Clinical Description
Zellweger spectrum disorder (ZSD) is defined by a continuum of three phenotypes described before the biochemical and molecular bases of these disorders had been fully determined: Zellweger syndrome (ZS), neonatal adrenoleukodystrophy (NALD), and infantile Refsum disease (IRD) [Braverman et al 2016].
The ZSD phenotypic spectrum is broad; some affected individuals have mild manifestations, mainly sensory deficits and/or mild developmental delay. Recently, individuals with normal intellect have been identified [Ratbi et al 2015, Ratbi et al 2016, Smith et al 2016] by genomic testing methods. Nonetheless, all of the peroxisome assembly disorders cause significant morbidity, frequently resulting in death in childhood.
Although the phenotypic designations listed above may be useful when evaluating undiagnosed individuals and counseling their families, one should not place too much emphasis on assigning a phenotypic label to an affected individual given that these phenotypes lie on a continuum. Thus, the terms "severe," "intermediate," and "milder" ZSD are now preferred. Because of the breadth of the phenotypic spectrum, individuals with ZSD mainly come to clinical attention in the newborn period or later in childhood. Occasionally, the subtlety of symptoms delays diagnosis until adulthood.
Newborns are hypotonic with resultant poor feeding. Neonatal seizures are frequent and caused by underlying neuronal migration defects. Liver dysfunction may be evident as neonatal jaundice and elevation in liver function tests. Distinctive craniofacial features include flat face, broad nasal bridge, large anterior fontanelle, and widely split sutures. In severely affected children, bony stippling (chondrodysplasia punctata) at the patella(e) and the long bones may be noted, as well as renal cysts.
Older children manifest retinal dystrophy, sensorineural hearing loss, developmental delay with hypotonia, and liver dysfunction. Children may first come to attention because of a failed hearing screen. Onset and severity of the hearing and visual problems vary. A few children with a clinical diagnosis of neonatal adrenoleukodystrophy had transient leopard spot pigmentary retinopathy [Lyons et al 2004]. Liver dysfunction may be first identified in children with severe bleeding episodes caused by a vitamin K-responsive coagulopathy. Older children may develop adrenal insufficiency [Berendse et al 2014] and osteopenia [Rush et al 2016].
Adults are rarely diagnosed with ZSD, but exceptions have been reported. Usually these are individuals with predominantly sensory deficits but normal neurologic development [Moser et al 1995, Raas-Rothschild et al 2002, Majewski et al 2011, Ratbi et al 2015, Ratbi et al 2016, Smith et al 2016].
Severe ZSD
Severe ZSD (previously called Zellweger syndrome [ZS]) typically presents in the neonatal period with profound hypotonia, characteristic facies, gyral malformations, seizures, inability to feed, renal cysts, hepatic dysfunction, and chondrodysplasia punctata. Infants with severe ZSD are significantly impaired and usually die during the first year of life, usually having made no developmental progress. Death is usually secondary to progressive apnea or respiratory compromise from infection.
Other
Individuals with atypical ZSD do not show sensory losses but have ataxia and peripheral neuropathy, and may have congenital cataracts (e.g., those with PEX2-ZSD [Sevin et al 2011], PEX11B-ZSD [Ebberink et al 2012], PEX10-ZSD [Steinberg et al 2009], PEX12-ZSD [Gootjes et al 2004], and PEX16-ZSD [Ebberink et al 2010]).
Note that although Heimler syndrome [Ratbi et al 2015, Ratbi et al 2016, Smith et al 2016] and ataxia (see Régal et al [2010], Renaud et al [2016]) have been reported as unique phenotypes associated with PEX gene defects, the authors consider them part of the ZSD continuum. In general, screening assays of individuals described as having these milder phenotypes do not show the biochemical profile typical of ZSD (Table 1).
Neuroimaging
MRI may identify cortical gyral abnormalities and germinolytic cysts that are highly suggestive of severe ZSD. Other brain MRI findings have been identified over time in individuals with milder ZSD.
In a small number of individuals with ZSD, diffusion-weighted imaging and diffusion tensor imaging can be used to discern white matter damage not detected by standard imaging [Patay 2005]. A demyelinating leukodystrophy can occur, but it is not clear which affected individuals are at increased risk for this development, or how it progresses in the individual.
Genotype-Phenotype Correlations
A general relationship appears to exist among the genotype, cellular phenotype (i.e., import of peroxisomal matrix proteins), and clinical phenotype [Moser 1999]. PEX gene defects are associated with loss-of-function variants; hence, variants that abolish activity (e.g., large deletions, nonsense, frameshift variants) are most severe. In contrast, missense variants that retain some residual function have a less severe effect on peroxisome assembly; however, it should be noted that not all missense variants have residual activity.
Due to the overall rarity of ZSD the opportunities to rigorously assess genotype and phenotype are limited. The PEX1 variants p.Ile700TyrfsTer42 and p.Gly843Asp are exceptions, as hundreds of individuals homozygous or compound heterozygous for these variants have been identified (mostly in molecular research studies or clinical laboratories and not as part of a thorough natural history assessment).
Homozygosity for
PEX1
p.Gly843Asp has to date been associated with a milder ZSD phenotype and sometimes with an intermediate phenotype [
Poll-The et al 2004]. In addition, an adult with a normal neurologic examination and an ocular phenotype was reported [
Majewski et al 2011].
Nomenclature
Peroxisome biogenesis disorders (PBD) can be divided into two subtypes: the Zellweger spectrum disorder (ZSD) and the rhizomelic chondrodysplasia punctata spectrum, of which rhizomelic chondrodysplasia punctata type 1 (RCDP1) is one subtype. RCDP1 is caused by biallelic pathogenic variants in PEX7, the receptor that recognizes peroxisome enzymes containing peroxisomal targeting signal 2. While individuals with RCDP1 have a perturbation in matrix protein import consistent with a peroxisomal assembly defect, they have a biochemical, cellular, and clinical phenotype distinct from ZSD. (See Rhizomelic Chondrodysplasia Punctata Type 1 for an in-depth description.)
ZSD has also formerly been referred to as cerebrohepatorenal syndrome, generalized peroxisomal disorders, Zellweger syndrome, neonatal adrenoleukodystrophy, or infantile Refsum disease (also known as infantile phytanic acid oxidase deficiency). Some individuals later shown to have ZSD were initially described as having hyperpipecolatemia or Heimler syndrome. The current preferred terminology is ZSD of severe, intermediate, or milder phenotype in order to recognize the common etiology, variations, and atypical presentations now being documented in individuals with biallelic pathogenic variants in any one of the 13 ZSD-PEX genes.
Of note, although Heimler syndrome [Ratbi et al 2015, Ratbi et al 2016, Smith et al 2016] and ataxia (see Régal et al [2010], Renaud et al [2016]) have been reported as unique phenotypes associated with PEX gene defects, the authors consider them part of the ZSD continuum.
Note: Refsum disease is clinically and molecularly distinct from infantile Refsum disease.
Prevalence
ZSD occurs worldwide with varying prevalence. In the past the incidence of ZSD had been estimated at 1:50,000 [Gould et al 2001]. More recent data from the New York state newborn screening laboratory confirmed 11 individuals with ZSD in more than 1.4 million screened for X-ALD using a biochemical assay (C26:0-LPC) that also detects ZSD (see Table 1) [Hubbard et al 2006, Hubbard et al 2009]. Thus, the confirmed incidence of ZSD in this population is 1:133,000 births [JJ Orsini, M Caggana, NY State Newborn Screening Laboratory Staff, personal communication, 2020]. Any estimate relying on a biochemical assay will be an underestimate because such assays fail to detect mild ZSD not associated with a definitive biochemical phenotype.
The main diagnostic center for peroxisomal diseases in Japan reported only 31 affected individuals over a 20-year period, with an estimated birth prevalence of 1:500,000 [Shimozawa et al 2003]. This lower incidence in Japan is mainly due to the absence of the common European PEX1 variants p.Ile700TyrfsTer42 and p.Gly843Asp.