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Porphyria Overview
Q:
How does one get porphyria?
A: In each type of porphyria, there
is a deficiency of a specific "enzyme." These enzymes are involved
in the synthesis of "heme," a substance important to many body
functions and found in largest amounts in the bone marrow, red blood
cells, and the liver. Heme exists as hemoglobin in the bone marrow
and red blood cells but has other functions in other tissues such
as the liver. The type of porphyria present is determined by which
enzyme is deficient. These enzyme deficiencies usually are inherited.
Environmental factors, such as drugs, chemicals, diet, and sun exposure
can, depending on the type of porphyria, greatly influence the severity
of symptoms.
The inherited porphyrias are either autosomal dominant (inherited
from one parent) or autosomal recessive (inherited from both parents). "Autosomal" genes
always occur in pairs, with one coming from each parent. Individuals
with an autosomal dominant form of porphyria have one abnormal gene
paired with a normal one, and half of their offspring (on the average)
will inherit the gene for the disease, while the other half will
inherit the normal gene. Some of those who inherit the abnormal gene
will develop symptoms. Individuals with an autosomal recessive type
of porphyria have a pair of abnormal genes, and each of their children
will inherit one abnormal gene for that disease, which will be paired
with a normal gene from the other parent, and there will be no symptoms.
If two carriers of the same abnormal recessive gene have children,
approximately one fourth of their offspring will inherit two abnormal
genes, and these children will develop symptoms of the disease. Because
all porphyrias are uncommon, it is very unlikely that more than one
type will occur in the same family, or that someone with one type
of porphyria will go on to develop another.
Q: How are the porphyrias classified?
A: The best way to classify a case of porphyria is to determine which enzyme is
deficient. The enzymes that are deficient in the porphyrias normally act in
sequence to make heme from simpler molecules. Heme is a vital substance for
all body organs and consists of an iron atom surrounded by a porphyrin molecule.
The sequence of enzymes and intermediates in the pathway to produce heme is
shown in the diagram.
As shown, each of the eight types of porphyria is associated with
a deficiency of one of these enzymes.
Sometimes, other classifications are useful. For example, the porphyrias
often are divided into two groups, the "hepatic" and "erythropoietic" types.
Porphyrias with skin manifestations sometimes are called "cutaneous
porphyrias." The "acute porphyrias" are those characterized
by attacks of pain and other neurological manifestations. These "acute" symptoms
can be both rapidly appearing and severe. An individual may be considered "latent" if
he or she has the required enzyme deficiency but has never developed
symptoms. There can be a wide spectrum of severity between the "latent" and "active" cases
of any particular type of porphyria. |
