Cholesterol biosynthesis, peroxisomes and peroxisomal disorders: mevalonate kinase is not only deficient in Zellweger syndrome but also in rhizomelic chondrodysplasia punctata

Cervical spine stenosis in chondrodysplasia punctata

Chondrodysplasia punctata (CDP) is a rare skeletal dysplasia characterized by stippled epiphyses during infancy. The frequency is probably underdiagnosed because of the large heterogeneity in this group. Many genotypic variations exist. Although cervical instability is commonly seen in many skeletal dysplasias, cervical spine stenosis associated with CDP is very rare. We report a boy with phenotypic features of brachytelephalangic chondrodysplasia punctata (BCDP) who had severe cervical spine stenosis successfully corrected by vertebrectomies of C6 and C7 with a fibular strut graft. We discuss the significance of this association.

Spinal deformity in chondrodysplasia punctata

BACKGROUND DATA

Our purpose was to describe the deformities, natural history, and course of treatment in chondrodysplasia punctata, a skeletal dysplasia characterized by stippled epiphysis in infancy, short stature, and progressive kyphoscoliosis.

STUDY DESIGN

We identified 20 patients (15 female, 5 male) with chondrodysplasia punctata and spinal abnormalities.

RESULTS

The spinal deformity was rarely present in the neonatal radiographs but typically appeared during the first year. We identified three patterns of spinal deformity. The first was a C2 dentocentral synchondrosis separation, which was successfully fused. Scoliosis had two patterns: one slowly progressive without dysplastic vertebrae or significant kyphosis (two patients) and the other rapidly progressive with dysplastic vertebral bodies often appearing similar to a congenital kyphoscoliosis (17 patients). Follow-up averaged 7.9 years (range 2-20 years). The slowly progressive type was fused successfully with standard posterior techniques. The dysplastic type, however, required on average 2.5 procedures, including anteroposterior fusions, repeat fusions for pseudarthrosis repair, extension of the fusions for progression beyond the curves, and osteotomies for progression through the fusion mass. Unlike congenital deformities, they did not respond well to hemiepiphysiodesis.

CONCLUSION

Three types of spinal deformities were identified in children with chondrodysplasia punctata, including cervical bony disruption, a slowly progressive, nondysplastic scoliosis responding well to standard fusion techniques and a dysplastic kyphoscoliosis, which is rapidly progressive and resistant to fusion. The best results in dysplastic curves are obtained with an anterior strut graft and a posterior fusion. The patients must be observed long-term for further vertebral dysplasia and progressive kyphoscoliosis.

Inborn errors of sterol biosynthesis

The known disorders of cholesterol biosynthesis have expanded rapidly since the discovery that Smith-Lemli-Opitz syndrome is caused by a deficiency of 7-dehydrocholesterol. Each of the six now recognized sterol disorders-mevalonic aciduria, Smith-Lemli-Opitz syndrome, desmosterolosis, Conradi-Hünermann syndrome, CHILD syndrome, and Greenberg dysplasia-has added to our knowledge of the relationship between cholesterol metabolism and embryogenesis. One of the most important lessons learned from the study of these disorders is that abnormal cholesterol metabolism impairs the function of the hedgehog class of embryonic signaling proteins, which help execute the vertebrate body plan during the earliest weeks of gestation. The study of the enzymes and genes in these several syndromes has also expanded and better delineated an important class of enzymes and proteins with diverse structural functions and metabolic actions that include sterol biosynthesis, nuclear transcriptional signaling, regulation of meiosis, and even behavioral modulation.

PTS2 protein import into mammalian peroxisomes

Peroxisome targeting signal (PTS)2 directs proteins from their site of synthesis in the cytosol to the lumen of the peroxisome. Unlike PTS1 which is present in the great majority of peroxisomal matrix proteins and whose import mechanics have been dissected in considerable detail, PTS2 is a relatively rare topogenic signal whose import mechanisms are far less well understood. However, as is the case for PTS1 proteins, an inability to import PTS2 proteins leads to human disease. In this report, we describe the biochemical characterization of mammalian PTS2 protein import using a semi-permeabilized cell system. We show that a PTS2-containing reporter molecule is taken up by peroxisomes in a reaction that is time-, temperature-, ATP-, and cytosol-dependent. Furthermore, the import process is specific, saturable, and requires action of the chaperone Hsc70, the cochaperone Hsp40, and the peroxins Pex5p and Pex14p. We also demonstrate peroxisomal translocation of PTS2 reporter/antibody complexes confirming the import competence of higher order structures. Importantly, cultured fibroblasts from patients with the rhizomelic form of chondrodysplasia punctata (RCDP) which are deficient for the PTS2 receptor protein, Pex7p, are unable to import the PTS2 reporter in this assay. The ability to monitor PTS2 import in vitro will permit, for the first time, a detailed comparison of the biochemical properties of PTS1 and PTS2 protein import.

First-trimester increased nuchal translucency and fetal hypokinesia associated with Zellweger syndrome

We report the prenatal detection of increased nuchal translucency and decreased fetal movements, at 11 weeks of gestation, in a fetus at risk for Zellweger syndrome. The diagnosis of Zellweger syndrome was confirmed by metabolic studies on cultured chorionic villus sampling (CVS) cells and the pregnancy was terminated. The couple's subsequent pregnancy was monitored using the same method. In this pregnancy the nuchal translucency measured at 12 weeks' gestation was normal, the fetus was active, and biochemical studies using CVS and amniocentesis confirmed normal results. We believe this to be the first reported case of Zellweger syndrome followed prenatally in which an increased nuchal translucency and fetal hypokinesia were detected in the first trimester. During the pregnancy with the affected child the maternal serum screen (MSS) showed low estriol level. We believe this to be the second report of a low estriol level on MSS in a pregnancy affected with Zellweger syndrome.

PEX7 gene structure, alternative transcripts, and evidence for a founder haplotype for the frequent RCDP allele, L292ter

We recently reported cloning a cDNA encoding Pex7p, the peroxisomal PTS2 receptor. PEX7 mutations cause the peroxisome biogenesis disorder (PBD) rhizomelic chondrodysplasia punctata (RCDP). In a survey of 44 RCDP probands, we found that one PEX7 allele, L292ter, accounted for 50% of mutant PEX7 genes. Here we report the characterization of the PEX7 structural gene, which spans 102 kb on chromosome 6q21-q22.2 and contains at least 10 exons. In addition to the predominant full-length transcript, we identified eight smaller PEX7 transcripts generated by alternative exon splicing in several tissues. However, none of these splice forms was able to restore PTS2 protein import into peroxisomes when expressed in RCDP fibroblasts nor did they inhibit PTS2 protein import when expressed in normal fibroblasts. To determine whether the high frequency of the L292ter allele is due to a founder effect, we identified five polymorphic markers (four diallelic markers and one CA repeat) spanning the PEX7 gene. We show that all 12 L292ter homozygotes in our patient sample have an identical haplotype at these five sites, consistent with the hypothesis that the L292ter mutation arose once on an ancestral chromosome in the Caucasian population.

Rhizomelic chondrodysplasia punctata, a peroxisomal biogenesis disorder caused by defects in Pex7p, a peroxisomal protein import receptor: a minireview

Rhizomelic chondrodysplasia punctata (RCDP) is a lethal autosomal recessive disease corresponding to complementation group 11 (CG11), the second most common of the thirteen CGs of peroxisomal biogenesis disorders (PBDs). RCDP is characterized by proximal limb shortening, severely disturbed endochondrial bone formation, and mental retardation, but there is an absence of the neuronal migration defect found in the other PBDs. Plasmalogen biosynthesis and phytanic acid oxidation are deficient, but very long chain fatty acid (VLCFA) oxidation is normal. At the cellular level, RCDP is unique in that the biogenesis of most peroxisomal proteins is normal, but a specific subset of at least four, and maybe more, peroxisomal matrix proteins fail to be imported from the cytosol. In this review, we discuss recent advances in understanding RCDP, most prominently the cloning of the affected gene, PEX7, and identification of PEX7 mutations in RCDP patients. Human PEX7 was identified by virtue of its sequence similarity to its Saccharomyces cerevisiae ortholog, which had previously been shown to encode Pex7p, an import receptor for type 2 peroxisomal targeting sequences (PTS2). Normal human PEX7 expression rescues the cellular defects in cultured RCDP cells, and cDNA sequence analysis has identified a variety of PEX7 mutations in RCDP patients, including a deletion of 100 nucleotides, probably due to a splice site mutation, and a prevalent nonsense mutation which results in loss of the carboxyterminal 32 amino acids. Identification of RCDP as a PTS2 import disorder explains the observation that several, but not all, peroxisomal matrix proteins are mistargeted in this disease; three of the four proteins deficient in RCDP have now been shown to be PTS2-targeted.

Abnormal sterol metabolism in patients with Conradi-Hünermann-Happle syndrome and sporadic lethal chondrodysplasia punctata

The term, "chondrodysplasia punctata" (CDP) denotes a pattern of abnormal punctate calcification of dystrophic epiphyseal cartilage and certain other cartilaginous structures, such as the larynx. CDP occurs in a variety of genetic disorders associated with skeletal dwarfism and can also be caused by prenatal exposure to warfarin. Although the most studied clinical syndrome with CDP, rhizomelic chondrodysplasia punctata (RCDP), is known to be caused by several different abnormalities of plasmalogen biosynthesis, there are many other genetic disorders with CDP for which the biochemical cause is unknown. Because patients with Smith-Lemli-Opitz syndrome, a primary disorder of sterol biosynthesis, often have rhizomesomelic limb shortness and, less commonly, CDP, we assessed sterol levels and metabolism in patients with different clinical forms of CDP. By quantitative sterol analysis of a variety of tissues, we identified 5 patients with similar radiological findings and abnormally increased levels of 8-dehydrocholesterol and cholest-8(9)-en-3beta-ol, suggesting a deficiency of 3beta-hydroxysteroid-delta8,delta7-isomerase, a principal enzyme of cholesterol biosynthesis. Cultured cells available from one patient showed increased levels of the same two sterols, decreased synthesis of cholesterol, and a pattern of inhibition by triparanol and AY-9944 consistent with a deficiency of 3beta-hydroxysteroid-delta8,delta7-isomerase. Clinical diagnoses among the 5 patients included X-linked dominant Conradi-Hünermann-Happle syndrome and nonspecific lethal CDP. We conclude that abnormal cholesterol biosynthesis is a characteristic of some clinical syndromes with rhizomesomelic dwarfing and CDP.

Lipid metabolism in peroxisomes in relation to human disease

Peroxisomes were long believed to play only a minor role in cellular metabolism but it is now clear that they catalyze a number of important functions. The importance of peroxisomes in humans is stressed by the existence of a group of genetic diseases in man in which one or more peroxisomal functions are impaired. Most of the functions known to take place in peroxisomes have to do with lipids. Indeed, peroxisomes are capable of 1. fatty acid beta-oxidation 2. fatty acid alpha-oxidation 3. synthesis of cholesterol and other isoprenoids 4. ether-phospholipid synthesis and 5. biosynthesis of polyunsaturated fatty acids. In Chapters 2-6 we will discuss the functional organization and enzymology of these pathways in detail. Furthermore, attention is paid to the permeability properties of peroxisomes with special emphasis on recent studies which suggest that peroxisomes are closed structures containing specific membrane proteins for transport of metabolites. Finally, the disorders of peroxisomal lipid metabolism will be discussed.