What is Peroxisome Biogenesis Disorder Type 4?
Peroxisome biogenesis disorder type 4 (also known as PEX6-related Zellweger syndrome spectrum, ZSS) is an inherited disease that stops part of the body's cells, called a peroxisome, from forming correctly. Peroxisomes normally break down waste products in a cell. Not everyone with ZSS will have the same symptoms and some may have more severe symptoms than others. The disease is generally grouped into three subtypes: Zellweger syndrome (the most severe), neonatal adrenoleukodystrophy or NALD (intermediate severity), and infantile Refsum disease or IRD (the mildest form). Individuals with ZSS usually show symptoms of the disease as newborns or as children, but some some people may not have symptoms until they are older.
Zellweger Syndrome (ZS)
ZS is the most severe form of ZSS. Infants with ZS usually die before their first birthday without reaching many mental or physical developmental milestones. Infants born with ZS have developmental delay leading to severe intellectual disability. They often have seizures and typically have facial deformities such as a high forehead, abnormal ear lobes, a large "soft spot" on the top of their heads, and a small chin. In some cases, the infant cannot move and may not be able to suck or swallow because their muscles are too weak. They often show poor feeding. Their livers are usually enlarged and their skin and the whites of their eyes may have a yellowish tinge (jaundice). Some have bleeding in their digestive tract. Abnormally shaped bones are also common.
Neonatal Adrenoleukodystrophy (NALD) and Infantile Refsum Disease (IRD)
The symptoms of NALD and IRD are less severe than those of ZS, with IRD being the mildest form of the disease. Symptoms in these children often begin in late infancy or early childhood and may progress more slowly. Infants and children with NALD or IRD may have developmental delays with mild to severe intellectual disability. Hearing loss and vision impairment typically grow worse over time and may lead to blindness and/or deafness. Many people with the disease have liver problems and some have developed episodes of spontaneous bleeding, particularly around the brain. Some children with the disease learn to walk, while others lack the muscle tone needed for such movement. Similarly many children with the disease learn to talk, though some do not.
How common is Peroxisome Biogenesis Disorder Type 4?
Peroxisomal biogenesis disorders generally affect approximately 1 in 50,000 infants, and approximately 9-16% of cases are attributed to mutations in PEX6. In Japan, the incidence of all peroxisomal biogenesis disorders may be lower than 1 in 500,000.
How is Peroxisome Biogenesis Disorder Type 4 treated?
There is no cure for ZSS and there is no standard way to treat it. In children with severe forms of the disease, the main goal of treatment is to protect the child from infections and breathing problems. Physicians can also address certain symptoms as they arise, such as prescribing medication for seizures. Children with milder forms of the disease may benefit from hearing aids, glasses, and/or surgery to remove cataracts. In those who reach school age, special education is likely necessary. Modifications to the child's diet may also be recommended.
What is the prognosis for a person with Peroxisome Biogenesis Disorder Type 4?
ZSS usually reduces a person's lifespan. The prognosis for an infant with ZS is poor. Most pass away within the first year of life without reaching any physical or mental developmental milestones. One study showed that children with NALD or IRD who survive the first year of life have a 77% chance of reaching school age. These children will all have some degree of intellectual disability. Most people with NALD survive into childhood while those with IRD can live into their teens or 20s, and perhaps even longer.