autosomal-dominant peripheral neuropathy that results from deletion of a 1.5-Megabase pair (Mb) segment of the short arm (p) of chromosome 17. Hereditary neuropathy with liability to pressure palsies increases susceptibility of peripheral nerves to pressure and trauma and can be associated with symptoms at multiple anatomic entrapment sites. Many patients present with multiple upper-extremity entrapment neuropathies and the etiology is uncertain. We hypothesized that some of these patients have an underlying hereditary neuropathy. The purpose of this study was to determine the prevalence of HNPP in patients with multiple surgically treated upper-extremity entrapment neuropathies.

METHODS

The inclusion criterion for the study was history of more than 1 carpal tunnel release and/or ulnar nerve transposition. The exclusion criteria were history of diabetes or history of Charcot-Marie-Tooth neuropathy. Fifty-nine patients were in the study group. Two patients known to have the 17p11.2 deletion were used as controls. Genomic DNA was extracted from peripheral blood. Each sample was genotyped using polymerase chain reaction (PCR) amplification with short tandem repeat polymorphism markers within the 1.5-Mb region of 17p deleted in HNPP. Markers were scored as homozygous or heterozygous after resolution by polyacrylamide gel electrophoresis and silver staining.

RESULTS

The 2 control patients were homozygous for 11 markers. None of the 59 study patients were homozygous for all markers tested in the deleted region. No study patient had the 17p deletion diagnostic for HNPP. Based on the sample size of 59 patients the 95% confidence interval for the prevalence of the 17p11.2 deletion in this population is 0% to 5%.

CONCLUSIONS

We found no evidence for an association between HNPP and patients who have multiple surgical releases for upper-extremity entrapment neuropathies.

Molecular diagnosis of inheritable neuromuscular disorders. Part II: Application of genetic testing in neuromuscular disease

Molecular genetic advances have led to refinements in the classification of inherited neuromuscular disease, and to methods of molecular testing useful for diagnosis and management of selected patients. Testing should be performed as targeted studies, sometimes sequentially, but not as wasteful panels of multiple genetic tests performed simultaneously. Accurate diagnosis through molecular testing is available for the vast majority of patients with inherited neuropathies, resulting from mutations in three genes (PMP22, MPZ, and GJB1); the most common types of muscular dystrophies (Duchenne and Becker, facioscapulohumeral, and myotonic dystrophies); the inherited motor neuron disorders (spinal muscular atrophy, Kennedy's disease, and SOD1 related amyotrophic lateral sclerosis); and many other neuromuscular disorders. The role of potential multiple genetic influences on the development of acquired neuromuscular diseases is an increasingly active area of research.

Loss-of-function phenotype of hereditary neuropathy with liability to pressure palsies

Hereditary neuropathy with liability to pressure palsies (HNPP) provides a human model to investigate the role of PMP22 in myelinated peripheral nerve, since the disease is caused by a deletion of one of the two PMP22 alleles. To systematically characterize the phenotype of HNPP, we prospectively evaluated the clinical features and electrophysiological findings in 17 genetically confirmed patients, 7 men and 10 women, ranging in age from 9 to 66 years (mean, 41 +/- 13). Fifteen symptomatic patients presented with episodes of transient focal weakness or sensory loss that were usually related to particular activities causing nerve compression, including stretching or minor repetitive focal trauma. No patient sought medical attention for symptoms of a symmetric polyneuropathy. Neurological examinations were either normal or mildly abnormal. Neither focal slowing of nerve conduction studies, nor reduction in compound muscle action potential (CMAP) or sensory nerve action potential (SNAP) amplitudes consistently predicted the site of symptoms. We conclude that the majority of patients with HNPP present with transient, recurrent, focal symptoms of weakness or sensory loss in the distribution of individual nerves or plexus, and that a diffuse symmetric sensorimotor polyneuropathy is an unusual presentation of HNPP. These studies suggest that the function of PMP22, at least in part, is to stabilize myelin so that it will be protected from injuries resulting from repetitive, minor trauma.

Hereditary neuropathy with liability to pressure palsies in infancy

Hereditary neuropathy with liability to pressure palsies is an autosomal-dominant disorder, classically characterized by recurrent mononeuropathies, associated with a deletion at 17p11.2, encompassing the peripheral myelin protein 22 gene. The typical clinical episodes of pressure palsy are usually noted for the first time during the 2nd or 3rd decade of life. We found only few reports in prepubertal children. We report a case of a 7.5-year-old child with muscle weakness and severe hypotonia associated with developmental gross motor delay. We suspect that bilateral peroneal nerve palsies after birth were the first episode of pressure palsy. Nerve conduction studies demonstrated slightly prolonged distal latencies with normal conduction velocity. Typical features of hereditary neuropathy with liability to pressure palsies with recurrent mononeuropathies were found in the father. DNA analysis revealed 1.5-Mb deletion at 17p11.2 in both father and child. To the best of our knowledge, this patient is one of the youngest ever found with this disease.