Defining Zellweger Syndrome
Zellweger syndrome is one of a group of four diseases that are related and are referred to as, 'peroxisome biogenesis disorders (PBD). The diseases are a part of a larger group of disease known as the leukodystrophies and are inherited conditions that damage the white matter in an affected person's brain, as well as how the person's body metabolizes certain substances in their organ tissues and blood. Zellweger syndrome is the most severe of the PBD's. Of the PBD's, Infantile Refsum disease (IRD), is the mildest form. Neonatal Adrenoleukodystrophy and Rhizomelic Chondrodysplasia present similar, yet less severe symptoms. The PBD's are caused by defects in a person's genes that are active during the development of a person's brain and the formation of, 'myelin,' the whitish substance that is found in a person's cerebral cortex area.
Once a person with Zellweger syndrome has been born, defects in the their genes either reduce or entirely eliminate the presence of, 'peroxisomes,' or cell structures that break down toxic substances in the cells of their kidneys, liver and brain. The result is high-levels of copper and iron that build up in their blood and tissue, causing the symptoms that are characteristic of the disease. Zellweger syndrome is also known by the following names:
Causes of Zellweger Syndrome
Mutations in one of a number of genes are the cause of Zellweger syndrome; they lead to a dysfunctional protein that is important for a person's cells to be able to import newly-synthesized proteins into small cytoplasmic organelles called, 'peroxisomes.' Zellweger syndrome is characterized by either a reduction in, or the complete absence of, peroxisomes. There are key enzymes which are critical for different chemical reactions, particularly oxidation, that are contained within peroxisomes.Structural and functional abnormalities of peroxisomes can lead to disease development observed in Zellweger syndrome.
Peroxisomes are usually abundant in a person's kidney and liver; their lack leaves the organs of persons with Zellweger syndrome affected by the disease. Toxic molecules that enter the person's bloodstream fail to be detoxified because of a lack of peroxisomes, although there are additional mechanisms for detoxification. Peroxisomes may also function in the organic creation of key compounds and play important roles related to different chemical reactions in a person's body. The mutations in a number of genes and Zellweger syndrome that affect the function of peroxisome include peroxin-1 (PEX1), peroxin-2 (PEX2) peroxin-3 (PEX3), peroxin-5 (PEX5), peroxin-6 (PEX6), and peroxin-12 (PEX12). Each of these gene locations are genetically and biochemically unique and found on different chromosomes.
Symptoms of Zellweger Syndrome
There are a number of observable, clinical features associated with Zellweger syndrome. These symptoms can include facial, developmental, as well as eye defects. Features that are characteristic of the syndrome include up-slanting eyes, a high forehead, and 'epicanthal,' or skin folds along the person's nasal borders of the space between the upper and lower eyelids of their eyes. Babies with Zellweger syndrome commonly experience a loss of muscle tone, severe weakness, and many times experience seizure activity. Many also have severe ocular (eye) abnormalities that may affect their vision. The symptoms of Zellweger syndrome can include:
The more common features of Zellweger syndrome involve enlargement of the person's liver, vision issues, as well as increased levels of both copper and iron in their blood. Some infants with the syndrome experience prenatal growth failure. Symptoms that may be present at birth include a lack of muscle tone and an inability to move. Additional symptoms related to the syndrome may include intellectual disability, characteristic facial features, an inability to either suck or swallow, jaundice, and gastrointestinal bleeding.
The most common features of Zellweger syndrome include an enlarged liver, high levels of iron and copper in the blood, and vision disturbances. Some affected infants may show prenatal growth failure. Symptoms at birth may include lack of muscle tone and an inability to move. Other symptoms may include unusual facial characteristics, mental retardation, seizures, and an inability to suck and/or swallow. Jaundice and gastrointestinal bleeding may also occur.
Diagnosing Zellweger Syndrome
The absence of peroxisomes in persons with Zellweger syndrome was initially demonstrated by American pathologist S.L. Goldfischer in the year 1985. The absence of these organelles in affected person's livers is now thought to be the hallmark of the disorder. Persons with Zellweger syndrome have been found to present with noticeably fewer peroxisomes in their brain and in cultured skin fibroblasts. 'Fibroblasts,' are a type of skin cell; in Zellweger syndrome these cells appear to have, 'ghost-like,' peroxisomes, caused by the absence of particular proteins inside the organelles that are recruited into the membranes.
Peroxisomes play a crucial role in organ development. Brain abnormalities may be explained by the disrupted migration of, 'neurons,' or nerve cells, around the third month of gestation. There is a defect that occurs in a particular area of a person's brain called the, 'cerebrum,' that leads to small or thick convolutions in the person's brain tissue. The brain abnormality permits Zellweger syndrome to be identified and distinguished from other diseases involving brain abnormalities. Additional tissues that are involved in the syndrome include the person's kidney, liver, heart, cartilage, and muscle. The majority of persons with Zellweger syndrome have cysts on their kidneys.
A diagnosis of Zellweger syndrome can be reached by measuring the metabolic compounds in the person's blood samples. Various plasmalogens, fatty acids, bile acid intermediates, and pipecolic acid are commonly studied. Other than diminished levels of plasmologen, the compounds in the person's blood are commonly increased in persons affected by Zellweger syndrome. Another possibility is to detect plasmologen synthesis and fatty acid levels prior to the person's birth through obtaining cells in the fluid of the amnion, something referred to as, 'amniocentesis.' Pregnant mothers who have had a baby that was previously affected by Zellweger syndrome can choose to have a prenatal diagnosis in order to determine if their child is affected by the syndrome.
Treatment of Zellweger Syndrome
Medical science has not discovered a cure for Zellweger syndrome at this time, and there is no standard course of treatment for this syndrome. Zellweger syndrome and the neurological and metabolic abnormalities associated with it, are caused during fetal development. Treatments to correct them after a person is born are limited; the majority of treatment options are supportive and symptomatic.
The prognosis for infants with Zellweger syndrome is poor, with the majority of infants failing to survive their first six months of life; they usually die from gastrointestinal bleeding, respiratory distress, or liver failure. People with Zellweger syndrome often do not live more than a year after they are diagnosed. Because of this, genetic counseling and prenatal diagnosis are commonly given high-priority for parents who are either identified or concerned that they might be at risk of having a child with Zellweger syndrome.