Larry H. Yamaoka

Duchenne muscular dystrophy High frequency of deletions

DNA probes are available for Duchenne muscular dystrophy (DMD) carrier detection and prenatal diagnosis. With probes for about 25% of the proximal portion of the gene, we found the proximal probes detected deletions in 23% of nonselected DMD boys, while a single distal probe detected 17% more as deletions. The combined percentage was 39% for all probes tested. Prenatal diagnosis and carrier detection are more accurate if deletions are mapped rather than by use of restriction fragment length polymorphism analysis. The effort involved in screening all affected boys for deletions is considerably less, and provides an accurate genetic marker for subsequent prenatal diagnosis in the family and prospective counseling for female relatives. It seems likely that, once the entire gene (cDNA) is available for screening, most DMD boys will show deletions.

Genetic linkage studies in Alzheimer's disease families

Alzheimers disease is a devastating neurological disorder and the leading cause of dementia among the elderly. Recent studies have localized the gene for familial Alzheimers disease to chromosome 21 in a series of early onset AD families (mean age of onset less than 60). Familial late onset AD (mean age of onset greater than 60) is a more common clinical form of the disorder. Thirteen families with multiply affected Alzheimers disease family members were identified and sampled. Ten of these families were of the late onset Alzheimers disease type. Simulation studies were used to evaluate the usefulness of these pedigrees in linkage studies in familial Alzheimers disease. Linkage studies undertaken to test the localization of both early onset and late onset Alzheimers disease families to chromosome 21 failed to establish linkage and excluded linkage from a large portion of the region where the early onset Alzheimers disease gene was localized. These findings suggest that more than one etiology may exist for familial Alzheimers disease and indicate the need for continued screening of the genome in familial Alzheimers disease families.

Linkage of Charcot-Marie-Tooth neuropathy type 1a to chromosome 17

Charcot-Marie-Tooth disease Type 1 (CMT) is an inherited neuropathy with known genetic heterogeneity, with at least one autosomal dominant form (CMT Type 1b) linked to the Duffy region of chromosome 1. Autosomal dominant families not demonstrating linkage to the Duffy blood group marker have been designated CMT Type 1a. We report linkage of six CMT Type 1a families to the chromosome 17 markers EW301 (D17S58) and pA10-41 (D17S71) with maximum LOD scores of zeta = 10.49 at theta (maximum recombination fraction) = 0.05 and zeta = 7.36 at theta = 0.06, respectively.

Localization of Charcot-Marie-Tooth disease type 1a (CMT1A) to chromosome 17p11.2

Charcot-Marie-Tooth (CMT) disease type 1a has been previously localized to chromosome 17 using the markers D17S58 and D17S71. In that report we were unable to provide unequivocal localization of the CMT1A gene on either the proximal p or the q arm. Therefore, data from one additional CMT1A family and typing of other probes spanning the pericentromeric region of chromosome 17 (D17S73, D17S58, D17S122, D17S125, D17S124) were analyzed. Multipoint analysis demonstrates convincing evidence (log likelihood difference greater than 5) that the CMT1A gene lies within 17p11.2 and most likely between the flanking markers D17S122 and D17S124.

Neuropathological features of frontotemporal dementia and parkinsonism linked to chromosome 17q21-22 (FTDP-17): Duke family 1684

Frontotemporal dementia with parkinsonism (FTDP-17) is an autosomal dominant disorder that presents clinically with dementia, extrapyramidal signs, and behavioral disturbances in mid-life and progresses to death within 5 to 10 years. Pathologically, the disorder is characterized by variable neuronal loss and gliosis in the frontal and temporal lobes, limbic structures, and the midbrain. Autopsied individuals from some kindreds display abundant neurofibrillary change while others, including a single affected individual from Duke Family 1684, lack distinctive histological features and exhibit only mild neuronal loss and gliosis in limbic structures and subcortical nuclei when examined by routine silver stain. Recently, mutations in the microtubule associated protein tau have been shown to segregate with the disease in this family and in many other affected kindreds. In order to examine the distribution of tau deposits, we performed tau immunohistochemistry, immunoblotting, and immunoelectron microscopy of tau-containing filaments. Immunohistochemistry revealed numerous tau deposits within glial cells and within neurons. Twisted ribbon-like filaments observed by immunoelectron microscopy were immunodecorated with tau AT8 antibody. Sarkosyl-insoluble tau extracted from the hippocampus and cortex migrated as 2 major bands at 64 and 68 kilodaltons and a minor band at 72 kilodaltons, which after alkaline phosphatase treatment appeared to contain mainly tau isoforms with 4 repeats. Furthermore, the ratio of soluble tau with 4 to 3 microtubule-binding repeats was increased. The role of tau mutations in this disorder is discussed in this paper.

No genetic association between the LRP receptor and sporadic or late-onset familial Alzheimer disease.

ABSTRACT The low-density lipoprotein receptor-related protein gene (LRP1) is often mentioned as a candidate gene for Alzheimer disease (AD) because of its role as a receptor for apolipoprotein E (apoE), a major genetic risk factor for late-onset familial and sporadic AD. A recent association study of a tetranucleotide repeat polymorphism located 5′ to the LRP1 gene detected an increase in the 87 base pair allele in AD cases compared to unaffected controls. Additionally, an independent study involving a genomic screen for genes associated with late-onset AD identified a region as a possible location of a late-onset AD gene on chromosome 12p between D12S373 and D12S390, about 10 cM proximal to LRP1. We examined 144 late-onset multiplex AD families, 436 sporadic AD cases, and 240 controls and found no evidence of linkage or association of LRP1 and AD. Our data indicate that genetic variation of the LRP1 gene is not a major risk factor in the etiology of AD.

Tight linkage of apolipoprotein C2 to myotonic dystrophy on chromosome 19

The cDNA and genomic probes for apolipoprotein C2 detect two restriction fragment length polymorphisms on chromosome 19. The combined estimated percentage of heterozygosity, assuming equilibrium, is approximately 75%, ie, apolipoprotein C2 is informative in 75% of matings. We have analyzed over 350 individuals in large multigenerational families for linkage of apolipoprotein C2 to myotonic muscular dystrophy. The maximum lod score was 16.29 with the maximum recombination fraction (𝛉) of 0.02, with 95% confidence limits for 𝛉 of 0.001 to 0.065. Thus, apolipoprotein C2 is useful in carrier detection and prenatal diagnosis with an accuracy of about 98%.

No association or linkage between an intronic polymorphism of presenilin-1 and sporadic or late-onset familial alzheimer disease

Recent reports have shown an association between an intronic polymorphism of the presenilin‐1 (PSEN1) gene and late‐onset (age at onset > 65) familial and sporadic (no family history) Alzheimer disease (AD). The reported association was independent of the effect of the only previously identified gene associated with late‐onset AD, APOE. Blood samples were obtained from members of 122 multiplex AD families, 42 unrelated cases of AD with positive family histories of dementia, 456 sporadic cases of AD, and 317 controls of similar ages at examination to the cases. These samples were genotyped for an intronic polymorphism of the PSEN1 gene, located 3′ to exon 8, and the data analyzed for evidence of association or linkage. The samples were also genotyped for APOE and the data analyzed to see if the association or linkage changed when controlling for APOE genotype. There was no statistically significant increase (at α = .01) in allele 1 (199 bp) or genotype 1/1 in the sporadic AD cases, or in a random sample of one affected from each multiplex family, compared to controls. When examining the effect of the PSEN1 polymorphism while controlling for APOE genotype, APOE genotype was strongly associated with AD, but the PSEN1 polymorphism genotype was not. Model‐trait dependent (lod score) and independent (SimIBD) methods detected no evidence of linkage between PSEN1 and AD. In this independent dataset, the previously reported association between the intronic PSEN1 polymorphism and AD cannot be confirmed, and the conclusion that PSEN1 is a major susceptibility gene for late‐onset AD is not supported. Genet. Epidemiol. 14:307–315,1997.

No association between very low density lipoprotein receptor (VLDL-R) and Alzheimer disease in American Caucasians

The very low density lipoprotein receptor gene (VLDL-R) is a receptor for apolipoprotein-epsilon (APOE)-containing lipoproteins, and thus has been suggested as a possible risk factor for Alzheimer disease (AD). Recently, Okuizumi et al. [Nature Genet, II (1995) 207-209] reported an association between the 96 bp allele at the VLDL-R locus and AD in a Japanese population. The association resulted in a two-fold increase of risk that decreased with increasing age. We have examined this association in 316 Caucasian sporadic AD patients, comparing their findings to 160 Caucasian AD spouse controls. We also investigated 53 late-onset Caucasian AD families for association and linkage. Our data failed to confirm linkage and/or association to the VLDL-R locus. Stratification by age at onset or APOE genotype also failed to show significant results.

Development of a microsatellite genetic map spanning 5q31–q33 and subsequent placement of the LGMD1A locus between D5S178 and IL9

Limb-girdle muscular dystrophy (LGMD) is a genetically and clinically heterogeneous group of disorders. We previously localized an autosomal dominant form of the disorder (LGMD1A) to chromosome 5q22-31 by linkage analysis in a single large pedigree. After developing a microsatellite genetic map incorporating six loci in q31-33 of chromosome 5 and spanning 35 cM, we have refined the original localization. Using multipoint analysis, LGMD1A is localised to a 7 cM region between the markers IL9 and D5S178 with odds > 1000:1.