CAG Repeats In Sporadic Cases

CAG Repeats in a sporadic case of HD

- various studies

Nine studies between 1992 and 2000

These are older studies. Check the HD Lighthouse for the latest in HD Research

De novo expansion of a (CAG)n repeat in sporadic Huntington's disease.

Nat Genet 1993 Oct;5(2):168-73
Myers RH, MacDonald ME, Koroshetz WJ, Duyao MP, Ambrose CM, Taylor SA, Barnes G, Srinidhi J, Lin CS, Whaley WL, et al.- Department of Neurology, Boston University School of Medicine, Massachusetts 02118

Huntington's disease (HD) chromosomes contain an expanded unstable (CAG)n repeat in chromosome 4p16.3.

We have examined nine families with potential de novo expression of the disease. With one exception, all of the affected individuals had 42 or more repeat units, well above the normal range.

In four families, elderly unaffected relatives inherited the same chromosome as that containing the expanded repeat in the proband, but had repeat lengths of 34-38 units, spanning the gap between the normal and HD distributions. Thus, mutation to HD is usually associated with an expansion from an already large repeat.

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

J Med Genet 1993 Dec;30(12):987-90
Goldberg YP, Andrew SE, Theilmann J, Kremer B, Squitieri F, Telenius H, Brown JD, Hayden MR. - Department of Medical Genetics, University of British Columbia, Vancouver, Canada

Huntington's disease (HD) is associated with expansion of a CAG repeat in a new gene. We have recently defined a pre-mutation 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.

Mutation analysis in patients with possible but apparently sporadic Huntington's disease.

Lancet 1994 Sep 10;344(8924):714-7
Davis MB, Bateman D, Quinn NP, Marsden CD, Harding AE.- University Department of Clinical Neurology, Institute of Neurology, London, UK.

Until the advent of mutation analysis it was impossible to make a certain diagnosis of Huntington's disease (HD) in the absence of a positive family history, and sporadic cases of possible HD presented a substantial diagnostic dilemma.

We have looked for the characteristic expanded trinucleotide (CAG) repeat sequence in the HD gene in 44 patients with probable or possible HD who did not have similarly affected relatives. We used two methods, the traditional widely used method, which estimates both the CAG repeat and the flanking CCG repeat and gives the CAG length by subtraction, and the more precise CAG method, which estimates the repeat length directly.

With the CAG method, the HD mutation was detected in 25 (89%) of 28 patients with the typical clinical features of HD and 5 (31%) of 16 in whom the diagnosis was more doubtful. The CAG-CCG method gave results in the borderline abnormal range of repeats for 13 of the 33 patients eventually shown to have an unequivocal repeat expansion by the CAG method. Most of these patients had late onset of symptoms. There was evidence of expansion of an intermediate-length paternal allele in 1 patient and of non-paternity in another.

The identification of the mutation causing HD means that it is now possible to confirm or exclude the diagnosis with confidence, even in the absence of a family history, by analysis of DNA from a blood sample. The precise method of measuring the CAG repeat, which is technically more difficult than the traditional method, may be needed to clarify results in a substantial proportion of such patients.
PMID

Origins and evolution of Huntington disease chromosomes.
Neurodegeneration 1995 Sep;4(3):239-44
Andrew SE, Hayden MR-Biomedical Research Centre, University of British Columbia, Vancouver, Canada

Huntington disease (HD) is one of five neurodegenerative disorders resulting from an expansion of a CAG repeat located within the coding portion of a novel gene. CAG repeat expansion beyond a particular repeat size has been shown to be a specific and sensitive marker for the disease. A strong inverse correlation is evident between CAG length and age of onset.

Sporadic cases of HD have been shown to arise from intermediate sized alleles in the unaffected parent. The biochemical pathways underlying the relationship between CAG repeat length and specific cell death are not yet known. However, there is an increasing understanding of how and why specific chromosomes and not others expand into the disease range.

Haplotype analysis has demonstrated that certain normal chromosomes, with CAG lengths at the high range of normal, are prone to further expansion and eventually result in HD chromosomes. New mutations preferentially occur on normal chromosomes with these same haplotypes associated with higher CAG lengths. The distribution of different haplotypes on control chromosomes in different populations is thus one indication of the frequency of new mutations for HD within that population.

Analysis of normal chromosomes in different populations suggests that genetic factors contribute to expansion and account for the variation in prevalence rates for HD worldwide.

Expansion of a (CAG)n repeat region in a sporadic case of HD.

Acta Neurol Scand 1995 Aug; 92(2):132-4

Bozza A, Malagu S, Calzolari E, Novelletto A, Pavoni M, del Senno L. - Centro di Studi Biochimici delle Patologie del Genoma Umano, Universita, Ferrara

The genetic mutation underlying Huntington's disease (HD) has been identified as an expansion and instability of a specific CAG repeat sequence in a gene on chromosome 4. A simple polymerase chain reaction assay has been used for the assessment of the (CAG)n expansion in a 72-year-old woman with typical HD symptoms, but no family history of the disorder.

The DNA analysis showed that the patient had an allele with 41 repeat units, in the size range seen in HD chromosomes. Therefore, HD diagnosis is confirmed in this seemingly sporadic case and the disease is newly diagnosed in a large family. The risk of inheriting this unstable expanded allele is discussed.

INTRODUCTION

The discovery of an expansion of a trinucleotide (CAG) repeat region in the IT15 gene on the short arm of chromosome 4 has identified the mutational mechanism causing Huntington's disease (HD) and enables the direct diagnosis of affected subjects based on DNA analysis alone. Here a 72-year-old woman with typical HD symptoms, but no family history of the disorder, has been unambiguously diagnosed by using a quick DNA analysis. This is relevant because the disease is newly diagnosed in a large family.

MATERIAL AND METHODS

A labelled polymerase chain reaction (PCR) test has been used to amplify the repeat region of the IT15 gene and DNA fragments were analyzed by Polyacrylamide gel electrophoresis. RESULTS--The number the CAG repeats in the proband displayed two alleles of 23 and 41 repeats, respectively. Since normal chromosomes are reported to contain 11-34 repeats, the clinical appearance of HD in the proband is explained by the presence of the repeat expansion.

DISCUSSION

The parents of the proposita both died aged over 80 y apparently without neurological signs referable to HD. Hence, this is presumably a sporadic case of the disease. Because of the length of 41 repeats of this HD chromosome, offspring of this proband could inherit the expanded allele with 37 repeats, as expected for the reversal of the trinucleotide expansion.

A subject with this intermediate allele could be affected, but would not be affected if the HD IT gene with reduced triplets had recovered its normal function. Thus, in a seemingly sporadic case like the one reported here, despite the PCR analysis, the risk of transmission of HD to her offspring may remain uncertain.

Huntington chorea. Molecular genetic principles, mutation detection and predictive diagnosis]
Nervenarzt 1996 Jan;67(1):25-35
[Article in German]
Zuhlke C, Thies U. - Institut fur Humangenetik, Gottingen.

Predictive genetic testing offers the opportunity for persons at risk to obtain information about their carrier status concerning Huntington's disease (HD). HD is associated with the expansion of a (CAG) repeat in the gene IT15. HD chromosomes contain about 40-75 repeat units, whereas normal chromosomes show a range between 11 and 33 repeats.

The HD repeat is highly unstable during transmission, involving both increases and decreases in size with the largest expansions occurring in male meioses. The number of (CAG) copies is inversely correlated with the onset age of the disorder.

Investigation of sporadic cases revealed that new mutations for HD are more frequent than estimated. Analysis of the repeat length allows direct DNA diagnosis of affected individuals and asymptomatic persons at risk.

From 1989 to March 1993 in our institute predictive testing by indirect DNA methods was requested by 108 persons at risk. After cloning of the HD gene in 1993, direct testing was requested by 113 persons at risk between April 1993 and April 1994 in Gottingen. In about 400 patients differential diagnosis was performed with the direct method. In approximately 82% of the patients the expanded (CAG) repeat was found, and the HD diagnosis could be confirmed.

Reduced penetrance of the Huntington's disease mutation.
Hum Mol Genet 1997 May;6(5):775-9
McNeil SM, Novelletto A, Srinidhi J, Barnes G, Kornbluth I, Altherr MR, Wasmuth JJ, Gusella JF, MacDonald ME, Myers RH - Molecular Neurogenetics Unit, Massachusetts General Hospital, Harvard Medical School, Boston 02114, USA.

Controversy persists concerning the significance of Huntington disease (HD) alleles in the 36-39 repeat range. Although some clinically affected persons have been documented with repeats in this range, elderly unaffected individuals have also been reported.

We examined 10 paternal transmissions of HD alleles of 37-39 repeats in collateral branches of families with de novo HD. All 10 descendants, including many who are elderly, are without symptoms of HD. Forty percent of the transmissions were unstable, although none varied by more than one repeat.

The observation that individuals with alleles of 37-39 repeats may survive unaffected beyond common life expectancy supports the presence of reduced penetrance for HD among some persons with repeat sizes which overlap the clinical range.

Non-penetrance may be increased in the collateral branches of de novo mutation families when compared to penetrance estimates from patient series. There was no CAA-->CAG mutation for the penultimate glutamine in either a de novo expanded 42 repeat allele or the corresponding non-penetrant 38 repeat allele in a family with fresh mutation to HD.

Comparative semi-automated analysis of (CAG) repeats in the Huntington disease gene: use of internal standards.
Mol Cell Probes 1999 Aug;13(4):283-9
Williams LC, Hegde MR, Herrera G, Stapleton PM, Love DR - School of Biological Sciences, The University of Auckland, Auckland, New Zealand
Huntington disease (HD) belongs to the group of neurodegenerative disorders characterized by unstable expanded trinucleotide repeats. In the case of HD, the expansion of a CAG repeat occurs in the IT15 gene. The detection of the expanded CAG repeats has usually involved the electrophoretic separation of polymerase chain reaction (PCR) amplification products using conventional agarose and acrylamide gel electrophoresis.

We have undertaken the comparative analysis of sizing CAG repeats of the IT15 gene using radioactive and fluorescent PCR amplification, and the subsequent separation of these products by slab gel and capillary electrophoresis. The assays have been performed on both cloned and sequenced CAG repeats, as well as genomic DNA from HD patients with a wide range of repeat lengths. The mobility of the CAG repeat amplification products of the IT15 gene is greater using capillary electrophoresis compared to slab gel electrophoresis.

The analysis of 40 DNA samples from HD patients indicates that the mobility difference increases with the length of the repeat. However, we have devised an allele ladder for sizing the CAG repeats. This ladder provides a mandatory internal calibration system for diagnostic purposes and enables the confident use of either capillary or slab gel electrophoresis for sizing HD alleles..

Null alleles at the Huntington disease locus: implications for diagnostics and CAG repeat instability.
Genet Test 2000;4(1):55-60
Williams LC, Hegde MR, Nagappan R, Faull RL, Giles J, Winship I, Snow K, Love DR.School of Biological Sciences, University of Auckland, New Zealand

PCR amplification of the CAG repeat in exon 1 of the IT15 gene is routinely undertaken to confirm a clinical diagnosis of Huntington disease (HD) and to provide predictive testing for at-risk relatives of affected individuals.

Our studies have detected null alleles on the chromosome carrying the expanded repeat in three of 91 apparently unrelated HD families. Sequence analysis of these alleles has revealed the same mutation event, leading to the juxtaposition of uninterrupted CAG and CCG repeats.

These data suggest that a mutation-prone region exists in the IT15 gene bounded by the CAG and CCG repeats and that caution should be exercised in designing primers that anneal to the region bounded by these repeats. Two of the HD families segregated null alleles with expanded uninterrupted CAG repeats at the lower end of the zone of reduced penetrance. The expanded repeats are meiotically unstable in these families, although this instability is within a small range of repeat lengths.

The haplotypes of the disease-causing chromosomes in these two families differ, only one of which is similar to that reported previously as being specific for new HD mutations. Finally, no apparent mitotic instability of the uninterrupted CAG repeat was observed in the brain of one of the HD individuals.