¶ … Genetics Case Study Part II: Genetics

Describe if chromosomal analysis is/was indicated.

Huntington disease, also known as HD [MIM 143100] is dominantly inherited gradually neurodegenerative disorder. It is caused by a mutation; which leads to the expansion of the CAG or polymorphic trinucleotide HTT tract. Normatively, the size of the control CAG among ordinary people should be between 17 and 20 repeats. In HD patients, 1-2 duplicate genes have an expanded GAC tract to at least 36 repeats (Kremer, et al., 1994).The polymorphic trinucleotide tract size can be uneven and is more likely to expand, especially if it is passed on by a male germline. At first, the new rate of mutation for Huntington disease was estimated to be extremely low. The illness only affected those families that had history with HD. Current estimates have discovered that the expansion of CAG into the illness range has become more rampant than predicted earlier. The new rate of mutation may be at least 10% (Warby, et al., 2009).

Many factors are believed to cause CAG instability, such as CAG tract size, interruptions of the CAG tract, age and sex of the parent transmitting it, environmental factors and genetic trans-factors and cis-elements. Though a bigger CAG tract and transmission by a male germline are clearly seen to cause high CAG instability, such trans-factors as machinery for DNA repair are also said to be major contributing factors. For example, the CAG unsteadiness of transgenic mice with HD was saved after it was crossed with other mice lacking MSH2 (MIM 609309) (enzyme for mismatch repair) or OGG1 (MIM 601982) (repair enzyme for base excision). Though it is believed that cis-elements modify CAG unsteadiness in different genes, previous information has stated that cis-elements has no role in CAG unsteadiness in HTT (the HD gene) (Warby, et al., 2009).

Many studies have examined the HD origins coming up with constructing haplotypes meant for the HTT area in specific tribal populations....

This is because besides the CAG, few HTT polymorphisms have been characterized before. Most of the studies discovered positive connections between disease chromosomes and specific markers. They concluded that mutation of the HD comes from the same descent, but not automatically from one founder (Warby, et al., 2009).
Detail the causes of the disorder.

HD is a dominantly hereditary autosomal disease whose cause is a long CAG repeat found on the shorter chromosome 4p16.3 arm within Huntingtine genes. These genes have huntingtin protein codes and have GAC tracts in exon 1. The wild-type has a repeat of the CAG repeat, with polyglutamine stretch coding in the available protein between 6 and 16. The disease is linked with at least 36 repeats. Definite clinical appearance occurs if the repeats exceed 40. A range of 36-39 causes incomplete entrance of Huntingtons disease or leads to late onset. At 29-35, there is instability of the intermediate alleles; hence, the alleles are likely to change when reproduction takes place. Duplicating the gene could cause mistakes and in most cases, elongation takes place. The cases of shortening are few. This occurrence mainly takes place in male reproduction (Roos, 2010).

The life span of a Huntington's disease patient can be categorized into 3 stages: at-risk, A (preclinical) and B (clinical). The first phase; at-risk, ends after it is established if the patient has the increased repeat on CAG on the fourth chromosome. If the patient has the gene, they will be in stages A and B. until the end (Roos, 2010).

Describe the disorder in terms of its origin as either a single gene inheritance, or as a complex inheritance and considerations for practice and patient education.

The interaction between symptomatology (chorea/rigidity) in HD and onset age was studied using information gathered from the Hungtington's Disease Victims and Families Research Roster. The research showed that the onset age is different among families and between maternal and paternal transmission. It was also found that rigidity is specifically linked with onset at a very young age, paternal transmission, young parental onset and major anticipation. It is suggested that onset age is reliant on the condition of the disease's locus methylation, which is distinct among familial traits. It is caused by 'genomic imprinting', which depends on parental transmission. Early familial onset age and male parental imprinting interact and occasionally produce a great change in expression of the gene;…