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Below is a selection of the questions we are
most frequently asked. If you need clarification or additional
information then please contact our Family Advisers on 0800 652 3181.
What is a
Metabolic Disorder?
The chemical processes that
take place in the body are known collectively as metabolism. These
processes are largely controlled by the bodies DNA and Enzyme. Metabolic disorders can also be caused by
underlying conditions affecting major organs that regulate metabolism such
as the liver and the kidneys.
One important group of
metabolic disorders is due to faulty or blocked chemical pathways, which
cause the build-up of a chemical that is usually eliminated from the body.
At the chemical accumulates, its may damage such organs as the brain and the
liver. Many of these disorders, such as Haemochromatosis, are due to
genetic defects, some of which tend to run in families.
Most metabolic disorders can
be diagnosed using blood or urine tests to measure levels of hormones or
other specific chemicals. If the disorders are recognised early they can
sometimes by treated by a change in diet or the replacement of missing
hormones or other chemicals. The sooner treatment is started the better the
prospects are for avoiding permanent damage.
It should be noted that
treatments are not available for all metabolic disorders. There are
few, if any, cures for metabolic disorders.
What part do Genes Play
in this?
Genes play a part in the
cause of may common diseases such as asthma and diabetes. Some rarer
disorders are caused solely by defective genes or abnormal chromosomes and
may be passed on from parent to child.
Genes provide instructions
to the cells to make enzymes and other proteins and molecules that the body
needs to grow and function. A effective gene may have mild, moderate or
potentially fatal consequences or no effect at all depending on the role of
the protein or molecule for which the gene codes.
Several thousand disorders
are the result of abnormalities in single genes but the majority of these
conditions are extremely rare. About 1 in 100 babies is born with a
disorder that is caused by a fault in a single gene.
Some communities have
particularly high frequencies of abnormal genes for certain disorders. For
example the abnormal gene that produces Thalassaemia is more common in
people from Mediterranean countries and Asia, the gene that produces Tay
Sachs disease is more common among Ashkenazi Jews.
Some gene disorders are
obvious soon after birth or in the first few weeks/months of life. Symptoms
of other disorders caused by a single abnormal gene do not appear until adult
life (Huntingdons disease).
What are the causes?
In most gene disorders an
individual has the defective gene in all body cells from the first stages in
the development of the embryo. There are two possible reasons for the
presence of the abnormal gene.
First,
and most commonly, the defective gene may have been passed on from parent to
child.
Second a
normal gene may have become faulty (mutated) during meiosis, the process of
division by which eggs and sperm form. In this way certain gene disorders
can occur without a family history.
Are there different
types?
Yes, genes occur in pairs –
one of each pair is inherited from the mother, the other from the father.
The genes may be dominant or recessive. Dominant genes override the effects
of genes that are recessive. Genes are found on the 22 pairs of chromosomes
known as autosomes and also on the X and Y sex chromosomes (predominantly on
the X chromosome). Single gene disorders are classified by their pattern of
inheritance:
Autosomal
dominant
Autosomal
recessive
X linked
Multifactorial
In addition many common
disorders are the result of an interaction between genes, the environment
and lifestyle. These are known as multifactorial disorders.
Autosomal
Dominant
In these disorders an
abnormal gene needs to be inherited from only one parent to lead to a
disorder. A normal gene inherited from the other parent does not mask the
effect of a dominant gene mutation. Each child of a person with an
autosomal dominant disorder has a 1 in 2 chance of inheriting the abnormal
gene and developing the disorder. Familial hypercholesterolaemia is one of
the most common conditions to follow this pattern of inheritance. In this
disorder high levels of blood cholesterol lead to an increased risk of early
coronary artery disease. About 1 in 500 people of European descent has this
gene and is affected by the disorder.
In the example (above) one
of the parents has the abnormal gene and the other parent is unaffected.
Each child has a 1 in 2 chance of inheriting the faulty gene and therefore
developing the disorder.
Autosomal
Recessive
A person must inherit two
copies of a faulty gene, one from each parent, to develop an autosomal
recessive disorder. If a person inherits one copy of the faulty gene from a
parent but the gene from the other parent is normal the person will be a
carrier.
A carrier does not develop
the disease but may pass the gene on to his or her children. Since two
genes, one from each parent, are necessary to produce the disease, autosomal
recessive disorders are rare. Medium Chain Acyl CoA Dehydrogenase
Deficiency (MCADD) is the most common autosomal recessive disorder that we
deal with.
For example Cystic Fibrosis,
a disease that affects certain glands, is most common in white Europeans.
In the UK population about 1 in 25 is a carrier of the disease. The chance
of two carriers meeting as approximately 1 on 625 so if two carriers have a
child there is a 1 in 4 risk of the child inheriting two faulty genes.
Sickle-cell is the most
common autosomal recessive disorder among black people, about 1 in 10 are
carriers and the disease affects about 1 in 400.
In this example both parents
carry the abnormal gene but do not have the disease. Their children may be
unaffected (1 in 4 chance), may be carriers (1 in 2 chance) or may have the
disorder (1 in 4).
X linked recessive
In a disorder of this type
the faulty gene is carried on the X chromosome. Women who have just one
faulty gene are unaffected carriers because they have a second X chromosome
that usually carries a normal gene masking the affect of the abnormal one.
They may pass on the faulty gene to their children. Since each child
inherits one x chromosome from the mother each child of a carrier has a 1 in
2 chance of inheriting the faulty gene. If a boy inherits the faulty gene
he will develop the disorder because he has a Y chromosome and not a second
X chromosome. Girls are carriers because the normal gene on the X
chromosome inherited from their father masks the faulty gene. An affected
man will pass on the faulty gene to all his daughters but not to his sons.
In the above example a
mother carries the abnormal gene on the X chromosome but is unaffected. The
sons have a 1 in 2 chance of inheriting the disease, the daughters have a 1
in 2 chance o being carriers but will not have the disease.
Multifactorial
There are many common
disorders, including asthma, that run in families but for which no single
gene appears to be responsible. In these disorders it is likely that
several different genes interact with lifestyle and environmental factors to
cause the disease. Certain disorders are known to be associated with a
group of proteins called HLAs, which form part of the body's immune system
and determine a person's tissue type. HLAs are inherited and each
individual has a unique combination of them.
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