A compound (AC)n repeat located 1,000 bp downstream from the human synapsin I gene and within the last intron of the A-raf-1 gene has been identified. DNA data-base comparisons of the sequences surrounding the repeat indicate that the synapsin I gene and the A-raf-1 gene lie immediately adjacent to each other, in opposite orientation. PCR amplification of this synapsin I/A-raf-1 associated repeat by using total genomic DNA from members of the 40 reference pedigree families of the Centre d'Etude du Polymorphisme Humaine showed it to be highly polymorphic, with a PIC value of .84 and a minimum of eight alleles. Because the synapsin I gene has been mapped previously to the short arm of the human X chromosome at Xp11.2, linkage analysis was performed with markers on the proximal short arm of the X chromosome. The most likely gene order is DXS7SYN/ARAF1TIMPDXS255DXS146, with a relative probability of 5 x 10(8) as compared with the next most likely order. This highly informative repeat should serve as a valuable marker for disease loci mapped to the Xp11 region.

In this dissertation I describe the isolation and characterization of genomic clones for the human synapsin I gene, the establishment of a linkage map for the human synapsin I gene locus, and studies of the possible involvement of this gene in neurological disease. Synapsin I is a neuron-specific phosphoprotein which is concentrated at the presynaptic terminal. Evidence suggests that it plays a fundamental role in the regulation of neurotransmitter release. Altogether 27,500 bp of the human synapsin I gene have been isolated, and the gene structure has been partially determined. DNA sequence comparisons between human and rat genes show a high degree of conservation. Sequenced exons display an 87% identity to each other. The synapsin I genomic clones were employed in the search for a polymorphic marker. A compound (AC)n repeat located 1000 base pairs downstream from the human synapsin I gene and within the last intron of the A-raf-1 gene has been identified. DNA database comparisons of the sequences surrounding the repeat indicate that the synapsin I gene and the A-raf-1 gene lie immediately adjacent to each other, in opposite orientation. Polymerase chain reaction amplification of this synapsin I / A-raf-1 associated repeat using total genomic DNA from members of the 40 reference pedigree families of the Centre d'Etude du Polymorphisme Humaine showed it to be highly polymorphic, with a polymorphic information content value of 0.84 and a minimum of eight alleles. Because the synapsin I gene had been mapped previously to the short arm of the human X chromosome at Xp11.2, linkage analysis was performed with markers on the proximal short arm of the X chromosome. The most likely gene order is: DXS7 - SYN/ARAF1 - TIMP - DXS255 - DXS146 with a relative probability of 5 x 108 compared with the next most likely order. The SynI/Araf marker was next utilized in a linkage study aimed at establishing a more accurate placement of the genetic locus responsible for the ocular disorder Congenital stationary night blindness, which had been mapped previously close to DXS7. Our results confirm this prior localization and also exclude any placement proximal to the SYN/ARAF1 locus. Finally, the inheritance of the different alleles of the SynI/Araf marker in three families with Rett syndrome, a severe neurodegenerative disorder, which has been assigned to the X chromosome, was studied. In at least one of the families in which two half sisters with the same mother suffer from the disease, the inheritance of Rett syndrome was discordant with the inheritance of the same allele for the SynI/Araf marker. Thus, this highly informative repeat has proven already effective in the study of X-linked diseases and should serve as a valuable marker for disease loci mapped to the Xp11 region.

The characteristics of Rett syndrome suggest that it is an X-linked neurodegenerative disorder. Laboratory investigations to date have not revealed any metabolic abnormalities in affected individuals. Synapsin I is a neuron-specific protein thought to play a fundamental role in neuronal function. In this report we summarize the circumstantial evidence suggesting that a defect in synapsin I gene structure or expression might be involved in Rett syndrome. This evidence includes analysis of structural and functional aspects of synapsin I primary structure, characterization of synapsin I messenger RNAs, location of the synapsin I gene on the human X chromosome and preliminary analysis of synapsin I gene structure in Rett individuals.