Rhizomelic chondrodysplasia punctata with isolated DHAP-AT deficiency

A Pex7 Deficient Mouse Series Correlates Biochemical and Neurobehavioral Markers to Genotype Severity—Implications for the Disease Spectrum of Rhizomelic Chondrodysplasia Punctata Type 1

Rhizomelic chondrodysplasia punctata type 1 (RCDP1) is a peroxisome biogenesis disorder caused by defects in PEX7 leading to impairment in plasmalogen (Pls) biosynthesis and phytanic acid (PA) oxidation. Pls deficiency is the main pathogenic factor that determines the severity of RCDP. Severe (classic) RCDP patients have negligible Pls levels, congenital cataracts, skeletal dysplasia, growth and neurodevelopmental deficits, and cerebral hypomyelination and cerebellar atrophy on brain MRI. Individuals with milder or nonclassic RCDP have higher Pls levels, better growth and cognitive outcomes. To better understand the pathophysiology of RCDP disorders, we generated an allelic series of Pex7 mice either homozygous for the hypomorphic allele, compound heterozygous for the hypomorphic and null alleles or homozygous for the null allele. Pex7 transcript and protein were almost undetectable in the hypomorphic model, and negligible in the compound heterozygous and null mice. Pex7 deficient mice showed a graded reduction in Pls and increases in C26:0-LPC and PA in plasma and brain according to genotype. Neuropathological evaluation showed significant loss of cerebellar Purkinje cells over time and a decrease in brain myelin basic protein (MBP) content in Pex7 deficient models, with more severe effects correlating with Pex7 genotype. All Pex7 deficient mice exhibited a hyperactive behavior in the open field environment. Brain neurotransmitters analysis of Pex7 deficient mice showed a significant reduction in levels of dopamine, norepinephrine, serotonin and GABA. Also, a significant correlation was found between brain neurotransmitter levels, the hyperactivity phenotype, Pls level and the severity of Pex7 genotype. In conclusion, our study showed evidence of a genotype-phenotype correlation between the severity of Pex7 deficiency and several clinical and neurobiochemical phenotypes in RCDP1 mouse models. We propose that PA accumulation may underlie the cerebellar atrophy seen in older RCDP1 patients, as even relatively low tissue levels were strongly associated with Purkinje cells loss over time in the murine models. Also, our data demonstrate the interrelation between Pls, brain neurotransmitter deficiencies and the neurobehavioral phenotype, which could be further used as a valuable clinical endpoint for therapeutic interventions. Finally, these models show that incremental increases in Pex7 levels result in dramatic improvements in phenotype.

Using Whole Exome Sequencing to Identify Candidate Genes With Rare Variants In Nonsyndromic Cleft Lip and Palate

Studies suggest that nonsyndromic cleft lip and palate (NSCLP) is polygenic with variable penetrance, presenting a challenge in identifying all causal genetic variants. Despite relatively high prevalence of NSCLP among Amerindian populations, no large whole exome sequencing (WES) studies have been completed in this population. Our goal was to identify candidate genes with rare genetic variants for NSCLP in a Honduran population using WES. WES was performed on two to four members of 27 multiplex Honduran families. Genetic variants with a minor allele frequency > 1% in reference databases were removed. Heterozygous variants consistent with dominant disease with incomplete penetrance were ascertained, and variants with predicted functional consequence were prioritized for analysis. Pedigree-specific P-values were calculated as the probability of all affected members in the pedigree being carriers, given that at least one is a carrier. Preliminary results identified 3,727 heterozygous rare variants; 1,282 were predicted to be functionally consequential. Twenty-three genes had variants of interest in ≥3 families, where some genes had different variants in each family, giving a total of 50 variants. Variant validation via Sanger sequencing of the families and unrelated unaffected controls excluded variants that were sequencing errors or common variants not in databases, leaving four genes with candidate variants in ≥3 families. Of these, candidate variants in two genes consistently segregate with NSCLP as a dominant variant with incomplete penetrance: ACSS2 and PHYH. Rare variants found at the same gene in all affected individuals in several families are likely to be directly related to NSCLP.

TESTING THE EFFECT OF THE EPIDEMIOLOGIC PARADOX: BIRTH WEIGHT OF NEWBORNS OF IMMIGRANT AND NON-IMMIGRANT MOTHERS IN THE REGION OF VALENCIA, SPAIN

The epidemiological paradox and ‘healthy migrant effect’ refer to the favourable health outcomes in unprivileged groups under unfavourable socioeconomic conditions. Weight at birth is associated with the epidemiological paradox. However, differences in fertility structure (mainly mother's age and first maternity) might be the cause of the difference in weight at birth between children of immigrant and non-immigrant mothers. This paper aims to analyse the impact of the epidemiologic paradox by distinguishing between the factors related to fertility structure, in addition to other socio-cultural factors. The importance of fertility structure as the cause of weight-at-birth differences of the newborns of immigrant and non-immigrant women, and between those of subgroups of immigrant mothers, is tested. Based on data from birth registries for the period 1998–2009, a variance analysis was performed for Spanish mothers and for those of five major immigrant subgroups living in the region of Valencia, Spain, which experienced significant migrant inflows within a short period of time. A Scheffé test between pairs of nationalities was carried out. Finally, linear regression models were built. The results suggest that the most relevant factors are those related to fertility structure, and that consequently the epidemiological paradox does not apply for immigrant mothers as a whole, although Bolivian immigrant offspring may be an exception. This unexpected result requires further research to test to what extent this is due to the special adaptation of multigenerational high-altitude populations in pregnancy. The factors associated with fertility structure must be controlled when trying to relate birth weight differences between ethnic groups to socioeconomic factors.

An Atypical form Rhizomelic Chondrodysplasia Punctata in a Newborn

Rhizomelic Chondrodysplasia punctata (RCDP) is an autosomal recessive metabolic disorder affecting mainly peroxisomal function. We describe a case of RCDP in a 12 days old newborn based on the clinical and radiological ground without any major systemic structural or functional abnormalities.

Defective lipid remodeling of GPI anchors in peroxisomal disorders, Zellweger syndrome, and rhizomelic chondrodysplasia punctata

Many cell surface proteins in mammalian cells are anchored to the plasma membrane via glycosylphosphatidylinositol (GPI). The predominant form of mammalian GPI contains 1-alkyl-2-acyl phosphatidylinositol (PI), which is generated by lipid remodeling from diacyl PI. The conversion of diacyl PI to 1-alkyl-2-acyl PI occurs in the ER at the third intermediate in the GPI biosynthetic pathway. This lipid remodeling requires the alkyl-phospholipid biosynthetic pathway in peroxisome. Indeed, cells defective in dihydroxyacetone phosphate acyltransferase (DHAP-AT) or alkyl-DHAP synthase express only the diacyl form of GPI-anchored proteins. A defect in the alkyl-phospholipid biosynthetic pathway causes a peroxisomal disorder, rhizomelic chondrodysplasia punctata (RCDP), and defective biogenesis of peroxisomes causes Zellweger syndrome, both of which are lethal genetic diseases with multiple clinical phenotypes such as psychomotor defects, mental retardation, and skeletal abnormalities. Here, we report that GPI lipid remodeling is defective in cells from patients with Zellweger syndrome having mutations in the peroxisomal biogenesis factors PEX5, PEX16, and PEX19 and in cells from patients with RCDP types 1, 2, and 3 caused by mutations in PEX7, DHAP-AT, and alkyl-DHAP synthase, respectively. Absence of the 1-alkyl-2-acyl form of GPI-anchored proteins might account for some of the complex phenotypes of these two major peroxisomal disorders.

Peroxisomal disorders: the single peroxisomal enzyme deficiencies

Peroxisomal disorders are a group of inherited diseases in man in which either peroxisome biogenesis or one or more peroxisomal functions are impaired. The peroxisomal disorders identified to date are usually classified in two groups including: (1) the disorders of peroxisome biogenesis, and (2) the single peroxisomal enzyme deficiencies. This review is focused on the second group of disorders, which currently includes ten different diseases in which the mutant gene affects a protein involved in one of the following peroxisomal functions: (1) ether phospholipid (plasmalogen) biosynthesis; (2) fatty acid beta-oxidation; (3) peroxisomal alpha-oxidation; (4) glyoxylate detoxification, and (5) H2O2 metabolism.

Metabolic and molecular basis of peroxisomal disorders: a review

The group of peroxisomal disorders now includes 17 different disorders with Zellweger syndrome as prototype. Thanks to the explosion of new information about the functions and biogenesis of peroxisomes, the metabolic and molecular basis of most of the peroxisomal disorders has been resolved. A review of peroxisomal disorders is provided in this paper.

Etherphospholipid biosynthesis and dihydroxyactetone-phosphate acyltransferase: resolution of the genomic organization of the human gnpat gene and its use in the identification of novel mutations

Etherphospholipids are characterised by the occurrence of an alkyl- or alkenyl-group at the sn-1 position of the glycerol backbone. Peroxisomes play an essential role in the formation of etherphospholipids since the first two enzymes of the biosynthetic pathway are strictly peroxisomal. The function of plasmalogens is still an enigma but the recent identification of patients suffering from an isolated defect in either dihydroxyacetone phosphate acyltransferase (GNPAT) or alkyldihydroxyacetone phosphate synthase provides conclusive evidence that plasmalogens play an essential role for human survival and functioning. In this paper we report the complete genomic organisation of the GNPAT gene coding for the peroxisomal dihydroxyacetone phosphate acyltransferase. The gene is located on chromosome 1q42.12-43. It spans approximately 28 kb and consists of 16 exons and 15 introns. This information was used to analyse the GNPAT gene in 12 patients with GNPAT deficiency. All patients analysed were found to have mutations in their GNPAT gene. Of the 9 different mutations found, 2 were missense mutations, 2 small deletions, 1 insertion and 3 mutations were within splice donor/acceptor-sites. Another mutation created an alternative splice donor-site causing the partial deletion of an exon. The data obtained provide conclusive evidence for the major role of GNPAT in etherphospholipid biosynthesis.

Immunological analyses of alkyl-dihydroxyacetone-phosphate synthase in human peroxisomal disorders

Alkyl-dihydroxyacetonephosphate synthase (alkyl-DHAP synthase) is a peroxisomal enzyme involved in the biosynthesis of ether phospholipids. To localize the enzyme in human peroxisomal disorders, indirect immunofluorescence and immunoblot analysis was performed. In Zellweger syndrome and rhizomelic chondrodysplasia punctata fibroblast cell lines, alkyl-DHAP synthase protein levels on immunoblots were strongly decreased and residual immunofluorescence was diffusely localized throughout the cytoplasm. In a particular neonatal adrenoleukodystrophy cell line, characterized by the absence of a functional peroxisomal targeting signal 1 receptor, the precursor form of the enzyme was detected in Western blots at levels comparable to that of the mature enzyme in control fibroblasts. Similarly, fibroblasts from patients with a single deficiency in the activity of either alkyl-DHAP synthase or DHAP-acyltransferase showed normal levels of the mature alkyl-DHAP synthase protein on immunoblots. Immunofluorescence experiments revealed a peroxisomal localization of both the precursor and the mature form of the enzyme. Collectively, these results visualize the peroxisomal localization of alkyl-DHAP synthase, indicate that the enzyme is unstable outside its target organelle and explain that normal enzyme protein levels found in some peroxisomal disorders result from protection against cytoplasmic degradation through import into peroxisomes. Additionally, alkyl-DHAP synthase could be detected in rat mesangial cells and murine NIH-3R3 fibroblasts by immunofluorescence as well as immunoblot analysis. Immunoelectron microscopy showed that the enzyme is predominantly located on the lumenal side of the peroxisomal membrane in rat and guinea pig liver.

Chondrodysplasia punctata stemming from maternal lupus erythematosus

The finding of stippled epiphyses on a neonatal radiograph generates a wide differential diagnosis, including genetic and teratogenic causes. We report the case of a male infant with stippled epiphyses evident on neonatal radiographs in whom a typical rash of lupus erythematosus developed. The skin abnormalities in the infant resulted in a diagnosis of systemic lupus erythematosus in his mother. Over a 3-year follow-up period, the child has demonstrated strikingly short stature, midface hypoplasia, anomalous digital development, slow resolution of the stippled epiphyses, and near normal cognitive development. The differential diagnosis of chondrodysplasia punctata and the literature supporting maternal lupus as one cause are reviewed.

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.

Pattern of central nervous system anomalies in a population with a high rate of consanguineous marriages

Nine thousand six hundred and ten births were prospectively studied in the three major hospitals in Al-Ain, United Arab Emirates (UAE) between October 1995 and January 1997. Babies suspected of, or diagnosed, as having central nervous system (CNS) abnormalities were evaluated by a neonatologist, a clinical geneticist and a pediatric neurologist. Brain computerized tomography/magnetic resonance imaging (CT/MRI) was performed on all babies suspected of having CNS abnormalities. In addition, metabolic screening and chromosome analysis were also performed when indicated. Of the 225 babies with congenital anomalies identified, 31 had CNS abnormalities (3.2/1000). Syndromic abnormalities of the CNS were present in 13 cases (42%), chromosomal abnormalities in one case (3.2%) and the rest included: neural tube defect (NTD) in 11 cases (36%), holoprosencephaly in two cases (6.4%) and hydrocephalus in four cases (12.9%). Detailed analysis of the syndromic types revealed that out of the 13 cases, 12 were inherited as autosomal recessive (AR) and in one case the inheritance was undetermined. Consanguinity with high level of inbreeding was present in 12 cases and the majority of the syndromes identified were extremely rare. The study indicates that CNS anomalies are fairly common in the UAE, particularly, the recessive syndromic types. Careful and detailed analysis of such anomalies is required so that accurate genetic advice can be given.

Alkyl-dihydroxyacetonephosphate synthase. Fate in peroxisome biogenesis disorders and identification of the point mutation underlying a single enzyme deficiency

Peroxisomes play an indispensible role in ether lipid biosynthesis as evidenced by the deficiency of ether phospholipids in fibroblasts and tissues from patients suffering from a number of peroxisomal disorders. Alkyl-dihydroxyacetonephosphate synthase, a peroxisomal enzyme playing a key role in the biosynthesis of ether phospholipids, contains the peroxisomal targeting signal type 2 in a N-terminal cleavable presequence. Using a polyclonal antiserum raised against alkyl-dihydroxyacetonephosphate synthase, levels of this enzyme were examined in fibroblast cell lines from patients affected by peroxisomal disorders. Strongly reduced levels were found in fibroblasts of Zellweger syndrome and rhizomelic chondrodysplasia punctata patients, indicating that the enzyme is not stable in the cytoplasm as a result of defective import into peroxisomes. In a neonatal adrenoleukodystrophy patient with an isolated import deficiency of proteins carrying the peroxisomal targeting signal type 1, the precursor form of alkyl-dihydroxyacetonephosphate synthase was detected at a level comparable to that of the mature form in control fibroblasts, in line with an intraperoxisomal localization. A patient with an isolated deficiency in alkyl-dihydroxyacetonephosphate (DHAP) synthase activity had normal levels of this protein. Analysis at the cDNA level revealed a missense mutation leading to a R419H substitution in the enzyme of this patient. Expression of a recombinant protein carrying this mutation in Escherichia coli yielded an inactive enzyme, whereas a comparable control recombinant enzyme was active, providing further proof that this substitution is responsible for the inactivity of the enzyme and the phenotype. In line with this result is the observation that wild-type alkyl-DHAP synthase activity can be inactivated by the arginine-modifying agent phenylglyoxal. The enzyme is efficiently protected against this inactivation when the substrate palmitoyl-DHAP is present at a saturating concentration. The gene encoding human alkyl-dihydroxyacetonephosphate synthase was mapped on chromosome 2q31.

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.

Zellweger syndrome and associated phenotypes

Until recently, the peroxisome was considered a "reactor chamber" for H2O2 producing oxidases, and it is now recognised as a versatile organelle performing complex catabolic and biosynthetic roles in the cell. Zellweger syndrome (ZS), the paradigm of human peroxisomal disorders, is characterised by neonatal hypotonia, severe neuro-developmental delay, hepatomegaly, renal cysts, senorineural deafness, retinal dysfunction, and facial dysmorphism. It is now clear that ZS is at the severe end of a phenotypic spectrum of Zellweger-like syndromes which may present for diagnosis later in childhood and even in adult life. It is important that clinical geneticists are aware of these milder clinical variants as the availability of sensitive and specific biochemical assays of peroxisomal function (for example, serum VLCFA ratios, platelet DHAP-AT activity) makes their diagnosis relatively straightforward.

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.

The profile of major congenital abnormalities in the United Arab Emirates (UAE) population

The aim of this study was to establish the profile of major congenital malformations in the United Arab Emirates (UAE) population which has a high rate of consanguinity. All births with birth weight above 500 g in the three hospitals in the Al Ain Medical District of UAE were prospectively studied from January 1992 to January 1994. About 98% of the births in the district occur in these three hospitals. Detailed family history and clinical and relevant laboratory investigations were recorded in each case. Necropsy was not permitted. The major malformations were classified as multiple or isolated single system abnormalities as well as genetic or non-genetic disorders. Of the 16,419 births which occurred during the two year period, 173 (10.5/1000 births) had major malformations, 90 (52%) had multiple malformations, and 83 (47.97%) had involvement of a single system. Of the infants with multiple malformations, 43 had recognised syndromes, most of which are autosomal recessive disorders with a high frequency of rare syndromes. Twenty eight (31%) had chromosomal abnormalities. The most common systems involved in infants with isolated single system malformations include gastrointestinal (33), central nervous system (17), and cardiovascular (10). While the consanguinity rate was similar (57% v 54%), the frequency of first cousin marriages was much higher (51% v 30%) in the study group compared with the figures for the general population. The consanguinity rate was highest among the syndrome cases, and related parents were more likely to have infants with multiple malformations than an isolated single system abnormality with a relative risk of 1.69 (95% CL 1.27-2.24). Genetic factors could be implicated in 116 (67%) of the 173 cases of major malformations and 49 (28%) were potentially preventable. The study suggests that genetic disorders account for a significant proportion of congenital malformation in the UAE and, thus, a genetic service should be provided as part of the preventive cae programme.

Measurement of dihydroxyacetone-phosphate acyltransferase (DHAPAT) in chorionic villous samples, blood cells and cultured cells

Dihydroxyacetone-phosphate acyltransferase (DHAPAT) is a peroxisomal enzyme catalysing the first step in ether-phospholipid biosynthesis. DHAPAT is deficient in cells from patients suffering from a variety of peroxisomal disorders. Accurate measurement of the activity of this enzyme is of great importance, especially since it is a central parameter in the prenatal diagnosis of the disorders of peroxisome biogenesis, rhizomelic chondrodysplasia punctata and DHAPAT-deficiency. We describe a straightforward and accurate assay allowing the activity of DHAPAT to be measured reliably in chorionic villus samples, blood cells, cultured skin fibroblasts, cultured chorionic villus fibroblasts and cultured amniocytes.

Chondrodysplasia punctata with a mild clinical course

We report a 7-year-old patient with chondrodysplasia punctata but without rhizomelia. He was born with typical clinical and radiological symptoms of this disease. He developed slowly with considerable psychomotor retardation but improved later, gaining some speech and psychosocial contacts. Joint contractures and bilateral cataracts are still major problems. De novo plasmalogen synthesis in fibroblasts was greatly reduced and DHAP-AT activity was at the lower limit of controls. Peroxisomal thiolase was present in its precursor form only. Membrane fluidity (measured by TMA-DPH fluorescence anisotropy) was increased in erythrocyte ghosts and in lymphocytes. Plasma phytanic acid concentration was elevated 5-fold. The patient represents a mild clinical course of chondrodysplasia punctata, resembling Conradi-Hünermann syndrome, but biochemically he has the typical peroxisomal dysfunction of rhizomelic chondrodysplasia punctata except for a high residual activity of DHAP-AT.

Isolated dihydroxyacetonephosphate acyltransferase deficiency presenting with developmental delay

A boy aged 21 months who was being investigated for developmental delay and failure to thrive was found to have punctate epiphyseal calcification. He had no evidence of rhizomelic shortening of the limbs or cataracts. Investigation revealed defective plasmalogen synthesis due to isolated deficiency of dihydroxyacetonephosphate acyltransferase (DHAP-AT). The parents were consanguineous and a sister was similarly affected, suggesting autosomal recessive inheritance. Hitherto, recessively inherited isolated DHAP-AT deficiency has only been described in patients with a phenotype similar to that of rhizomelic chondrodysplasia punctata. This report indicates that the same biochemical disorder can be associated with a less severe phenotype.