Widespread expression of the peripheral myelin protein-22 gene (PMP22) in neural and non-neural tissues during murine development.
The gene encoding the peripheral myelin protein PMP22 is affected by various mutations in the hereditary peripheral neuropathies Charcot-Marie-Tooth disease type 1A (CMT1A), Déjérine-Sottas syndrome (DSS) and hereditary neuropathy with liability to pressure palsies (HNPP). In contrast to the recent remarkable progress in the genetics of the PMP22 gene, the biological function of PMP22 remains largely unknown. In this report, we have confirmed by using in situ hybridization techniques that high levels of PMP22 mRNA are present in maturing peripheral nerves of the 2-week-old mouse, a finding consistent with the PNS-specific defect observed in hereditary peripheral neuropathies. However, high levels of PMP22 transcripts were also found in the villi of the adult gut, and PMP22 expression was detected in various non-neural tissues during embryonic mouse development. In early embryogenesis (9.5 days postconception, dpc), PMP22 RNA expression appears restricted to the epithelial ectodermal layer. During early organogenesis (11.5 dpc), particularly high levels of expression are present in the capsule surrounding the liver and in the forming gut, while low levels of PMP22 mRNA can be found in precartilagous condensations forming the vertebrae and the ventricular layer of the myelencephalon. During midgestation development (14.5 dpc to 16.5 dpc), the number of PMP22-positive tissues increases, and high expression is detected in several mesoderm-derived tissues, in particular connective tissues of the face region, bones including the vertebrae, the lung mesenchym, and in muscles. In addition, high expression is also found in ectoderm-derived tissues, especially the epithelia of the lens and the skin. These findings strongly suggest that PMP22 serves not only a PNS-specific function but is also of broader biological significance in cell proliferation and/or differentiation.