Down syndrome clinical symptoms are manifested in transfected cells and transgenic mice overexpressing the human Cu/Zn-superoxide dismutase gene

Down syndrome (DS), the phenotypic expression of human trisomy 21, is presumed to result from overexpression 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 from that region and its activity is elevated in DS patients. To investigate the possible involvement of CuZnSOD gene dosage in the etiology of the syndrome we have developed both cellular and animal models which enabled us to investigate the physiological consequences resulting from overexpression of the CuZnSOD gene. 1) Rat PC12 cells expressing elevated levels of transfected human CuZnSOD gene were generated. These transformants (designated PC12-hSOD) closely resembled the parental cells in their morphology, growth rate, and response to nerve growth factor, but showed impaired neurotransmitter uptake. The lesion was localized to the chromaffin granule transport mechanism. We found that the pH gradient (ΔpH) across the membrane, which is the main driving force for amine transport, was diminished in PC12-hSOD granules. These results show that elevation of CuZnSOD activity interferes with the trasport of biogenic amines into chromaffin granules. Since neurotransmitter uptake plays an important role in many processes of the central nervous system, CuZnSOD gene-dosage may contribute to the neurobiological abnormalities of Down's syndrome. 2) As an approach to the development of an animal model for Down syndrome, several strains of transgenic mice that carry the human CuZnSOD gene have been prepared. These animals express the transgene in a manner similar to that of humans, with 0.9 and 0.7-kilobase transcripts in a 1 : 4 ratio, and synthesize the human enzyme in an active form capable of forming human-mouse enzyme heterodimers. CuZnSOD activity is increased from 1.6 to 6.0-fold in the brains of four transgenic strains and to an equal or lesser extent in several other tissues. 3) To investigate the possible involvement of CuZnSOD gene dosage in the neuropathological symtpoms of Down's syndrome, we analyzed the tongue muscle of the transgenic mice that express elevated levels of human CuZnSOD. The tongue neuromuscular junctions (NMJ) in the transgenic animals exhibited significant pathological changes, namely, withdrawal and destruction of some terminal axons and the development of multiple small terminals. The ratio of terminal axons area to postsynaptic membrane decreased, and secondary folds were often complex and hyperplastic. The morphological changes in the transgenic NMJ were similar to those previously seen in muscles of aging mice and rats as well as in tongue muscle of patients with Down's syndrome. The findings suggest that CuZnSOD gene dosage is involved in the pathological abnormalities of tongue NMJ observed in Down's syndrome patients. 4) Reduced levels of the neurotransmitter serotonin in blood platelets is a clinical symptom characteristic of individuals with Down syndrome. To investigate the possible involvement of the CuZnSOD gene, in the etiology of that symptom, we examined blood platelets of the transgenic mice harboring the human CuZnSOD gene. It was found that platelets of transgenic CuZnSOD animals which overexpress the transgene contain lower levels of serotonin than non-transgenic littermate mice, due to a reduced rate of uptake of the neurotransmitter by the dense granules of the platelets. Furthermore, significantly lower than normal serotonin accumulation rate was also detected in dense granules isolated from blood platelets of DS individuals. These findings suggest taht CuZnSOD gene dosage is affecting the dense granule transport system and thereby involved in the depressed level of blood serotonin found in patients born with Down syndrome.