Mutation of the gene for the human lysosomal serine protease cathepsin G is not the cause of aberrant APP processing in familial Alzheimer disease

Genetic polymorphism in the cathepsin G gene and the risk of Alzheimer's disease

Alzheimer's disease (AD) is a complex disease with the possible involvement of several genes. The APOE*4 allele has been documented to be a major risk factor for sporadic late-onset AD, but it is neither necessary nor sufficient to cause the disease. Cathepsin G, a serine protease found commonly in the azurophillic granules of neutrophils, has been reported to possess some beta-secretase like properties, and thus may be involved in the processing of amyloid precursor protein (APP). Recently, an A-->G polymorphism has been reported in exon 4 of the cathepsin G gene, which changes the codon AAC ((125) Asp) to AGC ((125)Ser). In this study, we have investigated the association of this polymorphism with sporadic late-onset AD. We screened DNA samples from 464 late-onset AD cases and 310 age-matched controls. No significant association was seen between this polymorphism and AD. When the data were stratified by the APOE*4 carrier status, no significant difference was seen either. Our data show no effect of this cathepsin G polymorphism in AD. Characterization of additional polymorphisms in this gene may provide more conclusive answers.

Cloning of a gene bearing missense mutations in early-onset familial Alzheimer's disease

Some cases of Alzheimer's disease are inherited as an autosomal dominant trait. Genetic linkage studies have mapped a locus (AD3) associated with susceptibility to a very aggressive form of Alzheimer's disease to chromosome 14q24.3. We have defined a minimal cosegregating region containing the AD3 gene, and isolated at least 19 different transcripts encoded within this region. One of these transcripts (S182) corresponds to a novel gene whose product is predicted to contain multiple transmembrane domains and resembles an integral membrane protein. Five different missense mutations have been found that cosegregate with early-onset familial Alzheimer's disease. Because these changes occurred in conserved domains of this gene, and are not present in normal controls, they are likely to be causative of AD3.

Beta APP mRNA transcription is increased in cultured fibroblasts from the familial Alzheimer's disease-1 family

Familial (autosomal dominant) Alzheimer's disease (FAD) is a genetically heterogeneous disorder. Mutations in exons 16 and 17 of the amyloid beta-protein precursor (beta PP) gene currently account for less than 2% of FAD kindreds. No known defect in beta PP quantity, structure, or processing accounts for disease-associated beta-amyloid deposition in the majority of early-onset FAD kindreds. Only two out of a sample of 48 pedigrees (particularly the early onset FAD 4 kindred) contributed noticeably to evidence of linkage at the D21S16/13 and S1/S11 loci in the chromosomal region 21q21 [75]. Many early onset FAD pedigrees (including the FAD 1 and FAD 4 kindreds) show strong evidence of linkage to markers in the chromosome 14q24.3 region. Patients with trisomy 21 (Down's syndrome, DS) virtually always develop a histopathological phenotype indistinguishable from FAD, presumably on the basis of increased beta PP gene dosage and transcription. Whereas no beta PP gene duplication has been found in FAD, other mechanisms that augment beta PP production by effects at the transcriptional level could explain some FAD cases. Here, we report that cultured fibroblasts from affected members of the FAD 1 pedigree show a approximately 1.9 fold increase (P = 0.007) in beta PP mRNA levels compared to unaffected members when the cells are grown under stressed conditions in 0.5% serum. The elevated levels of beta PP mRNA in cells cultured in 0.5% serum also cosegregate with haplotypes in the 14q24.3 region when analyzed by linkage methods (LOD score = 3.26 at theta = 0.001). This is the chromosomal region to which FAD in this family has previously been mapped. As expected, fibroblasts from patients with DS used as a control show a similar beta PP mRNA increase. Fibroblasts from the FAD 4 pedigree did not show this defect under the conditions utilized here. beta PP and A beta protein levels were determined quantitatively after metabolic labeling and immunoprecipitation and found to increase 2.0 and 2.5 fold, respectively, in the fibroblasts from affected FAD 1 members. Finally, transient transfections of a beta PP promoter/chloramphenicol acetyl transferase reporter gene construct demonstrated a approximately 3-4 fold increase in beta PP promoter activity in affected fibroblasts from the FAD 1 but not the FAD 4 pedigree. Taken together, these data raise the possibility that an increase in beta PP transcription may underlie the AD phenotype in at least some of the chromosome 14-linked FAD families.

Analysis of the functional effects of a mutation in SOD1 associated with familial amyotrophic lateral sclerosis

Mutations in the Cu, Zn superoxide dismutase (SOD1) gene have been reported in some pedigrees with Familial Amyotrophic Lateral Sclerosis (FALS). We have investigated the functional and structural effects of a Gly-->Ser mutation at codon 41 of SOD1 in a pedigree with FALS and the topography of SOD1 expression in the mammalian CNS. These analyses show that the 41Gly-->Ser mutation causes a 27% reduction in Cu, Zn SOD activity. SOD1 is transcribed at high levels in rat motoneurons and four other types of neurons homologous to upper motoneurons that degenerate in human ALS. However, SOD1 is transcribed at lower levels in other types of neurons, such as cerebellar Purkinje cells, which are not usually involved significantly in human ALS. On the other hand, immunocytochemical studies indicate that most types of rat neurons contain similar levels of Cu, Zn SOD immunoreactive protein. Nevertheless, these results suggest that the essential feature causing this subtype of ALS is either a reduction in Cu, Zn SOD activity in cell types that presumably critically require Cu, Zn SOD for protection against oxidative damage or the fact that the mutation in SOD1 associated with FALS results in a novel gain of function that is particularly deleterious to those cell types expressing SOD1 at high levels.