FINAL DIAGNOSIS:
EXPANDED HUNTINGTIN ALLELE WITH 39 CAG REPEATS, CONSISTENT WITH HUNTINGTON DISEASE. THIS RESULT WAS CONFIRMED BY REPEAT ANALYSIS IN A SUBSEQUENT ASSAY.
DISCUSSION:
Huntington disease (HD) is an autosomal dominant disorder affecting about 1 in 20,000 individuals in Western populations. The average age of onset is during the fifth decade of life, although penetrance is high (virtually 100 % by age 80). Symptoms consist of personality changes, memory loss, and peculiar series of motor problems including chorea.
HD was the first genetic disorder to be mapped through a systematic screening of polymorphic genetic markers for linkage. The linkage of the G8 marker (mapped to the tip of the short arm of the chromosome 4) to HD was first reported in 1983. It took another 10 years to complete the positional cloning process leading to the identification of the HD gene, Huntingtin, in 1993 and characterization of the causative genetic mutation.
HD results from expansion of an unstable CAG triplet-repeat, encoding for a polyglutamine tract in exon 1 of the IT15 gene, on chromosome 4p16.3 (1). The CAG expansion is thought to result in a toxic "gain of function" in the huntingtin protein. Huntingtin is widely expressed throughout the brain, but only certain subsets of neurons are vulnerable to its dysfunction. The characteristic pathologic change is dramatic neuronal loss in basal ganglia of the brain, especially the caudate nucleus. Electron microscopy reveals cytoplasmic and nuclear abnormalities, including large inclusions or aggregates composed of amino-terminal fragments of mutant huntingtin protein.
Expansion of the HD CAG triplet repeat in our laboratory is detected directly by PCR amplification and sizing of resultant fragments. Two parallel PCR reactions are used to amplify the area encompassing the trinucleotide repeat region. A fluorescently-labeled common 5' primer, upstream of the CAG repeat region is paired either with a primer located just downstream of the CAG region to produce a 'short' fragment (Figs. 1) or with a second primer located further downstream beyond a short CGG polymorphic region to produce a 'long' fragment. Fragments are sized automatically on an ABI 377 automatic sequencer or ABI 3100 capillary electrophoresis instrument using Genescan analysis.
Interpretation is based primarily on allele size detected by PCR juxtaposed with clinical information. Huntingtin CAG repeat sizes range from 10-30 copies in normal chromosomes. The presence of 36 or more CAG repeats results in a likely disease-causing allele (2). Individuals with a CAG repeat size of 30 to 35 do not manifest HD themselves; however, there is a small risk that offspring may develop HD due to further CAG expansion, especially in paternal transmissions. CAG expansions in the range of 36 to 40 repeats show incomplete penetrance, and this interval is considered a "gray zone" or borderline range. In one large study, all individuals who demonstrated 42 or more CAG repeats developed disease (Table 1). There is a general inverse correlation between the length of the CAG repeat and the age of onset of symptoms (Table 2), although these statistics are not precise enough to be used clinically due to other modifying factors. Examples of such factors include: the sex of the affected parent and grandparent (paternal inheritance leads to a lower age at onset than maternal inheritance), the sex of the patients themselves, symptoms of persons within the families and other unknown factors (3, 4). Juvenile forms of HD are associated with alleles containing more than 70 repeats, which in some cases exceed 100. Studies show that homozygosity for expanded Huntingtin alleles does not lower the age of onset of symptoms but affects the phenotype and rate of disease progression (5).
Interpretation of results for our patients is based on literature findings combined with the clinical impression. A Canadian study (6) developed age-dependent likelihood of onset curves which differed significantly for CAG repeat lengths in the 39-50 range. As illustrated in Table 1, 8 of 9 patients with 39 CAG repeats developed disease before the age of 75 or 80 years (males and females). Patients with 39 repeats generally developed disease at an older age (See table 2). Because there are some individuals with 39 repeats who do not develop HD, this is considered the upper limit of the borderline range for Huntington disease.
The presence of longstanding Behcet's disease in our patient raises the question whether neurologic symptoms could result instead from this disorder. Behcet's disease is a chronic, relapsing, inflammatory condition characterized by recurrent oral aphthae and any of several systemic manifestations including genital aphthae, ocular disease, skin lesions, neurologic disease, vascular disease, or arthritis. It is considered to be a multisystem vasculitis. Neurologic disease occurs in less than one-third of patients and can manifest as aseptic meningitis or encephalitis, peripheral neuropathy, cranial nerve palsies, and focal deficits such as paralysis or ataxia. Progressive personality changes, psychiatric disorders, and dementia also may be seen.
The case features a patient clinically symptomatic for HD where sizing of the huntingtin gene region indicated a CAG repeat length at the upper limit of the borderline range for diagnosis of HD. The combination of these two factors was felt to be most important although the patient is also known to suffer from Behcet's disease, which in one third of patients can involve the CNS and produce clinical symptoms resembling Huntington disease. The case is unusual because of the relatively early onset of symptoms (age 47) for most patients with 39 CAG repeats.
|
CAG Repeat Size |
No. of Affected Individuals |
No. of unaffected Individuals, Age>75 Years (Males) or >80 Years (Females) |
Total |
|
<29-35 |
0 |
9 |
9 |
|
36 |
1 |
2 |
3 |
|
37 |
4 |
0 |
4 |
|
38 |
2 |
1 |
3 |
|
39 |
8 |
1 |
9 |
|
40 |
64 |
1 |
65 |
|
41 |
74 |
1 |
75 |
|
>41 |
575 |
0 |
575 |
TABLE 2:
|
CAG Repeat Size |
Median Age at Onset (95% CI) (years) |
|
39 |
66 (72-59) |
|
40 |
59 (61-56) |
|
41 |
54 (56-52) |
|
42 |
49 (50-48) |
|
43 |
44 (45–42) |
|
44 |
42 (43-40) |
|
45 |
37 (39-36) |
|
46 |
36 (37-35) |
|
47 |
33 (35-31) |
|
48 |
32 (34-30) |
|
49 |
28 (32-25) |
|
50 |
27 (30-24) |
REFERENCES:
Contributed by Laurentia Nodit, MD, Melina Flanagan, MD, MSPH and Jeffrey Kant, MD, PhD
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