Huntington’s disease (HD) is a hereditary condition transmitted as an autosomal dominant trait during conception. It is a movement disorder that causes neurologic and behavioral symptoms that typically become evident from 30 to 50 years and aggravate in the next one to two decades of a person’s life (McColgan & Tabrizi, 2018). It is approximated that 30,000 individuals in the United States (US) have HD, and another 20,000 to 50,000 are assumed to carry the gene. Males and females are equally affected at a time in their lives when they are highly productive. HD usually causes chorea, neuropsychiatric symptoms, and dementia during middle age, and most patients ultimately require institutionalization (McColgan & Tabrizi, 2018). The purpose of this paper is to discuss the chromosomal analysis in HD, causes, and gene mutation.
The direct test for the HD gene involves cysteine-adenosine-guanine (CAG) analysis and repeat length. The chromosomal analysis enables healthcare providers to offer genetic counseling and psychological support services that facilitate predictive testing in a timely, sensitive, and informed fashion (Goldman et al., 2021). Indications for chromosomal analysis in HD include predictive testing in an asymptomatic person at risk for carrying the HD gene to confirm a suspected HD diagnosis and for prenatal diagnosis and preimplantation genetic diagnosis (PGD). The common reasons for predictive testing include making plans on marriage, reproduction, finances, and the need to alleviate uncertainty (Goldman et al., 2021). However, the choice to undergo a predictive test chromosomal analysis for HD must always be informed, deliberated, and freely chosen.
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Confirmatory testing by chromosomal analysis of the HD gene is indicated at or following a clinical diagnosis of HD. A CAG replicate expansion in a patient with HD symptoms validates the clinical impression and supports HD diagnosis. In prenatal diagnosis, Chorionic Villus Sampling (CVS) and amniocentesis indicate if the parent is at risk or is positive for the HD gene (Garrett et al., 2019). CVS is performed from 10-12th week gestation, while amniocentesis is done from 14th to 20th week. Furthermore, the PGD test is conducted on a single cell obtained through a needle biopsy from the eight-cell embryo. The chromosomal analysis is carried out on the DNA from the single-cell, facilitating the detection of the HD replicates sizes for the specific embryo. It is worth noting that children should not undergo chromosomal analysis for HD except if there is a medically convincing reason, like a clinical diagnosis or a strong clinical suspicion of HD (Garrett et al., 2019). In these circumstances, the chromosomal analysis should come after a thorough neurological and neuropsychological examination.
HD is attributed to selective dysfunction of the neurons and ensuing neuronal in the cerebral cortex, striatum, and other brain regions. It is attributed to the elongation of CAG replicates on the short arm of chromosome 4p16.3 in the Huntingtin (HTT) gene. The mutation results in an unusually long expansion of the polyglutamine in the HTT protein, resulting in neurodegeneration (Ghosh & Tabrizi, 2018). The HTT protein’s gene encodes are involved in synaptic function and have a major role in the post-embryonic period. Besides, it is supposed to have anti-apoptotic functions and protects against the toxic mutant HTT. Some evidence shows that the mutant protein causes an addition and a loss of function.
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The HD gene is evident from conception and is inherited in an autosomal dominant manner. This means that every offspring of an affected parent, regardless of sex, has a 50% probability of inheriting the HD gene. HD is a single gene disorder attributed to a mutation in the HD gene (IT15) on chromosome 4 (Ghosh & Tabrizi, 2018). This causes unusual replication of the DNA sequence CAG, which normally codes for the amino acid glutamine. It results in a large protein referred to as huntingtin, which has an extended stretch of polyglutamine residues that build up within neurons contributing to HD through unknown mechanisms. The more CAG replicates, the earlier the onset of HD and the more acute its expression (Ghosh &
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