A transgenic mouse model of hereditary motor and sensory neuropathy

Abstract

Congenital Hypomyelination (CH) IS the most severe demyelinating form of Hereditary Motor and Sensory Neuropathies and manifests at birth in human. Some subtypes of CH are due to dominant mutations in the gene coding for PO glycoprotein, which functions as a homophilic adhesion protein, responsible for compaction of opposing myelin lamellae. By homologous recombination in ES cells, we have generated a mouse containing a nonsense mutation in the intracellular portion of PO (Q215X) that, in the heterozygous state, is associated with CH neuropathy in humans. This mutation is predicted to encode a truncated PO protein, lacking part of the cytoplasmic domain. Expression analysis demonstrated that Q215X heterozygous mice produce both wildtype PO and a smaller, truncated PO glycoprotein; furthermore, the levels of mRNA and protein produced by the mutated allele are less abundant, relative to the wildtype. We demonstrated then that this reduction in mRNA and protein levels could be paIiially explained by inefficient transcription of the mutated allele, due to the presence of a LoxP site with intron 5. Behavioral analysis of the Q215X1+ mice revealed reduced motor performance at 11 days after birth. Morphological analysis performed on sciatic nerves of mice between PI and P 14 revealed defects in the process of axonal sorting by Schwann cells, with the presence of bundles of mixed large and small calibre axons surrounded, but not ensheathed, by single Schwann cells. These morphogical defects are rescued after the second week of life: sciatic nerves from adult mice, in fact, show only mild hypomyelination, which is much less severe than the morphology reported in patients. From these preliminary data, we conclude that the Q215X mutation results in a truncated PO protein; since the phenotype of Q215X1+ mice and PO +1- mice differs, Q215X probably produces a gain of function. Finally, we studied in vivo the intracellular location of the truncated protein, in order to clarify aspects of the pathogenetic mechanism of the Q215X PO mutation: we found that the mutated protein is not properly trafficked within Schwann cells, being partially retained in the ER compartment. The phenotype we have observed in the mutant mice presents similarities to other hypomyelinating mice that carry mutations in different genes, that are all involved in the laminin pathway: this suggests that mutant PO may interfere with laminin signaling, required for the correct timing of axonal sorting by Schwann cells. Thus, from these data we conclude that the Q215X mouse is a partial model of Congenital Hypomyelination, less severely affected, if compared to human patients. This difference in disease severity could be partially explained by the inefficient transcription of the Q215X allele. Thus, we believe that the mechanism of this hypomyelination is likely to be related to the mechanism of the more severe neuropathy in human. This mouse will be useful to reveal the pathogenesis of the mutation.