Zellweger Syndrome

Epidemiology and Genetics

Epidemiology

• Incidence Incidence The number of new cases of a given disease during a given period in a specified population. It also is used for the rate at which new events occur in a defined population. It is differentiated from prevalence, which refers to all cases in the population at a given time. Measures of Disease Frequency: 1:50,000–75,000
• Most common peroxisome biosynthesis Biosynthesis The biosynthesis of peptides and proteins on ribosomes, directed by messenger RNA, via transfer RNA that is charged with standard proteinogenic amino acids. Virology disorder is seen in infancy.
• Most infants born with Zellweger syndrome (ZWS) do not live past 6 months.

Genetics Genetics Genetics is the study of genes and their functions and behaviors. Basic Terms of Genetics

• Inheritance: autosomal recessive Autosomal recessive Autosomal inheritance, both dominant and recessive, refers to the transmission of genes from the 22 autosomal chromosomes. Autosomal recessive diseases are only expressed when 2 copies of the recessive allele are inherited. Autosomal Recessive and Autosomal Dominant Inheritance
• Due to a mutation Mutation Genetic mutations are errors in DNA that can cause protein misfolding and dysfunction. There are various types of mutations, including chromosomal, point, frameshift, and expansion mutations. Types of Mutations in 1 of 13 PEX genes Genes A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. DNA Types and Structure, which encode peroxins → proteins Proteins Linear polypeptides that are synthesized on ribosomes and may be further modified, crosslinked, cleaved, or assembled into complex proteins with several subunits. The specific sequence of amino acids determines the shape the polypeptide will take, during protein folding, and the function of the protein. Energy Homeostasis required for the normal assembly of peroxisomes Peroxisomes Microbodies which occur in animal and plant cells and in certain fungi and protozoa. They contain peroxidase, catalase, and allied enzymes. The Cell: Organelles:
  • Peroxisomes Peroxisomes Microbodies which occur in animal and plant cells and in certain fungi and protozoa. They contain peroxidase, catalase, and allied enzymes. The Cell: Organelles are membrane-bound organelles Organelles A cell is a complex unit that performs several complex functions. An organelle is a specialized subunit within a cell that fulfills a specific role or function. Organelles are enclosed within their own lipid bilayers or are unbound by membranes. The Cell: Organelles present in all cells, except mature erythrocytes Erythrocytes Erythrocytes, or red blood cells (RBCs), are the most abundant cells in the blood. While erythrocytes in the fetus are initially produced in the yolk sac then the liver, the bone marrow eventually becomes the main site of production. Erythrocytes: Histology:
    • Contain enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes necessary for multiple catabolic and anabolic metabolic functions 
    • Catalyze beta-oxidation of very long chain fatty acids Acids Chemical compounds which yield hydrogen ions or protons when dissolved in water, whose hydrogen can be replaced by metals or basic radicals, or which react with bases to form salts and water (neutralization). An extension of the term includes substances dissolved in media other than water. Acid-Base Balance (VLCFAs)
    • Catalyze oxidation of phytanic, pipecolic, pristanic, and other dicarboxylic acids Acids Chemical compounds which yield hydrogen ions or protons when dissolved in water, whose hydrogen can be replaced by metals or basic radicals, or which react with bases to form salts and water (neutralization). An extension of the term includes substances dissolved in media other than water. Acid-Base Balance
    • Involved in synthesis Synthesis Polymerase Chain Reaction (PCR) of bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy acids Acids Chemical compounds which yield hydrogen ions or protons when dissolved in water, whose hydrogen can be replaced by metals or basic radicals, or which react with bases to form salts and water (neutralization). An extension of the term includes substances dissolved in media other than water. Acid-Base Balance and plasmalogen
  • PEX gene Gene A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. Basic Terms of Genetics mutations result in a loss of peroxisomal function → preventing catabolism of VLCFAs → ↑ accumulation of VLCFAs in membranes of neuronal cells → abnormal function, atrophy Atrophy Decrease in the size of a cell, tissue, organ, or multiple organs, associated with a variety of pathological conditions such as abnormal cellular changes, ischemia, malnutrition, or hormonal changes. Cellular Adaptation, and death of cells → impaired neuronal migration and differentiation → abnormal brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification development → neurologic dysfunction and injury to variable Variable Variables represent information about something that can change. The design of the measurement scales, or of the methods for obtaining information, will determine the data gathered and the characteristics of that data. As a result, a variable can be qualitative or quantitative, and may be further classified into subgroups. Types of Variables extents 
• Most commonly a mutation Mutation Genetic mutations are errors in DNA that can cause protein misfolding and dysfunction. There are various types of mutations, including chromosomal, point, frameshift, and expansion mutations. Types of Mutations in the PEX1 gene Gene A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. Basic Terms of Genetics

Clinical Presentation

Clinical manifestations of ZWS are present at birth.

Craniofacial dysmorphism

• Separated cranial sutures
• Large anterior fontanelle Anterior Fontanelle Physical Examination of the Newborn
• High forehead Forehead The part of the face above the eyes. Melasma
• Flattened face
• Low and broad nasal bridge
• Upslanting palpebral fissures 
• Epicanthal folds ( skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions fold from the upper eyelid to the inner corner of the eye)
• Protruding tongue Tongue The tongue, on the other hand, is a complex muscular structure that permits tasting and facilitates the process of mastication and communication. The blood supply of the tongue originates from the external carotid artery, and the innervation is through cranial nerves. Lips and Tongue: Anatomy
• Deformed ear lobes
• High arched palate Palate The palate is the structure that forms the roof of the mouth and floor of the nasal cavity. This structure is divided into soft and hard palates. Palate: Anatomy

Neurologic abnormalities

• Severe hypotonia Hypotonia Duchenne Muscular Dystrophy and muscle weakness
• Epileptic seizures Seizures A seizure is abnormal electrical activity of the neurons in the cerebral cortex that can manifest in numerous ways depending on the region of the brain affected. Seizures consist of a sudden imbalance that occurs between the excitatory and inhibitory signals in cortical neurons, creating a net excitation. The 2 major classes of seizures are focal and generalized. Seizures
• Hearing loss Hearing loss Hearing loss, also known as hearing impairment, is any degree of impairment in the ability to apprehend sound as determined by audiometry to be below normal hearing thresholds. Clinical presentation may occur at birth or as a gradual loss of hearing with age, including a short-term or sudden loss at any point. Hearing Loss
• Absent reflexes
• Developmental delay

Ocular abnormalities

• Vision Vision Ophthalmic Exam impairment 
• Cataracts
• Pigmentary retinopathy Retinopathy Degenerative changes to the retina due to hypertension. Alport Syndrome
• Glaucoma Glaucoma Glaucoma is an optic neuropathy characterized by typical visual field defects and optic nerve atrophy seen as optic disc cupping on examination. The acute form of glaucoma is a medical emergency. Glaucoma is often, but not always, caused by increased intraocular pressure (IOP). Glaucoma

Hepatic abnormalities

• Hepatomegaly
• Jaundice Jaundice Jaundice is the abnormal yellowing of the skin and/or sclera caused by the accumulation of bilirubin. Hyperbilirubinemia is caused by either an increase in bilirubin production or a decrease in the hepatic uptake, conjugation, or excretion of bilirubin. Jaundice
• Cirrhosis Cirrhosis Cirrhosis is a late stage of hepatic parenchymal necrosis and scarring (fibrosis) most commonly due to hepatitis C infection and alcoholic liver disease. Patients may present with jaundice, ascites, and hepatosplenomegaly. Cirrhosis can also cause complications such as hepatic encephalopathy, portal hypertension, portal vein thrombosis, and hepatorenal syndrome. Cirrhosis
• Coagulopathy
• Biliary dysgenesis (affects the normal movement of bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy)

Renal abnormalities

• Glomerulocystic kidney disease
• Renal dysfunction

Associated conditions

• Chondrodysplasia punctata present in 50%–70% of cases:
  • Encompasses a group of skeletal anomalies characterized by calcific deposits in cartilage Cartilage Cartilage is a type of connective tissue derived from embryonic mesenchyme that is responsible for structural support, resilience, and the smoothness of physical actions. Perichondrium (connective tissue membrane surrounding cartilage) compensates for the absence of vasculature in cartilage by providing nutrition and support. Cartilage: Histology (calcific stippling), particularly in the patella Patella The flat, triangular bone situated at the anterior part of the knee. Knee Joint: Anatomy and long bones Long bones Length greater than width. Bones: Structure and Types
  • Associated with shortened limbs, joint contractures Contractures Prolonged shortening of the muscle or other soft tissue around a joint, preventing movement of the joint. Wound Healing, and spinal abnormalities
• Feeding difficulties due to profound hypotonia Hypotonia Duchenne Muscular Dystrophy

Diagnosis and Management

Diagnosis

• History and clinical examination
• Laboratory studies:
  • Blood:
    • ↑ Plasma Plasma The residual portion of blood that is left after removal of blood cells by centrifugation without prior blood coagulation. Transfusion Products VLCFA concentration (particularly C26:0-LPC)
    • ↑ Bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy acid intermediaries di- and trihydroxycholestanoic acid (DHCA, THCA)
    • ↑ Phytanic, pristanic, and pipecolic acid concentrations
  • Erythrocytes Erythrocytes Erythrocytes, or red blood cells (RBCs), are the most abundant cells in the blood. While erythrocytes in the fetus are initially produced in the yolk sac then the liver, the bone marrow eventually becomes the main site of production. Erythrocytes: Histology: ↓ plasmalogen concentration
  • Urine:
    • ↑ Bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy acids Acids Chemical compounds which yield hydrogen ions or protons when dissolved in water, whose hydrogen can be replaced by metals or basic radicals, or which react with bases to form salts and water (neutralization). An extension of the term includes substances dissolved in media other than water. Acid-Base Balance
    • ↑ Pipecolic acid concentration
• Imaging:
  • Brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification magnetic resonance imaging (MRI): may show various abnormalities including neocortical dysplasia, impaired myelination, abnormal cortical gyral patterns, decreased volume of white matter White Matter The region of central nervous system that appears lighter in color than the other type, gray matter. It mainly consists of myelinated nerve fibers and contains few neuronal cell bodies or dendrites. Brown-Séquard Syndrome, bilateral ventricular dilatation, and germinolytic cysts Cysts Any fluid-filled closed cavity or sac that is lined by an epithelium. Cysts can be of normal, abnormal, non-neoplastic, or neoplastic tissues. Fibrocystic Change 
  • Abdominal ultrasound: shows hepatomegaly and/or renal cysts Renal Cysts Imaging of the Urinary System
• Assays of peroxisomal-related processes in cultured skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions fibroblasts Fibroblasts Connective tissue cells which secrete an extracellular matrix rich in collagen and other macromolecules. Sarcoidosis help confirm the diagnosis.
• Genetic testing Genetic Testing Detection of a mutation; genotype; karyotype; or specific alleles associated with genetic traits, heritable diseases, or predisposition to a disease, or that may lead to the disease in descendants. It includes prenatal genetic testing. Myotonic Dystrophies: identifies the specific molecular defect to confirm the diagnosis 
• In utero diagnosis by chorionic villus sampling may be done if there is a suspicion of the disorder or a family history Family History Adult Health Maintenance.

Management

There is no cure and no effective treatment for ZWS. Treatment is supportive at best. 

• Feeding issues may be treated with a nasogastric tube Nasogastric tube Malnutrition in children in resource-limited countries.
• Therapeutic and genetic counseling Genetic Counseling An educational process that provides information and advice to individuals or families about a genetic condition that may affect them. The purpose is to help individuals make informed decisions about marriage, reproduction, and other health management issues based on information about the genetic disease, the available diagnostic tests, and management programs. Psychosocial support is usually offered. Myotonic Dystrophies should be offered to the family.
• Cholic acid supplementation:
  • Reduces toxic bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy acid intermediates
  • It may not improve clinical outcomes (e.g., liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy function, growth, survival)

Prognosis Prognosis A prediction of the probable outcome of a disease based on a individual’s condition and the usual course of the disease as seen in similar situations. Non-Hodgkin Lymphomas

Death typically occurs within 1 year of birth (usually < 6 months), typically due to respiratory failure Respiratory failure Respiratory failure is a syndrome that develops when the respiratory system is unable to maintain oxygenation and/or ventilation. Respiratory failure may be acute or chronic and is classified as hypoxemic, hypercapnic, or a combination of the two. Respiratory Failure. 

Clinical Relevance

Related conditions

Zellweger syndrome is part of a clinical continuum of peroxisome biogenesis disorders known as Zellweger spectrum disorder (ZSD). All of the following conditions also have autosomal recessive inheritance Autosomal recessive inheritance Autosomal Recessive and Autosomal Dominant Inheritance, and are diagnosed and managed similar to ZWS: 

• Neonatal adrenoleukodystrophy (NALD): considered a less-severe form of ZSD than ZWS. Craniofacial dysmorphism is present, but less severe, with mid-face hypoplasia Hypoplasia Hypoplastic Left Heart Syndrome (HLHS) usually the most pronounced. Individuals with NALD also have hepatic dysfunction and may have adrenal insufficiency Adrenal Insufficiency Conditions in which the production of adrenal corticosteroids falls below the requirement of the body. Adrenal insufficiency can be caused by defects in the adrenal glands, the pituitary gland, or the hypothalamus. Adrenal Insufficiency and Addison Disease, but cystic Cystic Fibrocystic Change renal disease and chondroplasia punctata are absent. Neurologic dysfunction with progressive leukodystrophy (degeneration of brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification white matter White Matter The region of central nervous system that appears lighter in color than the other type, gray matter. It mainly consists of myelinated nerve fibers and contains few neuronal cell bodies or dendrites. Brown-Séquard Syndrome) begins in the early years of life. Most children do not survive past 5 years of age. 
• Infantile Refsum disease (IRD): the clinical presentation is less severe than ZWS and NALD. Facial dysmorphic features are mild or may be absent. Hepatic dysfunction and neurologic features such as hypotonia Hypotonia Duchenne Muscular Dystrophy and vision Vision Ophthalmic Exam and hearing impairments occur. Severe developmental delay is noted within the 1st 6 months of life. Gastrointestinal symptoms also occur. Neurologic deterioration is slower and children with IRD may survive into adolescence. 
• Heimler syndrome: an extremely rare peroxisomal biogenesis disorder that is considered the mildest form of ZSD. Heimler syndrome is characterized by acquired sensorineural hearing loss Sensorineural hearing loss Hearing loss resulting from damage to the cochlea and the sensorineural elements which lie internally beyond the oval and round windows. These elements include the auditory nerve and its connections in the brainstem. Hearing Loss, retinal dystrophy or pigmentosa, nail abnormalities, and enamel Enamel A hard thin translucent layer of calcified substance which envelops and protects the dentin of the crown of the tooth. It is the hardest substance in the body and is almost entirely composed of calcium salts. Under the microscope, it is composed of thin rods (enamel prisms) held together by cementing substance, and surrounded by an enamel sheath. Teeth: Anatomy hypoplasia Hypoplasia Hypoplastic Left Heart Syndrome (HLHS) of permanent teeth Teeth Normally, an adult has 32 teeth: 16 maxillary and 16 mandibular. These teeth are divided into 4 quadrants with 8 teeth each. Each quadrant consists of 2 incisors (dentes incisivi), 1 canine (dens caninus), 2 premolars (dentes premolares), and 3 molars (dentes molares). Teeth are composed of enamel, dentin, and dental cement. Teeth: Anatomy. The remainder of development and cognition appear to be normal. The lifespan is unaffected. 

Differential diagnosis

The differential diagnosis for ZWS is vast. Clinical features, biochemical testing, imaging, and ultimately genetic testing Genetic Testing Detection of a mutation; genotype; karyotype; or specific alleles associated with genetic traits, heritable diseases, or predisposition to a disease, or that may lead to the disease in descendants. It includes prenatal genetic testing. Myotonic Dystrophies help to differentiate and provide a diagnosis. The following are disorders that should be considered in any neonate Neonate An infant during the first 28 days after birth. Physical Examination of the Newborn that presents with profound hypotonia Hypotonia Duchenne Muscular Dystrophy:

• Chromosomal abnormalities:
  • Down syndrome Down syndrome Down syndrome, or trisomy 21, is the most common chromosomal aberration and the most frequent genetic cause of developmental delay. Both boys and girls are affected and have characteristic craniofacial and musculoskeletal features, as well as multiple medical anomalies involving the cardiac, gastrointestinal, ocular, and auditory systems. Down syndrome (Trisomy 21) ( Trisomy 21 Trisomy 21 Down syndrome, or trisomy 21, is the most common chromosomal aberration and the most frequent genetic cause of developmental delay. Both boys and girls are affected and have characteristic craniofacial and musculoskeletal features, as well as multiple medical anomalies involving the cardiac, gastrointestinal, ocular, and auditory systems. Down syndrome (Trisomy 21))
  • Prader-Willi syndrome Prader-Willi syndrome Prader-Willi syndrome (PWS) is a rare autosomal neurodevelopmental genetic disorders mapped to a specific region of chromosome 15 attributed to genomic imprinting. A paternally derived chromosome 15 with this deletion results in 15q11-13 paternal deletion syndrome, or PWS. Prader-Willi Syndrome and Angelman Syndrome 
• Single enzyme peroxisomal disorders:
  • Acyl-CoA oxidase Oxidase Neisseria deficiency (also known as pseudo-NALD) 
  • D-bifunctional protein deficiency
• Other metabolic disorders:
  • Acid maltase Maltase Digestion and Absorption of Carbohydrates deficiency ( AMD AMD Macular Degeneration)
  • Carnitine Carnitine A constituent of striated muscle and liver. It is an amino acid derivative and an essential cofactor for fatty acid metabolism. Fatty Acid Metabolism deficiency
  • Cytochrome C oxidase Oxidase Neisseria deficiency
• Neuromuscular disorders:
  • Spinal muscular atrophy Atrophy Decrease in the size of a cell, tissue, organ, or multiple organs, associated with a variety of pathological conditions such as abnormal cellular changes, ischemia, malnutrition, or hormonal changes. Cellular Adaptation
  • Congenital muscular dystrophies
  • Congenital myopathies
• Infections Infections Invasion of the host organism by microorganisms or their toxins or by parasites that can cause pathological conditions or diseases. Chronic Granulomatous Disease:
  • Cytomegalovirus Cytomegalovirus CMV is a ubiquitous double-stranded DNA virus belonging to the Herpesviridae family. CMV infections can be transmitted in bodily fluids, such as blood, saliva, urine, semen, and breast milk. The initial infection is usually asymptomatic in the immunocompetent host, or it can present with symptoms of mononucleosis. Cytomegalovirus (CMV)
  • Rubella Rubella An acute infectious disease caused by the rubella virus. The virus enters the respiratory tract via airborne droplet and spreads to the lymphatic system. Rubella Virus virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology
  • Toxoplasmosis Toxoplasmosis Toxoplasmosis is an infectious disease caused by Toxoplasma gondii, an obligate intracellular protozoan parasite. Felines are the definitive host, but transmission to humans can occur through contact with cat feces or the consumption of contaminated foods. The clinical presentation and complications depend on the host’s immune status. Toxoplasma/Toxoplasmosis
  • Herpes simplex Herpes Simplex A group of acute infections caused by herpes simplex virus type 1 or type 2 that is characterized by the development of one or more small fluid-filled vesicles with a raised erythematous base on the skin or mucous membrane. It occurs as a primary infection or recurs due to a reactivation of a latent infection. Congenital TORCH Infections virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology ( HSV HSV Herpes simplex virus (HSV) is a double-stranded DNA virus belonging to the family Herpesviridae. Herpes simplex virus commonly causes recurrent infections involving the skin and mucosal surfaces, including the mouth, lips, eyes, and genitals. Herpes Simplex Virus 1 and 2)