Ultrastructure and immunocytochemistry of hepatic peroxisomes in rhizomelic chondrodysplasia punctata

Peroxisome biogenesis disorders

Defects in PEX genes impair peroxisome assembly and multiple metabolic pathways confined to this organelle, thus providing the biochemical and molecular bases of the peroxisome biogenesis disorders (PBD). PBD are divided into two types--Zellweger syndrome spectrum (ZSS) and rhizomelic chondrodysplasia punctata (RCDP). Biochemical studies performed in blood and urine are used to screen for the PBD. DNA testing is possible for all of the disorders, but is more challenging for the ZSS since 12 PEX genes are known to be associated with this spectrum of PBD. In contrast, PBD-RCDP is associated with defects in the PEX7 gene alone. Studies of the cellular and molecular defects in PBD patients have contributed significantly to our understanding of the role of each PEX gene in peroxisome assembly.

Pex13 inactivation in the mouse disrupts peroxisome biogenesis and leads to a Zellweger syndrome phenotype

Zellweger syndrome is the archetypical peroxisome biogenesis disorder and is characterized by defective import of proteins into the peroxisome, leading to peroxisomal metabolic dysfunction and widespread tissue pathology. In humans, mutations in the PEX13 gene, which encodes a peroxisomal membrane protein necessary for peroxisomal protein import, can lead to a Zellweger phenotype. To develop mouse models for this disorder, we have generated a targeted mouse with a loxP-modified Pex13 gene to enable conditional Cre recombinase-mediated inactivation of Pex13. In the studies reported here, we crossed these mice with transgenic mice that express Cre recombinase in all cells to generate progeny with ubiquitous disruption of Pex13. The mutant pups exhibited many of the clinical features of Zellweger syndrome patients, including intrauterine growth retardation, severe hypotonia, failure to feed, and neonatal death. These animals lacked morphologically intact peroxisomes and showed deficient import of matrix proteins containing either type 1 or type 2 targeting signals. Biochemical analyses of tissue and cultured skin fibroblasts from these animals indicated severe impairment of peroxisomal fatty acid oxidation and plasmalogen synthesis. The brains of these animals showed disordered lamination in the cerebral cortex, consistent with a neuronal migration defect. Thus, Pex13(-/-) mice reproduce many of the features of Zellweger syndrome and PEX13 deficiency in humans.

Human peroxisomal disorders

Peroxisomes are single membrane-bound cell organelles performing numerous metabolic functions. The present article aims to give an overview of our current knowledge about inherited peroxisomal disorders in which these organelles are lacking or one or more of their functions are impaired. They are multiorgan disorders and the nervous system is implicated in most. After a summary of the historical names and categories, each having distinct symptoms and prognosis, microscopic pathology is reviewed in detail. Data from the literature are added to experience in the authors' laboratory with 167 liver biopsy and autopsy samples from peroxisomal patients, and with a smaller number of chorion samples for prenatal diagnosis, adrenal-, kidney-, and brain samples. Various light and electron microscopic methods are used including enzyme- and immunocytochemistry, polarizing microscopy, and morphometry. Together with other laboratory investigations and clinical data, this approach continues to contribute to the diagnosis and further characterization of peroxisomal disorders, and the discovery of novel variants. When liver specimens are examined, three main groups including 9 novel variants (33 patients) are distinguished: (1) absence or (2) presence of peroxisomes, and (3) mosaic distribution of cells with and without peroxisomes (10 patients). Renal microcysts, polarizing trilamellar inclusions, and insoluble lipid in macrophages in liver, adrenal cortex, brain, and in interstitial cells of kidney are also valuable for classification. On a genetic basis, complementation of fibroblasts has classified peroxisome biogenesis disorders into 12 complementation groups. Peroxisome biogenesis genes (PEX), knock-out-mice, and induction of redundant genes are briefly reviewed, including some recent results with 4-phenylbutyrate. Finally, regulation of peroxisome expression during development and in cell cultures, and by physiological factors is discussed.

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.

Biogenesis of peroxisomes in fetal liver

Peroxisomes are single membrane-limited cell organelles that are involved in numerous metabolic functions. Peroxisomes do not contain DNA; the matrix and membrane proteins are encoded by the nuclear genome. It is assumed that new peroxisomes are formed by division of existing organelles. The present article gives an overview of microscopic studies and recent unpublished results dealing with peroxisome biogenesis in mammalian fetal liver and presents data on peroxisomes in oocytes. Cytochemical (catalase and D-aminoacid oxidase activity) and immunocytochemical data in rat and human liver (antigens of catalase, the three peroxisomal beta-oxidation enzymes, alanine: glyoxylate aminotransferase, peroxisomal membrane proteins with molecular weights of 42 and 70 kDa) indicate that during embryonic and fetal development the peroxisomal population undergoes a differentiation with respect to the composition of the matrix and to the size and number of the organelles. In the youngest stages, rare and small peroxisomes are present, into which the matrix components are imported in a sequential way. The import seems asynchronous in peroxisomes of the same hepatocyte. The size and number of the peroxisomes increase during liver development. In rat and human liver, no morphological or immunocytochemical evidence for an elaborate network of interconnected peroxisomes ("reticulum") was found. Instead, peroxisomes presented as individual organelles, which occasionally show membrane extensions. The importance of the metabolic functions of peroxisomes in human liver is emphasized by the peroxisomal disorders. In the liver of affected fetuses, the microscopic features associated with the defect can already be recognized; i.e., either catalase containing peroxisomes are absent and catalase is localized in the cytoplasm (in fetuses affected with Zellweger syndrome or with infantile Refsum disease) or peroxisomes are present but they are abnormally enlarged (e.g., a fetus affected with acyl-CoA oxidase deficiency). In the quail ovary, numerous peroxisomes are observed in the oocyte and in the granulosa cells during follicle maturation, but not in the full-grown egg. Thus, the mechanism of peroxisome inheritance remains unresolved.

Isolated dihydroxyacetonephosphate-acyl-transferase deficiency in rhizomelic chondrodysplasia punctata: clinical presentation, metabolic and histological findings

Rhizomelic chondrodysplasia punctata (RCDP) is clinically characterized by symmetrical shortening of the proximal limbs, contractures of joints, a characteristic dysmorphic face, and cataracts. In the classical form an impairment of several peroxisomal functions and enzymes (plasmalogen synthesis, phytanic acid oxidation, 3-oxoacyl-CoA thiolase) has been repeatedly shown. Recently a variant involving only the peroxisomal dihydroxyacetonephosphate acyltransferase (DHAP-AT) has been described. We present a patient with isolated DHAP-AT deficiency and all clinical, radiological and pathological features of classical RCDP. For the first time, microscopy and immunocytochemistry of hepatocytes could be performed.

CONCLUSION

In contrast to studies on classical rhizomelic chondrodysplasia punctata which have shown enlarged peroxisomes in numbers varying from hepatocyte to hepatocyte, the peroxisomes in our patient seem to be normal in size, number and shape.

Rhizomelic chondrodysplasia punctata: report of a case with review of the literature and correlation with other peroxisomal disorders

A 3-year-old girl with rhizomelic chondrodysplasia punctata (RCDP) had severe microcephaly but a normal gyral pattern, neuronal density, and cortical cytoarchitecture. The white matter was diffusely decreased in mass but normally myelinated. There was optic atrophy and cerebellar degeneration. Leukodystrophy in peroxisomal disorders is caused by elevated very long chain fatty acids. The absence of a fatty acid abnormality in RCDP explains the normal myelination. Cerebellar and retinal degeneration and possible stunted dendritogenesis may be due to plasmalogen deficiency, which is the most severe biochemical abnormality in RCDP.

Immunoblot analysis of peroxisomal proteins in liver and fibroblasts from patients

Identification of a patient as suffering from a peroxisomal disorder usually starts by the finding of elevated very long-chain fatty acids in plasma and/or serum. This is followed by more detailed studies in blood, fibroblasts and tissues, including immunoblot analysis. Indeed, immunoblot analysis has become a valuable tool in the correct diagnosis and assignment of individual patients, except for X-linked adrenoleukodystrophy (X-ALD). We describe a simple immunoblotting procedure applicable to liver and fibroblast homo-genates using antibodies raised against catalase and the three beta-oxidation enzyme proteins acyl-CoA oxidase I, bifunctional protein and peroxisomal thiolase. The same procedure can also be used for chorionic villus biopsy specimens and has now become the method of choice for the prenatal diagnosis of Zellweger syndrome (and other disorders of peroxisome biogenesis) and rhizomelic chondrodysplasia punctata.

Immunocytochemical localization of peroxisomal proteins in human liver and kidney

The sample preparation and immunocytochemical methods for investigating the presence and subcellular localization of peroxisomal proteins (catalase, the three beta-oxidation enzymes, alanine : glyoxylate aminotransferase and a peroxisomal membrane protein) in human liver biopsies are described. We present a protocol for immunolabelling on ultrathin and semithin sections from the same tissue block, with protein A-colloidal gold as a reporter system. For this purpose, the tissue is embedded in Unicryl, a hydrophilic acrylic resin that is cured by ultraviolet illumination at 2 degrees C. The limitations and possibilities of the methods are discussed together with methodological problems. Cryostat sections of prefixed material should be used for the visualization by light microscopy of cytoplasmic catalase. It is emphasized that immunolabelling for catalase in formalin-fixed archival liver samples and in liver autopsy tissue (in the latter also for the peroxisomal beta-oxidation enzymes) permits visualization of peroxisomes; this can be helpful in diagnosing an index case retrospectively.

Secondary alterations of human hepatocellular peroxisomes

The morphological and morphometric characteristics of peroxisomes in normal human liver and the peroxisomal alterations in the liver of patients with acquired or congenital non-peroxisomal diseases are reviewed. Secondary peroxisomal changes are observed in steatosis, hepatitis and cirrhosis induced by various agents (viruses, alcohol, drugs, etc.), in cholestasis, in hepatomas, in extra-hepatic cancer with or without liver metastasis, in extrahepatic inflammatory processes, in metabolic disorders affecting metabolism of carbohydrates, lipids and lipoproteins, glycoproteins, amino acids, bilirubin or copper, and in altered thyroid hormone levels. They are recognized as a proliferation of peroxisomes (increased in number and to a lesser extent in surface density and volume density) often accompanied by a minor reduction in size (at most to 68% of the mean diameter in control livers) but very rarely by an increase in mean peroxisomal diameter, and as proliferation-related changes in shape (tails, gastruloid cisternae, funnel-like constrictions, elongation, protrusions) in at least a few of the peroxisomes. These secondary alterations of the peroxisomes are clearly distinguishable from the primary changes in peroxisomes observed in the liver of patients with congenital peroxisomal disorders.

Establishment of a normal range of morphometric values for peroxisomes in paediatric liver

The size and number of hepatic peroxisomes was investigated in 16 control paediatric liver biopsies from patients ranging in age from 3 months to 18 years one fetal liver specimen and one paediatric autopsy liver. The area, diameter, volume density (Vv), numerical density (Nv) and surface density (Sv) of the peroxisomes was recorded using randomly selected electron micrographs. The mean diameter of peroxisomes in control paediatric liver was 0.56 microns, the mean Vv was 1.67%, the mean Nv was 0.125 per micron+3 and the mean Sv was 0.161 per micron. No correlation was found between the size and number of hepatic peroxisomes and the age or sex of the patient. Peroxisomes in the fetal liver were smaller than those in biopsy tissue and had a mean diameter of 0.42 micron. Peroxisomes were identified in autopsy tissue and were enlarged with a mean diameter of 0.75 micron, most probably due to post-mortem swelling. A range of morphometric values in paediatric liver has now been established.

Morphometry of peroxisomes and immunolocalization of peroxisomal proteins in the liver of patients with generalised peroxisomal disorders

Hepatic peroxisomes were studied by morphometric and immunocytochemical techniques in control patients and in four Zellweger syndrome patients, two infantile Refsum's (IRD) patients, one neonatal adrenoleukodystrophy (NALD) patient, and three patients with peroxisomal disorders (PD) which do not fit any currently recognised classification, but have disorders involving a defect in peroxisomal biogenesis. Peroxisomes which were ultrastructurally abnormal and greatly reduced in size and/or number were found in two of the Zellweger syndrome patients, and the NALD and IRD patients. There was variation in their numerical density ranging from none at all in two of the Zellweger syndrome patients to normal numbers in the IRD patients. In most patients there was a decrease in the immunolabelling of catalase over the peroxisomes. In the Zellweger syndrome and NALD patients, the small, abnormal peroxisomes did not label for any of the beta-oxidation proteins. The IRD patients and the PD patients however, were heterogeneous with respect to beta-oxidation labelling. The ultrastructural heterogeneity of peroxisomes in these peroxisomal disorders patients indicates there may be genotypic differences between the major groups and also within each group. The common factor in all the patients in this study where peroxisomes were present was the presence in the hepatic peroxisomes of an electron dense centre which did not label immunocytochemically for catalase or the beta-oxidation enzymes. This electron dense centre may indicate a structural abnormality in the peroxisomes in these patients.

Cytoplasmic catalase and ghostlike peroxisomes in the liver from a child with atypical chondrodysplasia punctata

In the liver biopsy from an 8.5-year-old girl with the biochemical characteristics of rhizomelic chondrodysplasia punctata (RCDP), but with normal limbs, normal catalase-containing peroxisomes were absent. Light microscopy after diaminobenzidine staining for catalase activity (the peroxisomal marker enzyme) and immunostaining against catalase protein indicated a cytosolic localization of the enzyme. By electron microscopy, rare and extremely large, irregularly shaped vesicles were found in the parenchymal cells. The three peroxisomal beta-oxidation enzymes (acyl-CoA oxidase, bi(tri)functional enzyme, and 3-ketoacyl-CoA thiolase) and alanine-glyoxylate aminotransferase were immunolocalized in these organelles. However, a weak to negative label was obtained after staining against catalase. Diaminobenzidine staining demonstrated a minimal catalase reaction product in some vesicles only. Morphometry revealed a corrected mean d-circle of 1.44 microns and a maximum d-circle of 2.767 microns (controls: 0.635 microns and 1.027 microns, respectively). Numerical, volume, and surface densities were reduced to 3%, 41%, and 17% of control values, respectively. The large size, irregular shape, and rarity of the organelles are morphologic features of peroxisomal "ghosts." It seems that in this patient, apart from the known peroxisomal defects in RCDP, catalase incorporation into the peroxisomes is impaired together with a normal proliferation (division) of the organelles. In the cultured skin fibroblasts from the patient, however, immuno-electron microscopy showed normal catalase-containing peroxisomes in apparently normal numbers.

Human liver pathology in peroxisomal diseases: a review including novel data

Results from electron microscopic morphometry, enzyme cytochemistry and immunolocalization in liver biopsies are reviewed. Emphasis is put on the following aspects: 1) relationship between peroxisomal size and enzyme concentration; 2) abnormal enlargement of peroxisomes in many congenital disorders with peroxisomal dysfunction; 3) normal localization of matrix enzymes in several patients with peroxisomal dysfunction, with the exception of catalase, which is mainly cytoplasmic; 4) ghost-like peroxisomes in the liver of several syndromes but not in nine cases labelled as Zellweger; 5) discrepancies between liver and cultured fibroblasts; 6) trilamellar, regularly spaced inclusions, large stacks of which are birefringent, indicate a peroxisomal dysfunction; their absence does not exclude it. The same rule holds for lipid in macrophages which is insoluble in acetone and n-hexane (after fixation). The chemical nature of these two storage materials remains unclear; and 7) proliferation of human peroxisomes is frequent in acquired liver diseases and drug toxicity, but is never accompanied by an increase in size, in contrast to the effect of the fibrates and phthalates in rat and mouse. Novel data from seven peroxisomal patients are included.