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.

Complete genomic screen in late-onset familial Alzheimer's disease.

Alzheimer's disease (AD) is a complex genetic disorder. Linkage analysis has helped unravel a portion of the genetic component of AD by identifying four loci that play a role in the genetics of AD (amyloid precursor protein, presenilin 1, presenilin 2, and apolipoprotein E). These loci account for approximately 50% of the genetic etiology of AD. A total genomic screen is an efficient way to identify additional genetic effects in AD. A series of multiplex late-onset (>60 years) AD families were ascertained (NINDS-ADRDA diagnostic criteria) and sampled. A subset (n = 16) of the largest families (52 affecteds with DNA, 83 unaffecteds with DNA) were used to rapidly screen the genome (n = 280 markers) for additional major genetic effects. Critical values for regional follow-up were p < or =0.05 for SimIBD or sibpair analysis and/or a LOD score > or = 1.00. Fifteen regions warranted initial follow-up based on these criteria. An additional screening set was used (n = 38 families, 89 affecteds with DNA, 216 unaffecteds with DNA) for the follow-up analysis. These analyses revealed four regions of continued interest on chromosomes 4, 6, 12, and 20. Chromosome 12 presented the strongest results. Peak two point "affecteds only" LOD scores were 1.3, 1.6, 2.7, and 2.2 and (affected relative pair SimIBD) p values were 0.04, 0.03, 0.14, and 0.04 for D12S373, D12S1057, D12S1042, and D12S390, respectively. These markers span approximately 30 cm near the centromeric region of chromosome 12. Sibpair analysis resulted in two point Maximum Lod Score (MLS) results of 0.4, 1.2, 3.2, and 1.0 for the above markers. Multipoint MLS analysis supported these findings. Saturation mapping of all available markers in the chromosome 12 region as well as further investigation of the regions on 4, 6, and 20 is ongoing with candidate gene analysis to follow.

Periodic vestibulocerebellar ataxia, an autosomal dominant ataxia with defective smooth pursuit, is genetically distinct from other autosomal dominant ataxias.

BACKGROUND: Periodic vestibulocerebellar ataxia is an autosomal dominant disorder characterized by defective smooth pursuit, gaze-evoked nystagmus, ataxia, and vertigo. The age of onset ranges from the third to the sixth decade. To date, all patients have originated from North Carolina, suggesting a single common founder. OBJECTIVE: To clarify the classification of periodic vestibulocerebellar ataxia by determining whether it is allelic to other autosomal dominant cerebellar ataxias for which genes have been either localized or identified. METHODS: Blood was collected and DNA isolated from 66 subjects (19 affected individuals) in two multigenerational families. The microsatellite markers used in the analysis either flanked or were tightly linked to the disease gene regions. Two-point and multipoint linkage analyses were performed to define the limits of exclusion. RESULTS: Periodic vestibulocerebellar ataxia was excluded from loci linked to spinocerebellar ataxia type 1 (chromosome 6p), type 2 (chromosome 12q) type 3/Machado/Joseph disease (chromosome 14q), type 4 (chromosome 16q), and type 5 (11cent) as well as to episodic ataxia with myokymia (chromosome 12p), episodic ataxia with nystagmus (chromosome 19p), acetazolamide-responsive hereditary paroxysmal cerebellar ataxia (chromosome 19p), and dentatorubral-pallidoluysian atrophy/Haw River syndrome (chromosome 12p). CONCLUSION: Periodic vestibulocerebellar ataxia is genetically distinct from those autosomal dominant ataxias for which chromosomal localization has been established.

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.

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 (theta) of 0.02, with 95% confidence limits for theta of 0.001 to 0.065. Thus, apolipoprotein C2 is useful in carrier detection and prenatal diagnosis with an accuracy of about 98%.

Tight linkage of creatine kinase (CKMM) to myotonic dystrophy on chromosome 19.

The myotonic dystrophy (DM) gene is localized to the proximal long arm of chromosome 19. There have been reports of tight linkage to a number of chromosome 19 markers, including APOC2 and creatine kinase muscle type (CKMM), but they did not establish orientation of the 2 markers to DM. We screened several large multi-generational DM families for linkage to a series of chromosome 19 markers including CKMM. CKMM is tightly linked to DM in these data with z(theta) = 28.41; theta = 0.01. Analysis of cross-over data indicates CKMM is on the same side and closer to DM than APOC2. Thus, CKMM is a useful probe for carrier detection studies in presymptomatic individuals as well as for prenatal diagnosis.

Linkage studies in Charcot-Marie-Tooth disease type 2: evidence that CMT types 1 and 2 are distinct genetic entities.

Charcot-Marie-Tooth disease (CMT), the most common inherited peripheral neuropathy, is a progressive sensorimotor neuropathy divided into types 1 and 2 based upon electrophysiologic and neuropathologic differences. The more common autosomal dominant form of CMT type 1 (hereditary motor and sensory neuropathy type I) is genetically heterogeneous, with genes located on chromosomes 1 (type 1B) or 17 (type 1A). However, no locus for CMT type 2 is known. We have performed linkage studies on three large multigenerational CMT type 2 families using probes from chromosome 1 and chromosome 17, which span their respective linkage regions. Multipoint analysis of the chromosome 17 markers excluded linkage over an area of 45 cM--15 cM proximal and 30 cM distal to the region containing CMT type 1A. Multipoint analysis of the chromosome 1 markers exclude linkage 15 cM proximal and 20 cM distal to FC-gamma-RII in the region of CMT 1B. These data indicate that CMT type 2 is genetically distinct from CMT type 1.

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.

Confirmation of locus heterogeneity in the pure form of familial spastic paraplegia.

Familial spastic paraplegia (FSP), characterized by progressive spasticity of the lower extremities, is in its "pure" form generally of autosomal dominant inheritance pattern. Hazan et al. [Nat Genet 5:163-167, 1993] reported tight linkage of a large FSP family to the highly polymorphic microsatellite marker D14S269 with z (theta) = 8.49 at theta = 0.00 They further demonstrated evidence for locus heterogeneity when they showed that 2 FSP families were unlinked to this region. We have subsequently studied 4 FSP families (3 American, one British) and excluded the disease locus in these families for approximately 30 cM on either side of D14S269, thereby confirming evidence for locus heterogeneity within the spastic paraplegia diagnostic classification.

No association of alpha1-antichymotrypsin flanking region polymorphism and Alzheimer disease risk in early- and late-onset Alzheimer disease patients.

The alpha1-antichymotrypsin (AACT)-155 allele was found elsewhere to have a significant effect on Alzheimer disease (AD) risk in individuals with at least one APOE-4 allele. We compared AACT genotypes of 284 cases of sporadic AD and 172 controls. The frequency of the AACT-155 allele did not differ significantly between cases and controls, either overall or when restricted to subjects with at least one APOE-4 allele. Logistic regression controlling for age and sex failed to show an effect due to AACT either alone or acting with APOE. There was no evidence of an interaction between APOE-4 and the AACT-155 allele to reduce age at onset. Thus, our data do not support an association of AACT-155 with risk or age at onset in AD.

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.

Myotonic muscular dystrophy. Calcium-dependent phosphatidate metabolism in the erythrocyte membrane.

It has been suggested that the erythrocytes of myotonic dystrophy (MyD) patients have a decreased calcium-stimulated phosphatidic acid (PA) accumulation. This could be the result of a defect in the calcium-stimulated hydrolysis of the polyphosphoinositides (calcium-dependent phosphodiesterase) or in the subsequent formation of PA from its precursors (diacylglycerol kinase). In vitro assays were established for both enzymes in erythrocyte membranes. Calcium-dependent phosphodiesterase activity was assayed with both endogenous 32P-labeled erythrocyte diphosphoinositide and triphosphoinositide and with the same phospholipids isolated from rat brain. No significant differences in activity were found between MyD patients and normal controls with either method of substrate preparation. No difference in diglyceride kinase activity was found between ghosts prepared from MyD patients and normal controls. Thus, there were no differences in either of the membrane-associated enzymes of phosphatidic acid metabolism.

Confirmation of linkage in von Hippel-Lindau disease.

Von Hippel-Lindau (VHL) disease was initially reported to be linked to the RAF1 oncogene (3p25). We have ascertained and sampled two large multigenerational VHL families for linkage studies, in order to confirm the localization of the VHL gene as a prelude to fine mapping studies. The probes used in the analysis were p627 (RAF1) and pHeA12 (thyroid hormone receptor B) (3p24.1-3p22). VHL was analyzed as an autosomal dominant trait with age-dependent penetrance. The maximum lod score combining both families was z(theta) = 2.16 at theta = 0.0 for RAF1 and z(theta) = 2.20 at theta = 0.05 for thyroid hormone receptor B. Multipoint analysis using the RAF1 and thyroid hormone receptor B loci resulted in a peak lod score of 3.1 confirming linkage of VHL to this region of chromosome 3. However, the position of VHL relative to the two loci could not be established with certainty.

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.

Evidence for genetic heterogeneity supports clinical differences in congenital myasthenic syndromes.

Congenital myasthenic syndromes (CMS) define a diverse group of disorders, all of which compromise neuromuscular transmission. Symptoms can be present at birth or appear during childhood, and can range in severity. Both autosomal dominant and recessive forms exist, and a number of clinical subtypes have been described. The cause of many cases of CMS has been traced to mutations in the genes for the acetylcholine receptor (AChR) subunits, previously mapped to chromosomes 2 and 17. Recently, an additional form of CMS known as familial infantile myasthenia (FIM) was linked to chromosome 17p. The gene for FIM has not yet been identified. We examined the DNA from 5 families of Iranian Jewish origin (6 affected individuals) who have been diagnosed with a phenotypically unique form of CMS. Four of the families are consanguinous, and all families originate from the same geographical region, thus it is highly likely that they would carry the same ancestral CMS mutation. We examined these families for linkage to the regions on chromosomes 2 and 17 containing the AChR subunit genes, and to the region on 17p to which FIM was localized. Our data excludes linkage to these regions, suggesting that the clinical differences seen among patients with CMS correlate with locus heterogeneity, and that a defect in a different gene is responsible for the CMS in these patients.

Duchenne muscular dystrophy: detection of deletion carriers by spectrophotometric densitometry.

DNA isolated from a family segregating a deletion in the Duchenne muscular dystrophy gene and control families was digested with restriction enzymes, Southern transferred, and probed with a radioactive dystrophin cDNA probe. The resulting autoradiographs were analyzed with a densitometric spectrophotometer to detect carriers of the deletion. The carrier status of females in the deletion pedigree was independently determined by genomic probes and confirmed by densitometry. In many Duchenne families, deletions will only be observed using cDNA probes which show few restriction fragment length polymorphisms (RFLPs). Such deletions would normally have to be detected using dosage gels. The spectrophotometric densitometry technique used by us does not require dosage gels, and avoids problems arising from non-informative meioses and cross-overs. It should be possible to screen every family with an exon deletion by spectrophotometric densitometry provided the presently available cDNA is suitably reduced to produce fewer bands on autoradiographs.