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- Nick Dennis
- Wessex Clinical Genetics Service
- Princess Anne Hospital
- Southampton
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- 23 chromosomes from mother
- 23 chromosomes from father
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- A set of 23 chromosomes carries about 20,000 genes
- So our cells contain 2 copies of all these genes
- EXCEPT most of the genes on the X chromosome in males, as most of th=
em
are not present on the Y chromosome
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- Most of them tell the cells how to make a specific protein
- Proteins are the nuts and bolts and motors that make our bodies work=
- For every protein we need to make, we need at least one gene to tell=
us
how to make it
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- Contains about 5.3% of all the DNA
- Contains about 4% of all the genes
- In contrast, chromosome 21
- Contains only 1.6% of the DNA
- And 1.2% of the genes
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- So why does having an extra chromosome this size lead to Down syndro=
me
- And having an extra chromosome this size lead to the much less obvio=
us
KS?
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- Normal females have two X chromosomes, but they switch off most of t=
he
genes on one or other of them
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- Males with Klinefelter syndrome do the same
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- So why are they any different from 46,XY males?
- Well, not all the genes on the “spare” X chromosome are
switched off
- And some of these genes are also present on the Y chromosome
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- 46,XX females and 46,XY males have TWO functioning copies
- 47,XXY KS males have THREE functioning copies
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- Genes in this category are comparatively few
- Most of the genes on the X are inactivated
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- Although most of these genes have been identified, most of the featu=
res
of KS have not yet been traced back to the activity of specific gene=
s
- An exception is the “tallness” gene (known as SHOX) at t=
he
tip of the X and Y chromosomes
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- How does it arise?
- Which genes on the X determine the clinical picture, and how do they=
do
it?
- Is the extra X passed on by mother or father, and does it matter whi=
ch?
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- The answers to these questions will have implications far beyond KS
itself
- KS is an “experiment of nature” that may help to reveal
normal processes otherwise very difficult to study
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- How common is it?
- What factors increase the likelihood of its happening?
- What are ALL the ways in which KS males differ from 46,XY males? (e.=
g.
the fetus, causes of death, subtle differences in behaviour)
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- Newborn surveys found 1 in 1000 males
- Danish study: 1 in 600
- They found the same in fetuses, suggesting no excess spontaneous
miscarriage
- Fewer than 10% diagnosed before puberty
- About 25% adults diagnosed
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- 3738 boys with special educational needs tested for fragile X syndro=
me
- These boys were drawn from a school population of about 82,300 males=
- 20 males with fragile X identified
- 15 KS males identified (1 in 250 tested)
- 13 of these were not previously known to have KS
- But original population should have contained at least 80 KS males=
li>
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- KS makes one more likely to have special educational needs
- But many KS males do not have SEN
- The SEN are usually mild enough not to be perceived as a problem
requiring medical investigation
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- Linked 3518 KS males to mortality and cancer statistics
- Confirmed increased risk of breast cancer and some lymphomas, reduced
risk of prostate cancer
- Analysis of non-cancer causes of death to be published shortly
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- Can identify increased risk of disease not shown by study of individ=
uals
- May indicate practical measures for surveillance, prevention and
treatment
- May throw light on how these diseases are caused when they occur in
non-KS individuals
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- Advances in scanning allow more precise measurements of small parts =
of
the brain (which may have specific functions)
- These need to be correlated with measures of behaviour, which are
necessarily imprecise, or risks of psychiatric disease
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- 45,X 1 copy
- 46,XY 1 copy
- 46,XX 2 copies
- 47,XXY 2 copies
- 47,XXX 3 copies
- Thus they may be responsible for some of the normal male-female
differences in behaviour
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- In 45,X (Turner syndrome) there is some evidence that behaviour diff=
ers
according to whether the X came from mother or father
- This would predict that KS males who have both Xs from their mothers=
may
differ from those who have one from each parent (approx 50% in each
category)
- So far, no differences found- but this may mean nobody has found a w=
ay
of measuring them yet
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- KS is a fruitful area of research
- This research will help affected individuals and families
- Clarify prognosis and allow prevention or treatment of complication=
s
- But it will also throw light on normal function and common disease
processes
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