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THE GENETICS OF HUNTINGTON’S DISEASE
HD was long known to be a genetic condition. The disease would
run in families and appeared to be passed directly from parent
to child, without skipping generations or showing a preference
for affecting one sex more than the other. This pattern of
inheritance is known as autosomal dominant. All genes (except
for those found on X chromosome of males) occur in two copies
within cells, i.e., one copy inherited from the father and
the other copy from the mother. To cause an autosomal dominant
disorder, only one of the two copies have to be mutated, and
the mutated gene must have come from one of the parents. The
chance of an affected parent passing the mutant gene to each
child is 50%. Either females or males can inherit the gene
from their mother or father. If a child inherits the normal
copy of the gene from the affected parent (also a 50% chance),
then the child should not develop the disorder, and will also
not pass on the condition to his or her children. If a child
inherits the mutated copy from the affected parent, then the
child can pass on the mutated gene to each of his or her child
with 50% chance.
Generally, all individuals who inherit a mutant HD gene will
develop signs and symptoms of the condition, if they live long
enough. There are rare exceptional cases, in which the mutant
gene may not manifest clinical or histological findings of
HD (see mutation in the “reduced penetrance range” in
the next paragraph). The age at onset can vary widely among
individuals, even within the same family. When an affected
father passes the diseases to his children, the age of onset
tends to be earlier in the children. This phenomenon is called "anticipation."
The mutation that causes HD is an expansion of a part of the
HD gene. The expanded part consists of tandemly repeated CAGs,
which is a genetic code for amino acid glutamine. In normal
individuals, the repeat contains less than 29 CAGs in both
genes (i.e., one inherited from the father and the other from
mother). However, HD patients have one gene with a normal number
of CAGs (<29), and the other with an expanded number (>35)
of CAGs. Occasionally, the CAG repeat length exceeds 70 and
it could expand beyond 100 in some cases. Individuals with
36 to 39 CAG repeats do not always develop HD during their
expected life span. Thus, this range is considered as the "reduced
penetrance range". Individuals with 29 to 35 CAG repeats
have never been documented with HD. However, some of these
individuals may produce children with the disease. Expanded
CAG repeats generally show repeat size instability when they
are passed on from one generation to the next. In some normal
individuals who have the CAG repeat in the range of 29-35,
the CAG repeat occasionally expand to the range >39 CAGs
in the children, who would develop HD. Therefore, 29 to 35
CAG repeats are considered "mutable normal" range.
The CAG repeat of this range can be found in a small fraction
of the normal population, and appears to play a role of reservoir
for the new cases of HD. The large mutant repeats with the
number of CAG s >70 are mostly found in children with juvenile
HD who had received the mutant HD gene from the affected father.
Since the CAG repeat size shows an inverse correlation with
the age of onset, this intergenerational expansion of the CAG
repeat size provides the molecular basis of anticipation. The
CAG repeats within the mutable normal range expand to >39
repeat only when the father is the transmitting parent. Thus,
the gender of transmitting parent is an important determinant
for the stability of the CAG repeat in HD.
With the identification of the HD gene, there has been a major
advance in predictive testing for HD. Direct testing for the
CAG repeat length can not only identify individuals who will
develop the disorder, but it also can identify those who have
not inherited the mutant gene from a parent. In addition, direct
mutation analysis of the CAG repeat length can be used for
prenatal testing for Huntington's disease, if parents choose
this as an option.
MECHANISMS THAT CAUSE BRAIN CELL DAMAGE IN HD
Involuntary movements, such as chorea, result from abnormalities
in the structures called basal ganglia, which are located deep
in the brain and regulate motor movements. One of these structures
called striatum shows a decreased volume in HD. The atrophy
is due to degeneration of a particular subpopulation of the
neurons (brain cells with electrical activities) called medium-size
spiny neurons located within the striatum. Dementia and psychiatric
abnormalities are due to degeneration of neurons outside the
basal ganglia. A loss of neurons in the cerebral cortex (the
surface layers of the brain) is particularly prominent in HD.
The mechanism of the degeneration is not fully understood.
However, the final process of brain cell death appears to be
mediated by a class of amino acids (called excitatory amino
acids) released from other neurons in which excessive excitation
of neurons causes "exhaustion" of the neurons and
eventually leads to cell death, especially when the neurons
already suffer from a disease process. This phenomenon is called "excitotoxic
cell death." Injecting chemical compounds that activate
the excitotoxic receptors into animals produces a selective
loss of neurons closely resembling HD. Biochemical changes
and behavioral changes in these animals are also very similar
to HD. These studies suggest that excitotoxic cell death may
play an important role in HD.
The energy metabolism appears to be affected in the medium-size
spiny neurons in the striatum in HD patients. The abnormal
energy metabolism quickly "exhausts" the neurons,
making them susceptible to excitotoxicity. Mitochondria are
the power house of cells and produce energy by a chain of chemical
reactions called oxidative phosphorylation. Inhibitors of oxidative
phosphorylation such as 3-nitropropionic acid (3NP) cause a
disease with similar clinical signs and increased lactate levels
similar to those seen in HD. Interestingly, 3NP causes damage
specifically in the medium size spiny neurons and, consequently,
in the cortical neurons. Thus, abnormal energy metabolism may
contribute to the disease-causing mechanisms in HD.
The key question is how the genetic abnormality in HD can
lead to these changes in the brain? In HD, the expanded CAG
repeat tract should be translated into an expanded glutamine
repeat tract. In fact, enlarged huntingtin protein molecules
were found in HD patients and they appeared to contain expanded
glutamine repeats. Since, in addition to this mutant gene,
HD patients have the other HD gene that has normal size CAG
repeat. This is the gene the patient inherited from the normal
parent. Therefore, they have the huntingtin protein with a
normal glutamine tract in an amount of 50% of normal individuals.
For most proteins, cells can function normally if there is
50% of the normal amount, and this is true for huntingtin.
Patients in whom a piece of DNA containing the entire HD gene
is deleted from one of the chromosomes 4 do not have any signs
of HD. Mice in which one of the two HD genes has been "knocked
out" also fail to show signs of HD. Thus, an idea that
the abnormal huntingtin with an expanded tract of glutamine
repeat gains new toxic function was introduced to explain these
observations. If this gain-of-function theory is right, what
is the new function? The amino acid sequence of huntingtin
has no resemblance to any known human protein. As a result,
what the normal and mutant huntingtin do in cells is unknown.
Furthermore, this gain of function must significantly affect
only certain cells in the brain (for example, medium-sized
spiny neurons in the area of the brain called striatum), since
no other tissues appear to be abnormal in HD despite the widespread
presence of huntingtin protein among various tissues.
Experimental data suggested that the normal huntingtin protein
does interact with other cellular proteins. The mutant huntingtin
protein with a long polyglutamine repeat appears to interact
a different set of proteins although it may still interact
with the proteins that interact with the normal huntingtin.
Since all these proteins that interact with normal and mutant
huntingtin proteins are expressed not only in the striatum
but also in other parts of the brain, regional distributions
of these proteins are an unlikely cause of the selective neuronal
death in HD.
However, by postulating another tissue-specific variable,
we may be able to explain why the medium-sized spiny neurons
are more vulnerable. Interesting hypotheses have been proposed
for each huntingtin-interacting protein. Whether any of these
proteins actually play a significant role in the disease mechanisms
of HD remains to be further investigated.
Transgenic mice, which have multiple and random integrations
of a transgene containing a part or whole HD gene with an expanded
CAG repeat in variety of size, exhibited clinical and pathological
features similar to HD. In other experimental mouse models,
an expanded CAG repeat was introduced into the mice’s
own gene that codes for a protein, which closely resembles
the human huntingtin protein. They, too, exhibited clinical
and pathological features similar to HD. It is expected that
these animal models reflect the disease mechanisms of human
HD. Such animal models have provided important insights into
understanding the disease mechanisms in HD, and are being used
to test promising drugs for treatment of HD before human trials.
DNA TESTING FOR HUNTINGTON’S DISEASE
Prior to the discovery of the HD mutation, some HD family
members underwent a less accurate and more complex DNA analysis,
termed linkage analysis. Linkage analysis examines the inheritance
of DNA markers that are in close proximity to the huntingtin
gene and requires the participation of several affected family
members and certain unaffected family members. The inaccuracy
of the analysis comes from the fact that the analysis indirectly
look at the HD gene status via nearby genetic markers that
are likely to be passed on to the next generation when the
HD gene is passed on. Thus, direct mutation detection was a
major improvement in the molecular evaluation of HD.
The direct gene test is now widely used for confirmation of
clinical diagnosis in adult patients who have symptoms and
signs compatible with HD. While all testing requires informed
consent, predictive testing follows a special protocol. Predictive
testing is available to individuals ages 18 and older at 50%
risk and requires that all subjects be enrolled in a center
with a written protocol for predictive testing. While molecular
analysis of the clinically affected relative is recommended,
it is not required.
Molecular analysis uses polymerase chain reaction (PCR) analysis
with or without Southern analysis of the DNA from an individual
todetermine the size of the CAG repeat in the HD gene. As already
mentioned in the section of Genetics of Huntington’s
Disease, the normal range is considered to be up to 28 copies
of the repeat. The range of 29 through 39 CAG repeats are inconclusive
and often called intermediate alleles. The intermediate range
consists of the HD range with reduced penetrance (36 through
39 CAG repeats) and the mutable normal range (28 through 35
CAG repeats). The robust HD (full HD expansion) range is 40
repeats and greater. Individuals who have a fully expanded
CAG repeat will eventually develop HD in their expected life
spans. The probability of future generations for the mutable
normal allele to expand into full expansion range cannot be
reliably determined. The factors that influence the penetrance
of the 36-39 CAG repeat range are also unknown. Additional
information about the intergenerational stability of these
intermediate alleles is necessary in order to provide useful
information to such families. While age of onset generally
shows an inverse correlation with the length of the expanded
CAG repeat, genotype/phenotype correlations are not precise
enough to be used clinically and may sometimes give a misleading
impression to the test subjects.
GENETIC COUNSELING IN THE DNA TESTING OF HUNTINGTON’S
DISEASE
While the long awaited discovery of the HD gene has led to
improved diagnostic and predictive testing for this disease,
this new technology has raised a number of challenging counseling,
clinical and ethical dilemmas. This section will address the
benefits and limitations of currently available molecular analyses
for HD.
Confirmation of Suspected Diagnosis of HD
Direct gene testing can be extremely helpful in determining
the differential diagnosis for a patient with a movement disorder.
While most individuals are aware when they have a family history
of HD, some are unclear or unaware of the existence of HD in
the family. With the direct gene testing, it became possible
to diagnose HD regardless of family history.
We recommend genetic counseling for every subject who has
DNA testing. Understanding the genetic issues of HD is beneficial
to the subjects and their family members. Psychological counseling
should also be made available for symptomatic individual, especially
those who are having difficulty accepting their diagnosis.
We strongly recommend psychological counseling for patients
who have signs of depression and other psychiatric problems,
even if the patients already carry the clinical diagnosis of
HD. For adult individuals with "soft signs" of HD
and a positive family history, extreme caution should be taken
to order a mutation analysis as a ''confirmatory'' test. It
should be noted that a positive result does not necessarily
confirm that the "symptoms" represent the onset of
HD and such information has significant psychological impacts.
These individuals would benefit from a predictive testing protocol.
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February 15, 2008
