Frequency of mutations in the early growth response 2 gene associated with peripheral demyelinating neuropathies.

Hereditary motor and sensory neuropathies (HMSN) comprise a wide clinical spectrum of related disorders with defects in peripheral nerve myelination. More than 250 distinct mutations of the peripheral myelin protein 22 (PMP22), myelin protein zero (MPZ/P0), and connexin 32 (Cx32/GJB1) genes have been reported in patients diagnosed with different forms of hereditary motor and sensory neuropathies, such as Charcot-Marie-Tooth disease (CMT), Dejerine-Sottas syndrome (DSS), congenital hypomyelination (CH), and hereditary neuropathy with liability to pressure palsies (HNPP). To date, some 20 linked CMT loci and nine genes have been identified. Gene products include structural proteins (PMP22, MPZ), a gap junction protein (Cx32), and a transcription factor (EGR2). Recently, point mutations in the early growth response 2 (EGR2) gene, located on human chromosome 10q21.1-q22.1, have been associated with hereditary neuropathies. The gene for EGR2 spans 4.3 kb, contains two coding exons, and is part of a multigene family encoding Cys2His2 type zinc finger proteins and may play a role in the regulation of cellular proliferation. EGR2 is a Schwann cell transcription factor that binds DNA through three zinc finger domains and is thought to regulate the expression of late myelin genes such as MPZ and myelin basic protein, thus playing a key role in myelogenesis. Human EGR2 is homologous to the mouse Krox20 gene, which is part of a transcriptional cascade involved in the development and segmentation of the hindbrain. Homozygous mice knocked out for Krox20 display abnormal rhombomere segmentation and neuronal migration in the developing hindbrain, resulting in anatomical abnormalities of the cranial nerves. Furthermore, mice homozygous for a targeted mutation deleting a major part of the gene including the entire zinc finger domain die shortly after birth. Surviving Krox20 mice show a trembling phenotype, have hypomyelination of the peripheral nervous system, and their Schwann cells are blocked at an early stage of differentiation. Heterozygous Krox20 mice have no functional impairment or neuropathological abnormalities. To date, eight different EGR2 mutations have been found in patients with clinical features of CH, CMT1, or DSS. We examined a collection of 101 patients, some with well established diagnostic criteria, and we also included a group with little or no clinical information. Our aim was to document the frequency of EGR2 mutations in the CMT population. We report a heterozygous de novo missense mutation in the EGR2 gene in a patient diagnosed with CH associated with cranial nerve deficits. This mutation was previously presented with preliminary data in abstract format and is now documented in detail. Furthermore, we identified three different, probably silent exonic polymorphisms in the EGR2 transcription factor. MATERIALS AND METHODS Selection and phenotypical classification of the patients One hundred and one unrelated probands presenting with peripheral neuropathy and without PMP22, MPZ, or GJB1 mutations were studied from the laboratory collections of Lyons (51 patients) and Cardiff (50 patients). They all presented with at least weakness and progressive wasting of the distal limb muscles, pes cavus, and absence of deep tendon reflexes. The criteria for the classification of different conditions included: • Congenital hypomyelination: hypotonia, delayed motor milestones, biopsy showing absence of myelination, NCV less than 10 m/s. • Dejerine-Sottas syndrome: isolated CMT cases compatible with autosomal recessive transmission, NCV less than 10 m/s, onset in childhood. Biopsy, when available, must show demyelination. • CMT1: NCV between 15 and 35 m/s associated with typical CMT phenotype. • Unspecified phenotype: with the presence of only one bit of clinical information, either NCV or transmission mode.