Pathology and molecular genetics of inherited neuropathy

Onion-bulb patterns predict acquired or inherited demyelinating polyneuropathy

INTRODUCTION

Onion-bulbs (OB) are concentrically layered Schwann-cell processes, surrounding nerve fibers, occurring in both inherited and acquired demyelinating polyneuropathies. We investigated whether OB patterns (generalized, mixed, or focal) correlate with acquired or inherited neuropathies.

METHODS

One hundred thirty-one OB-rich nerve biopsies were graded for OB pattern and inflammation without knowledge of clinical history. We classified inherited (n = 49) or acquired (n = 82) neuropathies based solely on clinical history.

RESULTS

Fifty-one biopsies had generalized (34 inherited vs. 17 acquired, P < 0.001), 54 mixed (48 acquired vs. 6 inherited, P < 0.001), and 26 focal/multifocal (inherited [n = 9], acquired [n = 17]) OB. Inflammation occurred more frequently in acquired (n = 54) than inherited (n = 14) neuropathy (P = 0.004).

DISCUSSION

Generalized OB correlates with inherited neuropathy; mixed OB with acquired. Inflammation occurs more in acquired neuropathy cases. OB patterns are best explained by ubiquitous Schwann-cell involvement in inherited and multifocal Schwann-cell involvement in acquired neuropathies and predict the electrophysiology of uniform demyelination in inherited and unequal demyelination in acquired neuropathies. Muscle Nerve 59:665-670, 2019.

Marked phenotypic variation in a family with a new myelin protein zero mutation

Myelin protein zero (MPZ) is a member of the immunoglobulin gene superfamily, which has a role in myelin compaction. MPZ gene mutations cause mostly demyelinating neuropathies of the Charcot-Marie-Tooth 1B type (CMT1B), but axonal CMT have been described as well. There is a broad spectrum of phenotypic manifestation of neuropathies caused by MPZ mutations. Some mutations of MPZ cause severe early-onset neuropathies such as Dejerine-Sottas disease, while others cause the classical CMT phenotype with normal early milestones but development of disability during the first two decades of life. We describe a family in which five members of three consecutive generations had a heterozygous mutation in nucleotide position 143 with a T-C transition in exon 2 of the MPZ gene. The resulting substitution of Leu48 with proline has not been previously described. The age of onset of symptoms varied from 8 months to 41 years. The marked variation of the age of disease onset and clinical phenotype in this one family, related to the same MPZ mutation, suggests that in addition to the type and intragenic location of the mutation, other putative modifying gene(s) are regulating MPZ gene expression, mRNA stability and posttranslational protein modification and may have an important effect on the ultimate clinical phenotype.