Download The Genetics and Molecular Biology of Huntington*s Disease

The Genetics and
Molecular Biology of
Huntington’s Disease
SON DONG, JUSTIN WISE, NIKET YADAV
History
Confusing disease in mid 1600s
Had different names based on symptoms
First mention was in 1842
Major 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 pattern.
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 huntingtin 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 Huntingtin protein:
-Laboratory techniques have shown that the normal huntingtin 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 Huntingtin 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?