Tag Archives: genetics

The Genetics Behind Huntington’s

Huntington Disease is a rare genetic condition that most people have never even heard of unless a) they study it b) they personally know someone with the disease or c) they are a fan of House. Luckily for me only a) and c) apply in this situation. However, I believe Huntington’s like a variety of other diseases is something the public needs to be educated about because awareness really is the greatest way to inspire research into any field.

As autosomal dominant disorder this makes Huntington’s especially dangerous because as a dominant trait a person only needs one affected allele to develop the disorder. Were the trait recessive such as the trait for hemophaelia, for example, then the likelihood of having Huntington’s is significantly lowered. When a trait is autosomal this means that it is no carried by any of the sex chromosomes (X or Y), rather is carried by any of the other 22 chromosomes the human body has. In Huntington’s the gene affected is located on chromosome 4, specially on the p (upper, shorter) arm.

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The symptoms of Huntington’s has already been discussed on a previous post on basal ganglia disorders; however, in summation it results in damage to the striatum and cerebral cortex causing changes in personality including mood swings, involuntary movements known as hypokinesia and eventually dementia. As is common with most genetic disorders, the symptoms do not appear until adulthood. In Huntington’s the symptoms usually arise around mid-age, but unfortunately it can arise earlier than our 30s or 40s if unlucky. Once symptoms start appearing the person usually has about another 5 to 15 years until death. The age at which symptoms appear directly correlates with the genetics behind the abnormal gene.

Huntington’s is part of numerous diseases including varies ataxias and fragile X syndrome that result due to trinucleotide repeat. Specifically, Huntington’s is due to a repeat of the CAG trinucleotide. Normal alleles carry about 10 to 35 copies, but those suffering from Huntington’s and various other neurodegenerative diseases have more than 40 repeats. People with around 60 repeats with develop Huntington’s around the age of 20. These repeats in CAG result in the production of a “mutant protein” that eventually fill the striatum and cerebral cortex causing degeneration and ultimately death of these brain cells. In healthy individuals the gene involved in Huntington’s encodes for a large protein known as huntingtin (Htt), which when normal enhances the production of a protein (BDNF) necessary for the survival of the cells in the striatum and cerebral cortex.

Stay tuned for a post later this week on current experimental treatment on Huntington’s! Thank you for reading :)

Citation:

Cummings, Michael. “Genetics of Behavior.” Human Heredity: Principles and Issues. 9th ed. Belmont: Brooks/Cole, 2011. 405-06. Print.

Low Functioning Autism’s Genetic Roots

I am currently taking at genetics course at Duke University, hence the inspiration for my last post. One of my assignments also looked at a correlation between genetics and autism I thought I would share it on here! I hope you enjoy reading :)

A new study published in the American Journal of Human Genetics has found evidence suggesting a recessive genetic component to low-functioning autism. Professor Eric Morrow and his colleges of Brown University analysed the DNA of over 2,100 children born with autism into “simplex” families, families where only one child suffers from autism and no other immediate family (Brown University, 2013). Morrow et al. investigated the genomes of the autistic children and their siblings for “runs of homozygosity”; runs of homozygosity refer to long strands of DNA that are contributed to a child’s DNA by both parents (ibid). Morrow and his colleges discovered that the children with low-functioning autism had longer runs of homozygosity compared to the DNA of their siblings. Normally, as humans we do share large blocks of DNA sequences; nearly all the participants had at least 1 million letters in common. In fact, about 1/3 of all the participants had about 2.5 million shared letters, which translates to a “shared common ancestor approximately 40 generations or 1,000 years ago” (ibid). However, in 500 of the participants, where the autistic children had an IQ below 70, the runs of homozygosity surpassed even the 2.5 million mark and the runs of their siblings. The original article, titled “Intellectual Disability Is Associated with Increased Runs of Homozygosity in Simplex Autism” stresses that increased runs of homozygosity do not predict low-functioning autism; yet, a greater number does increase the likelihood of carrying the shared “recessive variants” necessary for developing low-functioning autism (Morrow et al., 2013). Basically, Morrow et al. concluded that the longer runs of identical DNA lettering meant that a child was far more likely than other children, even their own siblings to inherit rare genetic traits.

These findings are relevant to the course as they illustrate that a pedigree for low-functioning autism exists even if the carried recessive trait is extremely illusive in simplex families. Unfortunately, the whole picture of the genetics behind autism is still a bit of a blur; however, the established correlation between low-functioning autism and runs of homozygosity does suggest that inheritance does play a role and that individual errors are far less likely to be the only possible perpetrators of autism.

The role of inheritance in low-functioning autism is a breakthrough considering the elusiveness of the disorder. Currently, most genetic explanations for low-functioning autism focus on “spontaneous mutations and having too many or too few copies of a gene”; however, in some families these hypotheses fail to explain their particular case of autism (Brown University, 2013). Runs of homozygosity at least allows for parents to look somewhere new for answers. In fact, if a test is run on the child at an early age, high runs of homozygosity might help health professionals guide the development of the child by focusing treatment on the particular associated issues such as problems with speech and social interaction. With all genetic testing it seems people are weary; however, given that autism is very difficult to diagnose without symptoms, I feel that testing for runs of homozygosity in the early stages of a diagnosis can only be beneficial for parents trying to understand their child’s diagnosis. Furthermore, continued genetic testing on children from simplex families can only increase the likelihood of fine-tuning the genetic component of low-functioning autism.

Citation:

Brown University (2013, July 3). DNA markers in low-IQ autism suggest       heredity. ScienceDaily. Retrieved July 4, 2013, from            http://www.sciencedaily.com­/releases/2013/07/130703140236.htm

Assignment: Part of Biology 156 at Duke University