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Preventing a Genetic Inherited Disease: Huntington's

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The Genetics of Huntington’s Disease

Huntington’s disease is an autosomal dominant neurodegenerative disorder whose symptoms can begin at a variable age, depending on the severity of the gene mutation. People who have the disease can show signs of the disorder at as early on as childhood1. Due to the late onset of this disease, it is worthwhile to begin screening of this monogenetic disease at a young age, especially if there is a family history of the disorder2. In some cases, while parents who carry the dominant diseased gene might not yet show signs of the disease, it can still pass be passed on to their offspring1. Advances in technology have enabled us to use cell-free fetal DNA to determine the parental genetic contributions of these single-gene disorders, like Huntington’s, for early screening and detection2.

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Huntington ’s disease is caused by the excessive repetition of a trinucleotide CAG in the coding regions of the Huntingtin gene (HTT) on chromosome 41. Normally, the gene codes for the Huntingtin protein (Htt) using a set length of CAG codons repeats1. However, longer than normal repeats associated with this gene shows a rather high level of mutation in the protein product (mHtt), which causes disease symptoms1. Due to the variability in the age of onset for this disease, one would normally not be able to determine when the symptoms would show just by simply inheriting the diseased gene. Studies have shown that there is a high correlation between the length of the nucleotide repeats and the age of onset3. DNA from 1,050 affected patients were examined and categorized by their age of onset for the disease3. It was found that the longer the repeat length of the CAG expansion in the diseased gene, the younger the patient would be to show symptoms of the disease3. Huntington’s is said to exhibit variable penetrance, as it is not enough just to inherit the gene, however, the length of the trinucleotide repeats would determine when symptoms of the disease would show itself, if ever1.

Since the gene is inherited from a diseased parent, one would assume that the length of the trinucleotide mutation in the inherited gene would be roughly the same as in the parental gene, which would result in roughly the same age of onsets from the parent to the child. However, genetic analysis of families who have Huntington’s have found that while the age of onset of the offspring of affected mothers did not differ much from their mother, the age of onset of the offspring of affected fathers was about 20 years younger than the age of onset for father, on average4. Since the age of onset for offspring who got the gene from their father will keep getting younger, the reproductive success in males who have Huntington’s will decrease dramatically in the population, affecting both the reproductive rate and the life expectancy of the young males with the disease4.

Due to the nature of the dominant disease, prevention is the best solution so far to combat the spreading of this disease to future generations1. The genetic nature of Huntington’s, being a single-gene defect allows direct genetic testing for the disease, especially predictive and prenatal testing, which is the best method to assist in making informed choices and to minimize offspring risk of inheriting this disease2.

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