Chromosomal
abnormality
Chromosomal abnormality,
aberrations or anomalies are disruptions in the normal chromosomal content of a
cell. Chromosomal aberrations are the changes in the structure of chromosomes.
It has a great role in evolution.It also involve the gain, loss or
rearrangement of visible amounts of genetic material.Some chromosomal
conditions are caused by changes in the number of chromosomes. These changes
are not inherited, but occur as random events during the formation of
reproductive cells (eggs and sperm). An error in cell division called non-disjunction
results in reproductive cells with an abnormal number of chromosomes. For
example, a reproductive cell may accidentally gain or lose one copy of a chromosome.
If one of these atypical reproductive cells contributes to the genetic makeup
of a child, the child will have an extra or missing chromosome in each of the
body’s cells.Changes in chromosome structure can also cause chromosomal disorders. Some changes in chromosome structure can be inherited, while others occur as random accidents during the formation of reproductive cells or in early fetal development.
Some chromosome abnormalities do not cause disease in carriers, such as translocations, or chromosomal inversions, although they may lead to a higher chance of birthing a child with a chromosome disorder. Abnormal numbers of chromosomes or chromosome sets, aneuploidy, may be lethal or give rise to genetic disorders.
Each of our
chromosomes has a characteristic structure and displays a specific pattern of
dark and light bands when stained with chemicals. These features are conserved
in all human beings, making the different chromosomes easy to identify and
distinguish from each other under a microscope. They define what is known as a
normal karyotype - as a full set of chromosomes from an individual which can be
compared to a "normal" karyotype for the species via genetic testing.
Any deviation from the normal karyotype, in terms of chromosome number or structure,
is known as a chromosome abnormality. A chromosome anomaly may be detected or
confirmed in this manner.
The gain or loss of DNA
from chromosomes can lead to a variety of genetic disorders. Human examples
include:- Cri du
chat, which is caused by the deletion of part of the short arm of
chromosome 5. "Cri du chat" means "cry of the cat" in
French, and the condition was so-named because affected babies make
high-pitched cries that sound like those of a cat. Affected individuals
have wide-set eyes, a small head and jaw, and are moderately to severely
mentally retarded and very short.
- Wolf-Hirschhorn
syndrome, which is caused by partial deletion of the short arm of
chromosome 4. It is characterized by severe growth retardation and severe
to profound mental retardation.
- Down
syndrome, usually is caused by an extra copy of chromosome 21 (trisomy
21). Characteristics include decreased muscle tone, stockier build,
asymmetrical skull, slanting eyes and mild to moderate mental retardation.
- Edwards
syndrome, which is the second-most-common trisomy; Down syndrome is the
most common. It is a trisomy of chromosome 18. Symptoms include mental and
motor retardation and numerous congenital anomalies causing serious health
problems. Ninety percent die in infancy; however, those that live past
their first birthday usually are quite healthy thereafter. They have a
characteristic clenched hands and overlapping fingers.
- Patau
Syndrome, also called D-Syndrome or trisomy-13. Symptoms are somewhat
similar to those of trisomy-18, but they do not have the characteristic
hand shape.
- Idic15,
abbreviation for Isodicentric 15 on chromosome 15; also called the
following names due to various researches, but they all mean the same;
IDIC(15), Inverted dupliction 15, extra Marker, Inv dup 15, partial
tetrasomy 15
- Jacobsen
syndrome, also called the terminal 11q deletion disorder. This is a very
rare disorder. Those affected have normal intelligence or mild mental
retardation, with poor expressive language skills. Most have a bleeding
disorder called Paris-Trousseau syndrome.
Types
of chromosomal abnormalties
There are many types of
chromosome anomalies. They can be organized into two basic groups, numerical and structural abnormalties.
1)
Numerical chromosomal
abnormality
This is called aneuploidy
(an abnormal number of chromosomes), and occurs when an individual is missing
either a chromosome from a pair (monosomy) or has more than two chromosomes of
a pair (trisomy,
tetrasomy,
etc.). These type of birth defects occur when there is a different number of
chromosomes in the cells of the body than is usually found. So, instead of the
usual 46 chromosomes in each cell of the body, there may be 45 or 47
chromosomes. Having too many or too few chromosomes is a cause of some birth
defects. Most human cells contain 23 pairs of chromosomes, for a a total of 46.
When sex cells are produced, these pairs normally separate, which is called
disjunction. A sex cell, or a gamete, thus
has 23 chromosomes when this happens as it should. Sometimes, however, a pair
will not separate — which is called nondisjunction — giving one gamete too many
chromosomes and another too few. When these abnormal sex cells are involved in
fertilization, the resulting cell will have the wrong number of chromosomes. If
the cell has too many chromosomes, this is called polyploidy.
Often, the cell only has one extra copy of a certain chromosome. If the cell
has one copy of a chromosome, it is called a monosomic cell.
In humans an example of a condition caused by a
numerical anomaly is Down Syndrome, also known as Trisomy 21 (an individual
with Down Syndrome has three copies of chromosome 21, rather than two). Turner
Syndrome is an example of a monosomy where the individual is born with only one
sex chromosome, an X.
Trisomy
The term "trisomy" is used to describe the presence
of three chromosomes, rather than the usual pair of chromosomes. For example,
if a baby is born with three #21 chromosomes, rather than the usual pair, the
baby would be said to have "trisomy 21." Trisomy 21 is also known as
Down syndrome. Other examples of trisomy include trisomy 18 and trisomy 13.
Again, trisomy 18 or trisomy 13 simply means there are three copies of the #18
chromosome (or of the #13 chromosome) present in each cell of the body, rather
than the usual pair. The effects range from moderate to severe, and people who
have Down syndrome have characteristic facial features, a short stature and heart
defects. They often suffer from respiratory diseases, have a shorter life span
and have some degree of mental retardation.
Patau syndrome results from
a trisomy of chromosome 13. It causes severe eye, brain and circulatory
defects. Cleft palate is often a result, and these children rarely live longer
than a few months. Children who have Edward’s syndrome also live for only a few
months, in most cases. This is because all of their organs are affected in some
way. Edward's syndrome is caused by trisomy 18. In people who have
Klinefelter’s syndrome, the sex-determining chromosomes — normally XX for
females and XY for males — are one Y chromosome and two X chromosomes. These
individuals are male, but the presence of an extra X chromosome causes body
proportions that are female and smaller testes, with no sperm production.
Similar chromosome disorders result in XYY males or XXYY males, but their
effects are much different and can vary widely in their nature and their
degree.Monosomy
The term "monosomy" is used to describe the absence of one member of a pair of chromosomes. Therefore, there is a total of 45 chromosomes in each cell of the body, rather than 46. For example, if a baby is born with only one X sex chromosome, rather than the usual pair (either two X's or one X and one Y sex chromosome), the baby would be said to have "monosomy X." Monosomy X is also known as Turner syndrome.
Children who
have Turner’s syndrome have only one X chromosome and no Y chromosome, so they
have only 45 chromosomes. This is the only monosomy that is not always lethal
in humans, although it usually results in miscarriage. These babies who are
born are female, but they are small in stature and do not mature sexually.
A few
numerical abnormalities support development to term either because the
chromosome has relatively few genes (13, 18, 21, Y-chromosome) or because there
is a natural mechanism to adjust gene dosage even in normal people
(X-chromosome). The most common numerical abnormalities are listed below
The
major numerical abnormalities that survive to term
Syndrome
|
Abnormality
|
Incidence per 10 000 births
|
Lifespan (years)
|
Down
|
Trisomy 21
|
15
|
40
|
Edward's
|
Trisomy 18
|
3
|
<1
|
Patau's
|
Trisomy 13
|
2
|
<1
|
Turner’s
|
Monosomy X
|
2 (female births)
|
30-40
|
Klinefelter’s
|
XXY
|
10 (male births)
|
Normal
|
XXX
|
XXX
|
10 (female births)
|
Normal
|
XXY
|
XYY
|
10 (male births)
|
Normal
|
2) Structural chromosomal abnormalty
Structural chromosome abnormalities occur when
there is a change in the structure or components of a chromosome. The total
number of chromosomes is usually normal (46 total per cell). Structural
chromosome abnormalities occur when part of a chromosome is missing, a part of
a chromosome is extra, or a part has switched places with another part.
Ultimately, this leads to having too much or too little genetic material, which
is a cause of some birth defects.Each chromosome has many segments which are
usually divided into a "short arm" and a "long arm" of the
chromosome. The short arm which is the upper half of the chromosome, is known
as the "p arm" and the long arm, which is the lower half of the
chromosome, is the "q arm." The centromere is the center part of a
chromosome that appears "pinched" between the p and q arms
Structural abnormalities can be
unbalanced or balanced. The former are similar to numerical abnormalities in
that genetic material is either gained or lost. The abnormalities range from
the loss or duplication of whole chromosome arms to the deletion or duplication
of tiny chromosome fragments barely visible under the microscope. However, even
these tiny deviations (microdeletions and microduplications) can encompass
several to many genes and have severe effects. Below is a list of some common
deletion and microdeletion syndromes.
Unbalanced structural abnormalities (p = short arm, q = long arm)
Syndrome
|
Abnormality
|
Incidence
|
Wolf-Hirschhorn
|
Deletion, tip of 4p
|
1 in 50 000
|
Cri-du-chat
|
Deletion, tip of 5p
|
1 in 50 000
|
WAGR
|
Microdeletion, 11p
|
|
Prader-Willi/Angelman
|
Microdeletion, 15p
|
|
DiGeorge
|
Microdeletion, 22q
|
Balanced structural abnormalities
involve the rearrangement of genetic material but no overall gain or loss.
Examples include inversions (where a segment is removed from a chromosome,
turned on its axis and sealed back in place), translocations (where part of one
chromosome becomes attached to another) and ring chromosomes (where the ends of
the long and short arms fuse together to form a circle).
Balanced abnormalities can have an
immediate effect if a gene is disrupted by the breakpoint, if two genes are
fused together, or if the relocation of a gene causes it to be expressed at a
higher or lower level than usual. However, the major consequence is to prevent
normal chromosome pairing at meiosis, leading to the production of sperm and
eggs with incomplete or partially duplicated chromosome sets.
Structural
chromosomal abnormalties can take the following form
- Deletions: A portion
of the chromosome is missing or deleted. Known disorders in humans include
Wolf-Hirschhorn syndrome, which is caused by partial deletion of the short
arm of chromosome 4; and Jacobsen syndrome, also called the terminal 11q
deletion disorder.
- Duplications: A portion
of the chromosome is duplicated, resulting in extra genetic material.
Known human disorders include Charcot-Marie-Tooth disease type 1A which
may be caused by duplication of the gene encoding peripheral myelin
protein 22 (PMP22) on chromosome 17.
- Translocations: A portion
of one chromosome is transferred to another chromosome. There are two main
types of translocations:
- Reciprocal translocation: Segments
from two different chromosomes have been exchanged.
- Robertsonian translocation: An
entire chromosome has attached to another at the centromere - in humans
these only occur with chromosomes 13, 14, 15, 21 and 22.
- Inversions: A portion
of the chromosome has broken off, turned upside down and reattached,
therefore the genetic material is inverted.
- Insertions: A portion
of one chromosome has been deleted from its normal place and inserted into
another chromosome.
- Rings: A portion
of a chromosome has broken off and formed a circle or ring. This can
happen with or without loss of genetic material.
- Isochromosome: Formed by
the mirror image copy of a chromosome segment including the centromere
CAUSES,
DIAGNOSIS, TREATMENT AND INHERITANCE
Causes
Chromosomal
abnormalities usually result from an error that occurs when an egg or sperm
cell develops. It is not known why these errors occur. As far as we know,
nothing that a parent does or doesn’t do before or during pregnancy can cause a
chromosomal abnormality in his or her child.The question of what causes
chromosomal abnormalities is a little more complex. The simplest answer is that
"it just happens." Cell division is a complex process with a lot of
things that can go wrong, so it follows that sometimes things do go wrong. A
sperm or egg cell may end up with the wrong number of chromosomes or with
chromosomes with missing or extra pieces, which ultimately go on to cause
problems such as miscarriage or stillbirth (or lead to genetic disorders).The
following factors have been implicated
1) late pregnancy made the baby inside the
mother have a higher risk of getting Down Syndrome
2) Electromagnetic waves like UV light or
X-ray cause our chromosome to mutate
3) Family inheritance
4) Non disjunction
Diagnosis
Screening tests
Screening
tests indicate whether an abnormality is likely, without risk to the foetus.
Serum screen
This is a blood test performed at 15 to 20 weeks. It
measures the levels of pregnancy hormones in the mother's blood.
By combining these results with the mother's age, it is possible to
calculate the risk of the baby having Down's syndrome. The blood test is not a foolproof and, occasionally, a Down's baby will be born to a mother with a low-risk result.
Nuchal translucency
measurement
Ultrasound is used to measure the fluid space at
the back of the foetus' neck at 11 to 14 weeks. (Nucha, pronounced nooka, is
the nape of the neck). Down's babies have a bigger space than unaffected
pregnancies. By combining this measurement with your age and sometimes with
hormone measurements in your blood, it is possible to calculate the risk of
carrying an affected baby. If the risk is high, you may want to consider a
diagnostic test.
Chromosomal abnormalities can
be diagnosed before birth using prenatal tests [amniocentesis or chorionic
villus sampling (CVS)] or after birth using a blood test. Cells obtained from
these tests are grown in the laboratory, and then their chromosomes are
examined under a microscope. The lab makes a picture (karyotype) of all the person’s
chromosomes, arranged in order from largest to smallest. The karyotype shows
the number, size and shape of the chromosomes and helps experts identify any
abnormalities.Analysing cells from the foetus is the only way a chromosome abnormality can be diagnosed with certainty. A diagnostic test is performed when:
- a woman is
considered at high risk because of the screening test.
- a woman is
considered at high risk because she has previously had a pregnancy
affected by a chromosomal or genetic disorder.
- an
ultrasound examination has detected features or abnormalities indicating
an increased risk of a chromosome abnormality.
- a woman
requests it because she is concerned that her baby has a chromosome
abnormality.
There are two types of diagnostic test: amniocentesis and chorionic villus sampling
(CVS). Which test you are offered will depend on the hospital where you
plan to give birth.
Amniocentesis
Amniocentesis analyses a sample of the fluid
surrounding the foetus in the womb (the amniotic fluid). It is performed from
15 weeks of pregnancy onwards.
Using ultrasound to guide the way, a fine needle is inserted through the
mother's abdomen and into the fluid surrounding the foetus. 20ml of amniotic
fluid is removed and sent for analysis. Chorionic villus sampling (CVS)
CVS analyses a sample of the placenta and is
performed from 11 weeks onwards. Using ultrasound to guide the way, a needle is
inserted through the mother's abdomen into the developing placenta. Suction is
applied and a small sample of tissue is sent to the laboratory.
The procedure usually takes 5 to 10 minutes. It is a little more
uncomfortable than amniocentesis for the mother. The foetus is unaware of the
procedure. With modern techniques, a preliminary result may be available within 48 hours. The final result will be ready in two weeks.
Treatment
There are are no treatment for chromosomal abnormalities.
Inheritance
Although it is possible to inherit some types of chromosomal abnormalities, most chromosomal disorders (such as Down syndrome and Turner syndrome) are not passed from one generation to the next.
Some chromosomal conditions are caused by changes in the number of chromosomes. These changes are not inherited, but occur as random events during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction results in reproductive cells with an abnormal number of chromosomes. For example, a reproductive cell may accidentally gain or lose one copy of a chromosome. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra or missing chromosome in each of the body’s cells.
Changes in chromosome structure can also cause chromosomal disorders. Some changes in chromosome structure can be inherited, while others occur as random accidents during the formation of reproductive cells or in early fetal development. Because the inheritance of these changes can be complex, people concerned about this type of chromosomal abnormality may want to talk with a genetics professional.
Some cancer cells also have changes in the number or structure of their chromosomes. Because these changes occur in somatic cells (cells other than eggs and sperm), they cannot be passed from one generation to the next
Chromosomal Abnormality
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