The Genetics and Molecular Biology of Huntington’s Disease - PowerPoint Presentation

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The Genetics and Molecular Biology of Huntington’s Disease

Son Dong, Justin Wise, Niket Yadav

History

Confusing disease

in mid

1600sHad different names based on symptomsFirst mention was in 1842Major discoveries occurred after: Charles Gorman, Johan Lund

George Huntington

*Wrote

first thorough description of HD in 1872

*First paper ever published*Considered to be most accurate description of a disease ever

History

*Sir

William Osler was impressed by Huntington’s paper

*HD awareness spread rapidly through Europe*By end of 18th century, most countries had research published on HD

History

*The

rediscovery of Mendelian Inheritance

*Scientists were able to discover source of HD in US*Pedigrees were constructed to discover more aspects of HD

Huntington’s Research Today

Hereditary Disease Foundation (HDF)

Developed DNA marking methods

Discovery of proteins involved with disease has led to massive improvements in potential drug treatments

HTT(Huntingtin) gene

•HTT gene provides instruction for making huntingtin protein.

•The exact function of this gene is unknown. It appears to play important role in neurons

•It can be found in many tissues, with the highest levels of activity in brain.Mutation in HTT gene causes Huntington's Disease

HTT(Huntingtin) gene

•

HTT gene contains DNA segment known as a CAG trinucleotide repeat. In its normal form, the CAG segment is repeated 10-35 times within a gene.

•People with HD have more than 36 CAG repeats. The severity of HD depends on the number of repeats.•People with adult-onset form of HD have 40-50 CAG repeats,while people with juvenile form of disease usually have more than 60 CAG repeats

Inheritance

HD is inherited in an autosomal dominant

pat

tern.Children of HD gene carriers have a 50% chance of inheriting the gene, and because penetrance is full.

As the altered HTT gene is passed from one generation to the next, the size of CAG trinucleotide repeats often increases .

Individuals with 27-35 CAG repeats in the HTT gene do not develop the disorder but their children might be affected.

Genetics testing

Using a blood sample, the genetic test analyzes DNA for the HD mutation by counting the number of CAG repeats in the huntingtin gene.

Prenatal testing can show whether the child will inherit the defective gene. To test the fetus, DNA is extracted from fetal cells. If the test is positive, parents can decide about whether terminate the pregnancy.

Genetics testing

The HD genetic test is widely available, and can be ordered as a clinical diagnostic procedure by sending a blood specimen to one of the many DNA diagnostic laboratories in North America.

Of the adults at risk for HD, approximately 5% to 7% have been tested

DNA testing companies in Ohio : Huntington's Disease clinic,University hospital of Cleveland, Children's Hospital,MetroHealth medical center.

Molecular Biology of HD-Background

*Exon 1 of the HD gene contains a segment of uninterrupted CAG trinucleotide repeats, which is translated into a polyglutamine tract in the

huntingt

in protein. -mouse studies have shown that CAG/polyglutamine mutations lead to deleterious functions on mutant HD proteins (Ordway et. al. 1997)*Asymptomatic individuals have 35 or less repeats, while symptomatic individuals have greater than 35 of these repeats (Rubinsztein

et. al

1996).

*

Inverse relationship

between # of CAG repeats and the age of symptom onset. In other words, a higher number of CAG repeats is associated with symptoms at an earlier age.

-Adult onset: 40-50 repeats

-Juvenile onset: > 55 repeats

-Mixed cases: 36-39 repeats; incomplete penetrance; some adults with this many repeats show no signs or symptoms of HD

MB: Experimental Evidence

*Novel mouse study by Bates

et. al.

in 1996 helped to elucidate the molecular pathology of HD: -Researchers successfully created a transgenic mouse that expressed exon 1 of the human HD gene (with differing numbers of CAG repeats)

-

Mice

expressing 18 repeats developed normally and were

healthy

-

Mice

expressing 113-156 repeats underwent progressive neurological

degeneration

-

Similar

results as those in humans: after crossing a threshold number of CAG repeats, the disease becomes degenerative. This implies that similar molecular mechanisms involved in neurodegeneration may be at play in humans and mice.

The Wild Type Huntingtin Protein

*Wild type

Huntingt

in protein: -Laboratory techniques have shown that the normal huntingt

i

n

protein binds to numerous proteins, especially in the brain

.

-Addition of an abnormally long polyglutamine tail to the protein can disrupt its binding affinities to accessory proteins, leading to many of the symptoms associated with HD

.

-Studies have shown the wild-type

Huntingt

i

n

protein to have a broad-spectrum of functions. These include:

-protein trafficking

-postsynaptic signaling in neurons

-regulation of gene transcription

-anti-apoptotic function (preventing cell death)

Molecular Mechanisms of Neurodegeneration

*Aggregates of the mutant HD protein can lead to neurodegeneration

-neuronal pathways degenerate

-degradation of regions of the cerebral cortex (involved in higher brain functions) and other brain regions.*Although the mutant HD protein is involved in neurodegeneration, there are a number of molecular mechanisms that do not directly invoke the HD protein

, but nevertheless lead to eventual

neurodegeneration

:

-metabolic impairment

-mitochondrial dysfunction

-oxidative stress

-apoptosis

***These mechanisms are not necessarily mutually exclusive, but may synergize with each other!

Some Mechanisms in more detail

*Metabolic dysfunction:

-significant decrease in glucose uptake in the cerebral cortex and striatum of brain of both presymptomatic and postsymptomatic HD patients

-significant reduction of aconitase (an enzyme) activity in cortex and striatum: indicator of reactive oxygen species (ROS) generation -significant decrease in activity of striatum mitochondrial complexes*Apoptosis:

-controversy regarding the role of apoptosis in HD pathology

-in general, it is agreed that apoptotic mechanisms influence the onset of HD symptoms to some degree, but the degree of influence is debatable.

Future Directions Editing of the human

huntingtin

gene to eliminate excess CAG repeats

-technology such as CRISPR/Cas?Which mechanism contributes most to neurodegneration -Synergistic effects?

References

Walker, Francis O. "Huntington's disease."

The Lancet

369.9557 (2007): 218-228.Myers, R. "Huntington's Disease Genetics." NeuroRx: 255-62.

Carmichael, L.W.HO, Swartz J., Wyttenbach A, Rankin J, Rubinsztein D.C. (2001). “The Molecular Biology of Huntington’s Disease.”

Psychological Medicine

: 31: 3-14

Gil J.M., Rego A.C. (2008). “Mechanisms of Neurodegeneration in Huntington’s Disease.”

European Journal of Neuroscience

: 27: 2803-2820.

Ordway, J. M., Tallaksen-Greene, S., Gutekunst, C. A., Bernstein,E. M., Cearley, J. A., Wiener, H. W., Dure, L. S. 4th, Lindsey, R.,Hersch, S. M., Jope, R. S., Albin, R. L. & Detloff, P. J. (1997).Ectopically expressed CAG repeats cause intranuclear inclusions and a progressive late onset neurological phenotype in the mouse. Cell 91, 753–763.

References...

Rubinsztein, D. C., Leggo, J., Coles, R., Almqvist, E., Biancalana,V., Cassiman, J.-J., Chotai, K., Connarty, M., Craufurd, D.,Curtis, A., Curtis, D.,Davidson, M. J., Differ, A.-M., Dode, C., Dodge, A., Frontali, M., Ranen, N. G., Stine, O. C., Sherr, M., Abbott, M. H., Franz, M. L., Graham, C. A., Harper, P. S., Hedreen, J. C., Jackson, A., Kaplan, J.-C., Losekoot, M., MacMillan, J. C., Morrison, P., Trottier, Y., Novelletto, A., Simpson, S. A., Theilmann, J., Whittaker, J. L., Folstein, S. E., Ross, C. A. & Hayden, M. R. (1996). Phenotypic characterisation of individuals with 30–40 CAG repeats in the Huntington disease (HD) gene reveals HD cases with 36 repeats and apparently normal elderly individuals with 36–39 repeats. American Journal of Human Genetics 59, 16–22

.

Mangiarini

L,

Sathasivam

K, Seller M, Cozens B, Harper A, et. al. (1996). “ Exon 1 of the HD gene with an expanded CAG repeat is sufficient to cause a progressive neurological phenotype in transgenic mice.”

Cell

. 87: 493-506.

Questions?