W.-Y. Hung

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.

Age of onset of amyotrophic lateral sclerosis is modulated by a locus on 1p34.1.

Amyotrophic lateral sclerosis (ALS) is the third most common adult-onset neurodegenerative disease. Individuals with ALS rapidly progress to paralysis and die from respiratory failure within 3 to 5 years after symptom onset. Epidemiological factors explain only a modest amount of the risk for ALS. However, there is growing evidence of a strong genetic component to both familial and sporadic ALS risk. The International Consortium on Amyotrophic Lateral Sclerosis Genetics was established to bring together existing genome-wide association cohorts and identify sporadic ALS susceptibility and age at symptom onset loci. Here, we report the results of a meta-analysis of the International Consortium on Amyotrophic Lateral Sclerosis Genetics genome-wide association samples, consisting of 4243 ALS cases and 5112 controls from 13 European ancestry cohorts from across the United States and Europe. Eight genomic regions provided evidence of association with ALS, including 9p21.2 (rs3849942, odds ratio [OR] = 1.21; p = 4.41 × 10(-7)), 17p11.2 (rs7477, OR = 1.30; p = 2.89 × 10(-7)), and 19p13 (rs12608932, OR = 1.37, p = 1.29 × 10(-7)). Six genomic regions were associated with age at onset of ALS. The strongest evidence for an age of onset locus was observed at 1p34.1, with comparable evidence at rs3011225 (R(2)(partial) = 0.0061; p = 6.59 × 10(-8)) and rs803675 (R(2)(partial) = 0.0060; p = 6.96 × 10(-8)). These associations were consistent across all 13 cohorts. For rs3011225, individuals with at least 1 copy of the minor allele had an earlier average age of onset of over 2 years. Identifying the underlying pathways influencing susceptibility to and age at onset of ALS may provide insight into the pathogenic mechanisms and motivate new pharmacologic targets for this fatal neurodegenerative disease.

Linkage analysis in familial amyotrophic lateral sclerosis

Familial amyotrophic lateral sclerosis (FALS) constitutes 5 to 10% of cases of ALS and, in most families, its inheritance is consistent with an autosomal dominant trait with age-dependent penetrance. The biochemical abnormality underlying the disorder is unknown. We analyzed DNA from 131 Members of 6 multigenerational ALS families, which included 13 affected Members, for genetic linkage to 39 expressed and DNA markers, using the techniques of 2-point linkage analysis, multilocus linkage analysis, and exclusion mapping. We identified FALS families with structures suitable for linkage, by computer simulation techniques. A DNA bank established to provide optimum use of available FALS families provided DNA from immortalized lymphoblast cell lines and frozen postmortem tissue. We could not link FALS to any of the markers studied, but excluded chromosome regions unlikely to be a locus of the FALS gene. With the help of this exclusion data, we will concentrate on regions of the human genome that remain unexcluded.

Age and founder effect of SOD1 A4V mutation causing ALS

Background: The alanine to valine mutation at codon 4 (A4V) of SOD1 causes a rapidly progressive dominant form of amyotrophic lateral sclerosis (ALS) with exclusively lower motor neuron disease and is responsible for 50% of SOD1 mutations associated with familial ALS in North America. This mutation is rare in Europe. The authors investigated the origin (geographic and time) of the A4V mutation. Methods: Several cohorts were genotyped: North American patients with confirmed A4V mutation (n = 54), Swedish (n = 3) and Italian (n = 6) A4V patients, patients with ALS with SOD1 non-A4V mutations (n = 66) and patients with sporadic ALS (n = 96), healthy white (n = 96), African American (n = 17), Chinese (n = 53), Amerindian (n = 11), and Hispanic (n = 12) subjects. High-throughput SNP genotyping was performed using Taqman assay in 384-well format. A novel biallelic CA repeat in exon 5 of SOD1, tightly linked to A4V, was genotyped on sequencing gels. Association statistics were estimated using Haploview. p Values less than 0.05 were considered significant. Age of A4V was estimated using a novel method based on r2 degeneration with genetic distance and a Bayesian method incorporated in DMLE+. Results: A single haplotype of 10 polymorphisms across a 5.86-cM region was associated with A4V (p = 3.0e-11) when white controls were used, suggesting a founder effect. The strength of association of this haplotype progressively decreased when African American, Chinese, Hispanic, and Amerindian subjects were used as controls. The associated European haplotype was different from the North American haplotype, indicating two founder effects for A4V (Amerindian and European). The estimated age of A4V with the r2 degeneration method was 458 ± 59 years (range 398–569) and in agreement with the Bayesian method (554–734 years with 80–90% posterior probability). Conclusions: North American SOD1 alanine to valine mutation at codon 4 descended from two founders (Amerindian and European) 400–500 years ago.

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%.

Absence of linkage of ABO blood group locus to familial tuberous sclerosis

A mutation leading to tuberous sclerosis was linked to the ABO blood group locus (9q34) on the long arm of chromosome 9. In an effort to confirm this assignment, nine multigenerational families with tuberous sclerosis, comprising 126 sampled individuals, were assessed for linkage of the ABO locus to tuberous sclerosis. Two-point linkage analysis and multilocus linkage analysis were used to evaluate linkage between tuberous sclerosis and the markers ABO, MCT136, and AblK2. Linkage of ABO to tuberous sclerosis was excluded for a distance of 20 centimorgans (cM) encompassing the region of the ABO locus. There was no evidence for genetic heterogeneity within this data set. Using 23 polymorphic markers, exclusion mapping demonstrated only a 1% probability that the tuberous sclerosis locus was on the distal short arm of chromosome 9 and provided no evidence in support of a tuberous sclerosis locus on the remainder of chromosome 9 including the area of the ABO locus.

Linkage studies in tuberous sclerosis. Chromosome 9?, 11?, or maybe 14!

Published reports show linkage of tuberous sclerosis (TSC) to either chromosome 9 in some families or chromosome 11 in other families. We studied 243 individuals (82 with TSC) from 16 multigenerational TSC families. The diagnosis of TSC conformed to the criteria of Gomez. Penetrance was estimated at 0.90. DNA markers were analyzed using Southern blotting, probe hybridization, autoradiography, and genetic linkage analysis. Two-point lod scores for TSC were calculated for 43 genetic markers distributed over 11 chromosomes. Tests for homogeneity rejected the null hypothesis of homogeneity. Linkage to TSC was excluded (z less than or equal to -2, theta greater than or equal to 0.05) for 23 of these markers including 9q34 and 11q markers. One family gave z(theta max) = 1.8, theta max = 0.001 with ABO (on 9q34), and two other families attained lod scores greater than 1 for 9q34-region markers. The lod score for TSC versus chromosome 14 marker pAW101 (D14S1) was z(theta max) = 1.98, theta max = 0.15. A single large family has overall negative lod scores for markers localized to both chromosome 9 and chromosome 11. These data confirm genetic heterogeneity in TSC and suggest linkage of some families to 9q34. Furthermore, the data suggest that 14q may be an interesting area.

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(𝛉) = 28.41; 𝛉 = 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.