CELL-DAMAGE BY EXCESS CUZNSOD AND DOWNS-SYNDROME

Down's Syndrome (DS), the phenotypic expression of human trisomy 21, i s presumed to result from over-expression of certain genes residing on chromosome 21 at the segment 21q22-the Down locus. The ''housekeeping '' enzyme CuZn-superoxide dismutase (CuZnSOD) is encoded by a gene fro m that region and its activity is elevated in DS patients. Moreover, t he recent discovery that familial ALS is associated with mutations in the gene encoding CuZnSOD, focused attention on the entanglement of ox ygen-free radicals in cell death and neuronal disorders. To investigat e the involvement of CuZnSOD gene dosage in the etiology of the syndro me we have developed both cellular and animal models which enabled us to investigate the physiological consequences resulting from overexpre ssion of the CuZnSOD gene. Rat PC12 cells expressing elevated levels o f transfected human CuZnSOD gene were generated. These transformants ( designated PC12-hSOD) closely resembled ty parental cells in their mor phology, growth rate, and response to nerve growth factor, but showed impaired neurotransmitter uptake. The lesion was localized to the chro maffin granule transport mechanism. These results show that elevation of CuZnSOD activity interferes with the transport of biogenic amines i nto chromaffin granules. Since neurotransmitter uptake plays an import ant role in many processes of the central nervous system, CuZnSOD gene -dosage may contribute to the neurobiological abnormalities of Down's Syndrome. As an approach to the development of an animal model for Dow n's Syndrome, several strains of transgenic mice which carry the human CuZnSOD gene have been prepared. These animals express the transgene as an active enzyme with increased activity from 1.6 to 6.0-fold in th e brains of four transgenic strains and to an equal or lesser extent i n several other tissues. To investigate the contribution of CuZnSOD ge ne dosage in the neuropathological symptoms of Down's Syndrome, we ana lyzed the tongue muscle of the transgenic-CuZnSOD mice. The tongue neu romuscular junctions (NMJ) in the transgenic animals exhibited signifi cant pathologic changes; withdrawal and destruction of some terminal a xons and the development of multiple small terminals. The ratio of ter minal axon area to postsynaptic membranes decreased, and secondary fol ds were often complex and hyperplastic. The morphological changes in t he transgenic NMJ were similar to those previously seen in the transge nic NMJ and were similar to those previously seen in muscles of aging mice and rats as well as in tongue muscles of patients with Down's Syn drome. The findings suggest that CuZnSOD gene dosage is involved in th e pathological abnormalities of tongue NMJ observed in Down's Syndrome patients. Reduced levels of the neurotransmitter serotonin in blood p latelets is a clinical symptom characteristic of individuals with Down 's Syndrome. To investigate the possible involvement of the CuZnSOD ge ne, in the etiology of that symptom, we examined blood platelets of th e transgenic mice harboring the human CuZnSOD gene. It was found that platelets of transgenic CuZnSOD animals which overexpress the transgen e contain lower levels of serotonin, due to a reduced rate of uptake o f the neurotransmitter by the dense granules of the platelets. Further more, significantly lower than normal serotonin accumulation rate was also detected in dense granules isolated from blood platelets of DS in dividuals. These findings suggest that CuZnSOD gene dosage affects the dense granule transport system and is thereby involved in the depress ed level of blood serotonin found in patients born with Down's Syndrom e.