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Hereditary demyelinating neuropathy of infancy: a genetically complex syndrome.

We read with interest the recent paper by Tyson et al. (1997). They report on nine patients with infantile hereditary demyelinating peripheral neuropathy. In two of them, nerve biopsy study revealed histological abnormalities characteristic of hereditary neuropathies with focally folded myelin sheaths (Ohnishi et al., 1989). The clinical picture they observed in these two individuals was extremely severe with involvement of both proximal and distal muscles of lower and upper limbs. The oldest patient aged 16 years was seriously handicapped and wheelchair-bound. Since both the patients’ parents were consanguineous, an autosomal recessive inheritance was presumed. The molecular study failed to demonstrate any of the mutations recently reported in hereditary demyelinating neuropathy (Raeymaekers et al., 1992; Nelis et al., 1994), namely duplication in chromosome 17p11.2–12, point mutations in the four exons of the PMP-22 and the six exons of the P0 myelin genes. Therefore, Tyson et al. (1997) argued that a mutation at another still unknown locus or loci seems possible. The observation of Tyson et al. (1997) further confirms that hereditary neuropathy with focally folded myelin sheaths, also called Charcot–Marie–Tooth (CMT) type 4B (CMT4B) (Thomas, 1994), represents a quite common disorder within the heterogeneous group of autosomal recessive demyelinating neuropathy of infancy. Of note, clinical and neuropathological findings of these two patients reported by Tyson et al. (1997) were strikingly similar to those of a large inbred pedigree with 10 patients reported by our group (Quattrone et al., 1996), as well as to most familial cases reported in the literature (for review, see Quattrone et al., 1996). Based on these findings, therefore, it seems that familial cases with CMT4B display a rather homogeneous clinical picture, characterized by infantile onset with progressive symmetrical distal and proximal muscular weakness, starting at the lower extremities and eventually involving the hands and forearms. A striking feature of all these familial cases is the non-random nature of age at onset, with the majority clustering around 2 years of age. Despite these phenotypic similarities, however, recent molecular studies have demonstrated that there are several