Chondrodysplasia punctata with a mild clinical course

Clinical, biochemical, and molecular characterization of mild (nonclassic) rhizomelic chondrodysplasia punctata

Rhizomelic chondrodysplasia punctata (RCDP) is a heterogenous group of disorders due to defects in genes encoding peroxisomal proteins required for plasmalogen (PL) biosynthesis, specifically PEX7 and PEX5 receptors, or GNPAT, AGPS and FAR1 enzymes. Most patients have congenital cataract and skeletal dysplasia. In the classic form, there is profound growth restriction and psychomotor delays, with most patients not advancing past infantile developmental milestones. Disease severity correlates to erythrocyte PL levels, which are almost undetectable in severe (classic) RCDP. In milder (nonclassic) forms, residual PL levels are associated with improved growth and development. However, the clinical course of this milder group remains largely unknown as only a few cases were reported. Using as inclusion criteria the ability to communicate and walk, we identified 16 individuals from five countries, ages 5-37 years, and describe their clinical, biochemical and molecular profiles. The average age at diagnosis was 2.6 years and most had cataract, growth deficiency, joint contractures, and developmental delays. Other major symptoms were learning disability (87%), behavioral issues (56%), seizures (43%), and cardiac defects (31%). All patients had decreased C16:0 PL levels that were higher than in classic RCDP, and up to 43% of average controls. Plasma phytanic acid levels were elevated in most patients. There were several common, and four novel, PEX7, and GNPAT hypomorphic alleles in this cohort. These results can be used to support earlier diagnosis and improve management in patients with mild RCDP.

Peroxisomes in brain development and function

Peroxisomes contain numerous enzymatic activities that are important for mammalian physiology. Patients lacking either all peroxisomal functions or a single enzyme or transporter function typically develop severe neurological deficits, which originate from aberrant development of the brain, demyelination and loss of axonal integrity, neuroinflammation or other neurodegenerative processes. Whilst correlating peroxisomal properties with a compilation of pathologies observed in human patients and mouse models lacking all or individual peroxisomal functions, we discuss the importance of peroxisomal metabolites and tissue- and cell type-specific contributions to the observed brain pathologies. This enables us to deconstruct the local and systemic contribution of individual metabolic pathways to specific brain functions. We also review the recently discovered variability of pathological symptoms in cases with unexpectedly mild presentation of peroxisome biogenesis disorders. Finally, we explore the emerging evidence linking peroxisomes to more common neurological disorders such as Alzheimer’s disease, autism and amyotrophic lateral sclerosis. This article is part of a Special Issue entitled: Peroxisomes edited by Ralf Erdmann.

Homeostasis of phospholipids - The level of phosphatidylethanolamine tightly adapts to changes in ethanolamine plasmalogens

Ethanolamine plasmalogens constitute a group of ether glycerophospholipids that, due to their unique biophysical and biochemical properties, are essential components of mammalian cellular membranes. Their importance is emphasized by the consequences of defects in plasmalogen biosynthesis, which in humans cause the fatal disease rhizomelic chondrodysplasia punctata (RCDP). In the present lipidomic study, we used fibroblasts derived from RCDP patients, as well as brain tissue from plasmalogen-deficient mice, to examine the compensatory mechanisms of lipid homeostasis in response to plasmalogen deficiency. Our results show that phosphatidylethanolamine (PE), a diacyl glycerophospholipid, which like ethanolamine plasmalogens carries the head group ethanolamine, is the main player in the adaptation to plasmalogen insufficiency. PE levels were tightly adjusted to the amount of ethanolamine plasmalogens so that their combined levels were kept constant. Similarly, the total amount of polyunsaturated fatty acids (PUFAs) in ethanolamine phospholipids was maintained upon plasmalogen deficiency. However, we found an increased incorporation of arachidonic acid at the expense of docosahexaenoic acid in the PE fraction of plasmalogen-deficient tissues. These data show that under conditions of reduced plasmalogen levels, the amount of total ethanolamine phospholipids is precisely maintained by a rise in PE. At the same time, a shift in the ratio between ω-6 and ω-3 PUFAs occurs, which might have unfavorable, long-term biological consequences. Therefore, our findings are not only of interest for RCDP but may have more widespread implications also for other disease conditions, as for example Alzheimer's disease, that have been associated with a decline in plasmalogens.

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PE accurately compensates for the lack of plasmalogens in vitro and in vivo.

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PE levels decrease to adapt to excess of ethanolamine plasmalogens (PlsEtn).

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Plasmalogen deficiency favors incorporation of arachidonic acid into PE.

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Docosahexaenoic acid in ethanolamine phospholipids decreases upon PlsEtn depletion.

The importance of ether-phospholipids: a view from the perspective of mouse models

Ether-phospholipids represent an important group of phospholipids characterized by an alkyl or an alkenyl bond at the sn-1 position of the glycerol backbone. Plasmalogens are the most abundant form of alkenyl-glycerophospholipids, and their synthesis requires functional peroxisomes. Defects in the biosynthesis of plasmalogens are the biochemical hallmark of the human peroxisomal disorder Rhizomelic Chondrodysplasia Punctata (RCDP), which is characterized by defects in eye, bone and nervous tissue. The generation and characterization of mouse models with defects in plasmalogen levels have significantly advanced our understanding of the role and importance of plasmalogens as well as pathogenetic mechanisms underlying RCDP. A review of the current mouse models and the description of the combined knowledge gathered from the histopathological and biochemical studies is presented and discussed. Further characterization of the role and functions of plasmalogens will contribute to the elucidation of disease pathogenesis in peroxisomal and non-peroxisomal disorders. This article is part of a Special Issue entitled: Metabolic Functions and Biogenesis of Peroxisomes in Health and Disease.

Functions and biosynthesis of plasmalogens in health and disease

Plasmalogens (1-O-alk-1'-enyl-2-acyl glycerophospholipids) constitute a special class of phospholipids characterized by the presence of a vinyl-ether bond at the sn-1 position. Although long considered as biological peculiarities, interest in this group of phospholipids has grown in recent years, thanks to the realization that plasmalogens are involved in different human diseases. In this review, we summarize the current state of knowledge with respect to the enzymatic synthesis of plasmalogens, the characteristic topology of the enzymes involved and the biological roles that have been assigned to plasmalogens.

Mutation analysis of PEX7 in 60 probands with rhizomelic chondrodysplasia punctata and functional correlations of genotype with phenotype

PEX7 encodes the cytosolic receptor for the set of peroxisomal matrix enzymes targeted to the organelle by the peroxisome targeting signal 2 (PTS2). Mutations in PEX7 cause rhizomelic chondrodysplasia punctata (RCDP), a distinct peroxisome biogenesis disorder. In previous work we described three novel PEX7 mutant alleles, including one, L292X, with a high frequency due to a founder effect. We have now extended our analysis to 60 RCDP probands and identified a total of 24 PEX7 alleles, accounting for 95% of the mutant PEX7 genes in our sample. Of these, 50% are L292X, 13% are IVS9+1G>C, and the remainder are mostly private. IVS9+1G>C occurs on at least three different haplotypes and thus appears to result from recurrent mutation. The phenotypic spectrum of RCDP is broader than commonly recognized and includes minimally affected individuals at the mild end of the spectrum. To relate PEX7 genotype and phenotype, we evaluated the consequence of the disease mutation on PEX7 RNA by Northern analysis and RT/PCR. We evaluated the function of the encoded Pex7 protein (Pex7p) by expressing selected alleles in fibroblasts from RCDP patients and assaying their ability to restore import of a PTS2 marker protein. We find that residual activity of mutant Pex7p and reduced amounts of normal Pex7p are associated with milder and variant phenotypes.

Mutational spectrum in the PEX7 gene and functional analysis of mutant alleles in 78 patients with rhizomelic chondrodysplasia punctata type 1

Rhizomelic chondrodysplasia punctata (RCDP) is a genetically heterogeneous, autosomal recessive disorder of peroxisomal metabolism that is clinically characterized by symmetrical shortening of the proximal long bones, cataracts, periarticular calcifications, multiple joint contractures, and psychomotor retardation. Most patients with RCDP have mutations in the PEX7 gene encoding peroxin 7, the cytosolic PTS2-receptor protein required for targeting a subset of enzymes to peroxisomes. These enzymes are deficient in cells of patients with RCDP, because of their mislocalization to the cytoplasm. We report the mutational spectrum in the PEX7 gene of 78 patients (including five pairs of sibs) clinically and biochemically diagnosed with RCDP type I. We found 22 different mutations, including 18 novel ones. Furthermore, we show by functional analysis that disease severity correlates with PEX7 allele activity: expression of eight different alleles from patients with severe RCDP failed to restore the targeting defect in RCDP fibroblasts, whereas two alleles found only in patients with mild disease complemented the targeting defect upon overexpression. Surprisingly, one of the mild alleles comprises a duplication of nucleotides 45-52, which is predicted to lead to a frameshift at codon 17 and an absence of functional peroxin 7. The ability of this allele to complement the targeting defect in RCDP cells suggests that frame restoration occurs, resulting in full-length functional peroxin 7, which leads to amelioration of the predicted severe phenotype. This was confirmed in vitro by expression of the eight-nucleotide duplication-containing sequence fused in different reading frames to the coding sequence of firefly luciferase in COS cells.

Peroxisomal disorders

Peroxisomes are subcellular organelles catalyzing a number of indispensable functions in cellular metabolism. The importance of peroxisomes is stressed by the existence of an expanding number of genetic diseases in which there is an impairment of one or more peroxisomal functions. The prototype of this group of diseases is the cerebro-hepato-renal syndrome of Zellweger (ZS), first described as a familial syndrome of multiple congenital defects in 1964. ZS is characterized by the presence of dysmorphias and polymalformative syndrome, severe neurologic abnormalities including neurosensory defects and hepato-intestinal dysfunction with failure to thrive and usually early death. Other peroxisomal disorders share some of these symptoms, but with varying degrees of organ involvement, severity of dysfunction and duration of survival. This paper provides an overview of the peroxisomal disorders including their clinical, biochemical and molecular characteristics with particular emphasis on the clinical presentation in neonates.

Congenital cataract and multisystem disorders

A knowledge of those syndromes associated with congenital cataract is essential for the paediatric ophthalmologist, as congenital cataracts are manifest in a large number of syndromes. It is important to have the correct diagnosis in such cases, not only for genetic and prognostic information, but also in order to help the parents to understand their child's condition. This paper describes the more common syndromes seen in association with congenital cataract, and emphasises the importance of looking at the whole child and family. We aim to provide a practical clinical guide to the diagnosis of hereditary and non-hereditary syndromes associated with congenital cataract.

Prenatal diagnosis of rhizomelic chondrodysplasia punctata due to isolated alkyldihydroacetonephosphate acyltransferase synthase deficiency

Current practices in prenatal diagnosis of rhizomelic chondrodysplasia punctata (RCDP) are reviewed. A case is presented with a family having one daughter affected with RCDP due to alkyldihydroacetonephosphate acyltransferase synthase (DHAPAT synthase) deficiency, and three subsequent pregnancies. Biochemical test values are presented for the pregnancies and daughter. Post-mortem tests of one fetus of a terminated pregnancy showed that radiologic examination could not make the diagnosis of RCDP. We conclude that biochemical or molecular testing is necessary to accurately diagnose this type of RCDP prenatally.

Human PEX7 encodes the peroxisomal PTS2 receptor and is responsible for rhizomelic chondrodysplasia punctata

Rhizomelic chondrodysplasia punctata (RCDP) is a rare autosomal recessive phenotype that comprises complementation group 11 of the peroxisome biogenesis disorders (PBD). PEX7, a candidate gene for RCDP identified in yeast, encodes the receptor for peroxisomal matrix proteins with the type-2 peroxisome targeting signal (PTS2). By homology probing we identified human and murine PEX7 genes and found that expression of either corrects the PTS2-import defect characteristic of RCDP cells. In a collection of 36 RCDP probands, we found two inactivating PEX7 mutations: one, L292ter, was present in 26 of the probands, all with a severe phenotype; the second, A218V, was present in three probands, including two with a milder phenotype. A third mutation, G217R, whose functional significance is yet to be determined, was present in five probands, all compound heterozygotes with L292ter. We conclude that PEX7 is responsible for RCDP (PBD CG11) and suggest a founder effect may explain the high frequency of L292ter.

Non-rhizomelic and rhizomelic chondrodysplasia punctata within a single complementation group

Several patients have been described recently who suffer from a non-rhizomelic type of chondrodysplasia punctata (CDP), but who show all the biochemical abnormalities characteristic of the rhizomelic form of chondrodysplasia punctata (RCDP), a peroxisomal disorder. We have used protease protection experiments and microinjection of reporter-protein-encoding expression plasmids to show that peroxisomal thiolase fails to be imported into peroxisomes in cells from non-rhizomelic CDP patients, as has already been found in cells from classical RCDP patients. Furthermore, complementation analysis after somatic cell fusion indicates that the non-rhizomelic CDP patients are impaired in the same gene as classical RCDP patients. We conclude that defects in a single gene can give rise to both clinical phenotypes.

Variant rhizomelic chondrodysplasia punctata (RCDP) with normal plasma phytanic acid: clinico-biochemical delineation of a subtype and complementation studies

Rhizomelic chondrodysplasia calcificans punctata (RCDP) is an autosomal recessive peroxisomal disorder which affects phytanic acid oxidation and de novo biosynthesis of plasmalogens in liver and fibroblasts. Peroxisomal thiolase is present in its unprocessed precursor form (44 kDa). We studied a mentally retarded 9-year-old girl with cataracts and atypical bone dysplasia. Neurological findings were mild compared to classic RCDP. Plasma phytanic acid was normal. Results of de novo plasmalogen synthesis and phytanic acid oxidation studied in cultured skin fibroblasts were intermediate between normal controls and classic RCDP. Peroxisomal thiolase was present only as the unprocessed 44 kDa protein. Taken together these results suggest that we are dealing with a variant form of RCDP with clinical and biochemical abnormalities much milder as compared to classic RCDP. In order to establish the genetic relationship between our patient and classic RCDP patients complementation studies were carried out. Earlier studies had already shown that fibroblasts from all RCDP patients studied belong to a single complementation group. Fibroblasts from our patient could also be assigned to this complementation group suggesting that the phenotypic variability results from different mutations within the same gene.