Peroxisomal disorder


Peroxisomal disorder

Description, Causes and Risk Factors:

Peroxisomal disorders are a group of congenital diseases characterized by the absence of normal peroxisomes in the cells of the body. Peroxisomes are membrane-bound organelle occurring in many eukaryotic cells that often has an electron-dense crystalline inclusion containing catalase, urate oxidase, and other oxidative enzymes relating to the formation and degradation of H2O2; thought to be important in detoxifying various molecules and in catalyzing the breakdown of fatty acids to acetyl-CoA; an absence of peroxisomes is found in individuals with Zellweger syndrome.

Peroxisomal disorders is caused by defects in peroxisome functions. This may be due to defects in single enzymes important for peroxisome function or in peroxins, proteins encoded by PEX genes that are critical for normal peroxisome assembly and biogenesis.

Most peroxisomal disorders are inherited autosomal recessive diseases. This means that both parents need to be carriers of the defective gene in order for a child to develop the disease. If both parents are carriers but do not show signs of disease, each child has a 25 percent chance of having the disease. If one parent has the disease and the other is a carrier, each child has a 50 percent chance of having the disease. As a sex-linked genetic disorder, the daughters of males affected with X-ALD become carriers and the sons are not affected. The children of female carriers have a 50 percent chance of having the genetic mutation, which means that sons who inherit the mutation have the disease, and daughters who inherit the mutation are carriers.

The combined incidence of peroxisomal disorders is in excess of 1 in 20,000 individuals. Patients hemizygous or heterozygous for adrenoleukodystrophy (ALD) that is X-linked (X-ALD) are by far the largest subset. Zellweger syndrome (ZWS) is the most common peroxisomal disorder to manifest itself in early infancy. Its incidence has been estimated to be 1 in 50,000-100,000.

Symptoms:

In general, developmental delay, mental retardation, and vision and hearing impairment are common in those who have these disorders. Acquisition of speech appears to be especially difficult, and because of the reduced communication abilities, autism is common in those who live longer. Peroxisomal disorder patients have decreased muscle tone (hypotonia), which in the most severe cases is generalized, while in less severe cases, is usually restricted to the neck and trunk muscles. Sometimes this lack of control is only noticeable by a curved back in the sitting position. Head control and independent sitting is delayed, with most patients unable to walk independently.

Failure to thrive is a common characteristic of patients with peroxisomal disorder, along with an enlarged liver, abnormalities in liver enzyme function, and loss of fats in stools (steatorrhea). Peroxisomal disorders are also associated with facial abnormalities, including high forehead, frontal bossing (swelling), small face, low set ears, and slanted eyes. These characteristics may not be prominent in some children and are especially difficult to identify in an infant.

Diagnosis:

Since hearing and vision deficiencies may be difficult to identify in infants, peroxisomal disorders are usually detected by observations of failure to thrive, hypotonia, mental retardation, widely open fontanel, abnormalities in liver enzymes, and an enlarged liver. If peroxisomal disorders are suspected, blood plasma assays for VLCFAs, phytanic acid, and pipecolic acid are conducted. Additional tests include plasmalogen biosynthesis potential.

It is possible to diagnose peroxisomal disorders in utero. For example, for X-ALD, diagnosis can be made from cultured skin fibroblasts or amniotic fluid cells. This allows prenatal diagnosis and carrier identification in 90 percent of those affected. As of the early 2000s it has been shown that biochemical diagnosis can be performed through chorionic villus testing, a procedure performed very early in the first trimester of pregnancy.

Treatment:

In general, most treatments that are attempted for peroxisomal disorders are dietary, whereby attempts are made to artificially correct biochemical abnormalities associated with the disorders. Therapies include supplementation of the diet with antioxidant vitamins or limitation of intake of fatty acids, especially VLCFAs.

Another area of dietary therapy that is being investigated is the supplementation of the diet with pure DHA, given as early in life as possible, in conjunction with a normal well-balanced diet. Some results have indicated that if given soon enough during development, DHA therapy may prevent some of the devastating consequences of peroxisomal disorders, including the loss of vision and brain damage.

Other treatment strategies include addition of important missing chemicals. For example, in disorders where there is faulty adrenal function, replacement adrenal hormone therapy is used.

Any dietary changes should be monitored biochemically to determine if the supplements are having their desired effects and are not causing additional adverse effects.

Some physicians have also treated peroxisomal patients with a mixture of trioleate-trierucate, popularly known as Lorenzo's Oil, a preparation widely used with X-ALD (X-linked adrenoleukodystrophy) patients to decrease the levels of VLCFA (very-long-chain fatty acid). Some fragmentary experience exists with other therapeutic diets. Plasmalogen precursors and cholic acid have been given to these patients in an effort to increase their plasmalogen levels and normalize bile acids, respectively. A patient treated with plasmapheresis (plasma exchange) was reported to improve in parallel with decreases in plasma phytanate levels. Attempts have also been made to improve peroxisomal-disorder patients by providing them with peroxisomal proliferators such as clofibrate, without success. No report showing clear evidence of improvement with any of these treatments has been published so far.

NOTE: The above information is for processing purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.

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