What enzyme is deficient in Zellweger syndrome?
The human body is indeed a marvel. Just imagine humans have between 20 000 to 25 000 genes. These genes work to make sure the trillion cells of the body work properly. It is only when the body works properly, a human can have a normal and healthy life. In this DotorOnCall’s article, we will be learning about a disease that is caused by a defect in genes known as Zellweger syndrome.
Zellweger syndrome, or also known as cerebrohepatorenal syndrome is a rare congenital disorder caused by the defects in any of the PEX genes. PEX genes are genes that are required for the normal formation and function of peroxisomes. Variants of the genes that cause Zellweger syndrome prevent peroxisomes from forming and working properly. A person with Zellweger syndrome will have deficiency of enzymes for fatty acid metabolism.
Peroxisomes are cell structures, very small and located within the gel-like fluid (cytoplasm) of cells that break down toxic substances and produce lipids that are important for cell function. All human cells except the red blood cells contain peroxisomes. Peroxisomes are the most concentrated in the liver and kidney. Peroxisomes is mainly required for normal brain development and formation of myelin (sheath that cover the nerve fibres, aid with nerve signalling). Peroxisomes are also required for normal function of the eye, liver, kidney and bone.
Zellweger syndrome is the most common peroxisomal disorder present in early infancy. It is part of the Zellweger spectrum disorders as result from dysfunctional lipid metabolism and defects of bile acids with lipids within cells. Zellweger syndrome is said to be the most severe form next to neonatal adrenoleukodystrophy and infantile Refsum disease as part of the Zellweger spectrum.
Zellweger syndrome, as a disorder, affects every organ system. Zellweger syndrome can be seen with severe craniofacial abnormalities, lack muscle tone (hypotonia), severe neurodevelopmental delay, hearing loss, eye (ocular) and dental (enamel) abnormalities. The severe craniofacial abnormalities include distinctive facial features such as flattened face, high forehead, broad nasal bridge and widely-set eyes (hypertelorism). It is common to see liver enlargement (hepatomegaly) with increased liver enzymes and bilirubin levels. Renal cyst is present in more than half cases of Zellweger syndrome. Jaundice, digestive tract bleeding and seizures can be found in patients with Zellweger syndrome.
A doctor would diagnose Zellweger syndrome based on symptoms and some lab testing. Test includes elevation of very-long-chain fatty acid (VLCFA) in the blood of the newborn and genetic testing of the PEX genes. Biochemical tests apart from VLCFA, such as bile acid and plasmalogen in red blood cells are usually done. Imagine tests such as ultrasound can give information of the organs affected by the syndrome. Zellweger syndrome may actually be diagnosed even before birth when parents are known to be carriers of the mutated PEX genes. In such cases, healthcare providers may do blood testing or imaging tests before the baby is born.
Zellweger syndrome is a rapidly progressive disorder. There is no cure available to treat this syndrome and most treatments available only aim as supportive care. This includes hearing aids, referral to eye specialist, standard antiepileptic drugs, cortisone for kidney issues, gastrostomy and vitamin supplementation such as vitamin K for coagulopathy issues and supplementation of fat-soluble vitamins (A, D, E).
Children with Zellweger syndrome with symptoms as young as during the neonatal period (under 28 days of age) usually have poor outcomes and usually die within the first year of life. Patients that develop symptoms later in childhood or late infants survive slightly longer than those in neonates with progressive liver diseases. Patients with symptoms presented in adolescence do survive longer but typically have progressive neurological symptoms such as neuropathy (dysfunction of the nerve outside the brain and spinal cord). Reports show that affected children live up to 2 years and most likely die from respiratory failure, breathing difficulties or infections.
Due to the poor outcomes and no specific treatments, genetic testing and family planning is the best way to prevent parents from passing the diseases to their children. This can help to detect potential carriers prior to pregnancy and be offered with advice on what steps should be taken next.
It can be concluded that Zellweger syndrome is a serious disease as it affects a lot of organs and the damages from it cause a person to have many disabilities or diseases throughout their lifetime. It can be difficult to live with Zellweger syndrome as it is a rare genetic disorder but treatments can help to ease symptoms and provide comfort. Always work closely with healthcare professionals if you or your child do have this disorder. The best way to avoid this condition is to make sure you are not passing the genes to your children. Hence, genetic counselling and testing can help you determine either it is safe or otherwise for you to have a child. Genetic counselling does not only apply for Zellweger syndrome but also many other diseases that could be passed down. Talk to your healthcare provider about genetic diseases that could affect your offspring.