Jane Theilmann

A Worldwide Study of the Huntington's Disease Mutation: The Sensitivity and Specificity of Measuring CAG Repeats

BACKGROUND Huntingtons disease is associated with an expanded sequence of CAG repeats in a gene on chromosome 4p16.3. However, neither the sensitivity of expanded CAG repeats in affected persons of different ethnic origins nor the specificity of such repeats for Huntingtons disease as compared with other neuropsychiatric disorders has been determined. METHODS We studied 1007 patients with diagnosed Huntingtons disease from 565 families and 43 national and ethnic groups. In addition, the length of the CAG repeat was determined in 113 control subjects with a family history of Alzheimers disease (44 patients), schizophrenia (39), major depression (16), senile chorea (5), benign hereditary chorea (5), neuroacanthocytosis (2), and dentatorubropallidoluysian atrophy (2). The number of CAG repeats was also assessed in 1595 control chromosomes, with the size of adjacent polymorphic CCG trinucleotide repeats taken into account. RESULTS Of 1007 patients with signs and symptoms compatible with a diagnosis of Huntingtons disease, 995 had an expanded CAG repeat that included from 36 to 121 repeats (median, 44) (sensitivity, 98.8 percent; 95 percent confidence interval, 97.7 to 99.4 percent). There were no significant differences among national and ethnic groups in the number of repeats. No CAG expansion was found in the 110 control subjects with other neuropsychiatric disorders (specificity, 100 percent; 95 percent confidence interval, 95.2 to 100 percent). In 1581 of the 1595 control chromosomes (99.1 percent), the number of CAG repeats ranged from 10 to 29 (median, 18). In 12 control chromosomes (0.75 percent), intermediate-sized CAG sequences with 30 to 35 repeats were found, and 2 normal chromosomes unexpectedly had expanded CAG sequences, of 39 and 37 repeats. CONCLUSIONS CAG trinucleotide expansion is the molecular basis of Huntingtons disease worldwide and is a highly sensitive and specific marker for inheritance of the disease mutation.

The psychological consequences of predictive testing for Huntington's disease.

Abstract Background. Advances in molecular genetics have led to the development of tests that can predict the risk of inheriting the genes for several adult-onset diseases. However, the psychological consequences of such testing are not well understood. Methods. The 135 participants in the Canadian program of genetic testing to predict the risk of Huntingtons disease were followed prospectively in three groups according to their test results: the increased-risk group (37 participants), the decreased-risk group (58 participants), and the group with no change in risk (the no-change group) (40 participants). All the participants received counseling before and after testing. Standard measures of psychological distress (the General Severity Index of the Symptom Check List 90-R), depression (the Beck Depression Inventory), and well-being (the General Well-Being Scale) were administered before genetic testing and again at intervals of 7 to 10 days, 6 months, and 12 months after the participants received their t...

Non-random association between alleles detected at D4S95 and D4S98 and the Huntington's disease gene.

Analysis of many families with linked DNA markers has provided support for the Huntingtons disease (HD) gene being close to the telomere on the short arm of chromosome 4. However, analysis of recombination events in particular families has provided conflicting results about the precise location of the HD gene relative to these closely linked DNA markers. Here we report an investigation of linkage disequilibrium between six DNA markers and the HD gene in 75 separate families of varied ancestry. We show significant non-random association between alleles detected at D4S95 and D4S98 and the mutant gene. These data suggest that it may be possible to construct high and low risk haplotypes, which may be helpful in DNA analysis and genetic counselling for HD, and represent independent evidence that the gene for HD is centromeric to more distally located DNA markers such as D4S90. This information may be helpful in defining a strategy to clone the gene for HD based on its location in the human genome.

Molecular analysis of late onset Huntington's disease.

Late onset Huntingtons disease is characterised by onset of symptoms after the age of 50 and is usually associated with a milder course. We have analysed the CAG trinucleotide repeat within the HD gene in 133 late onset patients from 107 extended families. The median upper allele size for the CAG repeat was 42 with a range of 38 to 48 repeats. A significant negative correlation (r = -0.29, p = 0.001) was found between the length of repeat and age of onset for the total cohort. However, for persons with age of onset greater than 60, no significant correlation was found. In addition, no significant correlation was found between age of onset and size of the lower allele and the sex of the affected parent or grandparent. There was no preponderance of maternal descent for late onset cases in this series. This study shows that variation in repeat length only accounts for approximately 7% of the variation in age of onset for persons beyond the age of 50 and clearly shows how with increasing onset age the effect of the repeat length on this onset age seems to diminish.

Familial predisposition to recurrent mutations causing Huntington's disease: genetic risk to sibs of sporadic cases.

Huntingtons disease (HD) is associated with expansion of a CAG repeat in a new gene. We have recently defined a premutation in a paternal allele of 30 to 38 CAG repeats in the HD gene which is greater than that seen in the general population (< 30 repeats) but below the range seen in patients with HD (> 38). These intermediate alleles are unstable during transmission through the germline and in sporadic cases expand to the full mutation associated with the clinical phenotype of HD. Here we have analysed three new mutation families where, in each, the proband and at least one sib have CAG sizes in the HD range. In one of these families, two sibs with expanded CAG repeats are both clinically affected with HD, thus presenting a pseudorecessive pattern of inheritance. In all three families the parental intermediate allele has expanded in more than one offspring, thus showing a previously unrecognised risk of inheriting HD to sibs of sporadic cases of HD.

Normal CAG repeat length in the Huntington's disease gene in senile chorea

There is a widely held belief that most patients presenting with senile chorea have late-onset Huntingtons disease (HD) with an unknown family history. We measured CAG trinucleotide repeat expansion in the HD gene in four patients with a clinical presentation of senile chorea and found that CAG repetition lengths were normal. These findings support senile chorea as being a distinct clinical entity that is nosologically separate from late-onset HD.

DNA analysis of distinct populations suggests multiple origins for the mutation causing Huntington disease

Andrew S, Theilmann J, Almqvist E, Norremolle A, Lucotte G, Anvret M, Sorensen SA, Turpin JC, Hayden MR. DNA analysis of distinct populations suggests multiple origins for the mutation causing Huntington disease.

Nonrandom association between huntington disease and two loci separated by about 3 Mb on 4p16.3

The gene for Huntington disease (HD) has been localized close to the telomere on the short arm of chromosome 4. However, refined mapping using recombinant HD chromosomes has resulted in conflicting findings and mutually exclusive candidate regions. Previously reported significant nonrandom allelic association between D4S95 and HD provided support for a more proximal location for the defective gene. In this paper, we have analyzed 17 markers, spanning approximately 6 Mb of DNA distal to locus D4S62, for nonrandom association to HD. We confirm the previous findings of nonrandom allelic association between D4S95 and HD. In addition, we provide new data showing significant nonrandom association between HD and 3 markers at D4S133 and D4S228, which are approximately 3 Mb telomeric to D4S95.

Localization of the cell death genes CPP32 and Mch-2 to human chromosome 4q.

Programmed cell death, manifested as apoptosis, is a deliberate and systematic means of cell suicide characterized by several distinct biochemical and morphological changes including cell shrinkage, membrane blebbing, and chromatin condensation (Wyllie et al. 1980). While this mechanism for self-destruction can be triggered by different pathogenic stimuli including infectious agents, it can also occur physiologically to halt the spread of neighboring cells or to make room for new cell types. Thus, apoptosis plays an important role during normal development, helping to maintain the delicate balance between cell death and survival. However, this balance can go astray, resulting in disease. For instance, excessive apoptosis has been implicated in neurodegenerative diseases and ischemic damage, while insufficient apoptosis has been postulated to occur in cancers and autoimmune diseases (reviewed in Nicholson 1996). In the nematode Caenorhabditis elegans, 131 cells die during normal development by apoptosis (Hengartner and Horovitz 1994). This process is under the control of several genes including Ced-3, which encodes a key cell death protease that is absolutely necessary for apoptosis (Hengartner and Horovitz 1994). CPP32/apopain appears to be a key mammalian Ced-3 homolog acting early in the cell death pathway. Relative to other mammalian cysteine proteases, it shares a high level of homology with Ced-3 (Frenandes-Alnemri et al. 1994). Moreover, it is specifically responsible for cleavage and inactivation of key homeostatic proteins during apoptosis. These proteins include poly (ADP-ribose) polymerase (PARP), an enzyme involved in DNA repair particularly in response to environmental stress (Nicholson et al. 1995; Tewari et al. 1995). In addition, the catalytic subunit of DNAdependent protein kinase (DNA-PKcs), an enzyme essential for repair of DNA double-stranded breaks (Casciola-Rosen et al., 1995), and the U1-70 kDa small ribonucleoprotein (CasciolaRosen et al. 1994), which is necessary for RNA splicing, are also involved. Huntingtin, the gene product for the gene associated with Huntingtons Disease, is also cleared by apopain (Goldberg et al. 1996). Over-expression of CPP32 in vitro leads to apoptosis, which can be blocked by a specific peptide aldehyde inhibitor of CPP32 (Nicholson et al. 1995). However, no mutations in CPP32 have been shown to underlie any disease, perhaps owing to either functional redundancy in this enzyme family or to embryonic lethality. Using fluorescence in situ hybridization (FISH; Lichter et al. 1990) of a genomic clone isolated from a P1 library (Ioannou et al. 1994), we have mapped CPP32 to the tip of the long arm of human Chr 4 (Fig. 1) and have further refined its localization against a YAC contig from this region spanning at least 2 megabases (Mb).

Interleukin-1α-converting enzyme (ICE) and related cell death genes ICErel-II and ICErel-III map to the same PAC clone at band 11q22.2-22.3

~Department of Medical Genetics and Centre for Molecular Medicine & Therapeutics (CMMT), University of British Columbia, 416-2125 East Mall, NCE Building, Vancouver B.C., V6T 1Z4, Canada 2Department of Biochemistry and Molecular Biology, Merck Frosst Centre for Therapeutic Research, Montreal, Canada 3Department of Genetics, The Hospital for Sick Children, Toronto, Canada 4Department of Pediatric Laboratory Medicine, The Hospital for Sick Children, Toronto, Canada