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Several multisystem diseases are the result of
inherited metabolic defects, the vast majority
of such disorders being inherited as recessive traits.
It is possible to
categorize such diseases
according to the main result of the metabolic defect
In some diseases, the metabolic defect results in the accumulation of a
metabolic product in cells,
giving rise to the term metabolic storage disease.
Storage diseases result in the accumulation of abnormal metabolic products in
Diseases that cause accumulation of metabolic substrates in cells are termed
In many cases, the metabolic product accumulates in lysosomal-derived vesicles
(lysosomal storage disease). In other instances,the abnormal product accumulates
free in the cell cytoplasm.
Storage disorders can be sub-classified according to the type of substance that
accumulates in cells:
• Lipidoses: defects in enzymes that degrade glycolipids,leading to the
accumulation of sphingolipids, e.g.
Gaucher's disease, Niemann Pick disease, Tay-Sachs disease.
• Mucopolysaccharidoses: the accumulation of mucopolysaccharides (glycosaminoglycans,
dermatan sulphate and heparin sulphate) in cells,e.g. Hunter's syndrome,
• Mucolipidoses: Types II-IV defects in lysosomal-enzyme transport systems.
• Glycoproteinoses: lack of enzyme that degrade glycoproteins, e.g. sialidosis,
• Glycogenoses: diseases characterized by the accumulation of glycogen in cells.
Diagnosis of glycogenoses
The glycogenoses result in abnormal accumulation of glycogen in tissues
The glycogenoses are a group of diseases in which there is an abnormal
accumulation of glycogen
in tissues; this is caused by diverse enzyme abnormalities in the enzymes
involved in the
glycogen metabolism. There are over 12 subtypes, the majority of which have
defined enzyme deficiencies
Familial defects in lipid metabolism cause hyperlipidaemias and are an important
cause of atheroma
Hyperlipidaemias are a set of conditions in which there is an abnormally high
level of one or more
lipoproteins in the plasma. In cases where this is the result of a systemic
it is termed a secondary hyperlipidaemia - the main causes being obesity, excess
diabetes mellitus, nephrotic syndrome and hypothyroidism. When hyperlipidaemia
by a genetic predisposition to an abnormal lipid metabolism, the term primary
is used. Such primary hyperlipidaemias have been shown to be due to a wide range
in genes that code for enzymes, apolipoproteins, or receptors involved in the
There are several types of hyperlipidaemia, which can be characterized by the
determination of the
lipoprotein profile in plasma. The WHO classification is based on that proposed
and divides cases into Types I-V. Unfortunately, this does not indicate the
cause of a hyperlipidaemia
(i.e. whether primary or secondary).
Recent developments in the biochemistry of these disorders is allowing precise
diagnosis to be made,
based on metabolic or genetic characterization.
One of the most important consequences of certain types of hyperlipidaemia
(mainly WHO Type IIa) is
a raised serum cholesterol concentration, mainly a reflection of serum LDL,
predisposing to atheroma.
The main types of hyperlipidaemia. In addition to the effects of hyperlipidaemia,
high levels of
one particular lipoprotein termed lipoprotein(a) confers a high risk of the
development of both
ischaemic heart disease and cerebrovascular disease, independent of LDL/cholesterol
This table shows the main types of hyperlipidaemia and the most important
features of each.
About 6/1000 of the population have an autosomal dominant predisposition to
develop an abnormality
of lipid metabolism that increases the risk of developing atheroma. In addition,
a polygenic increased risk. Some of these cases will come to medical attention
because of the trend for
the routine screening of blood for cholesterol levels.
Other cases will come to light having presented witha complication of atheroma
at a young age,
e.g. ischaemic heart disease or cerebrovascular disease.
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