David Valle

Mutations in the PTS1 receptor gene, PXR1, define complementation group 2 of the peroxisome biogenesis disorders.

The peroxisome biogenesis disorders (PBDs) are lethal recessive diseases caused by defects in peroxisome assembly. We have isolated PXR1, a human homologue of the yeast P. pastoris PAS8 (peroxisome assembly) gene. PXR1, like PAS8, encodes a receptor for proteins with the type–1 peroxisomal targeting signal (PTS1). Mutations in PXR1 define complementation group 2 of PBDs and expression of PXR1 rescues the PTS1 import defect of fibroblasts from these patients. Based on the observation that PXR1 exists both in the cytosol and in association with peroxisomes, we propose that PXR1 protein recognizes PTS1 –containing proteins in the cytosol and directs them to the peroxisome.

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.

Carnitine deficiency presenting as familial cardiomyopathy: a treatable defect in carnitine transport.

We studied a boy who presented at age 3 1/2 years with cardiomegaly, a distinctive electrocardiogram, and a history of a brother dying with cardiomyopathy. From age 3 1/2 to 5 years, cardiac disease progressed, resulting in intractable congestive heart failure. Skeletal muscle weakness developed and a muscle biopsy showed lipid myopathy. Muscle and plasma carnitine were reduced to 2 and 10% of the normal mean values, respectively. Therapy with L-carnitine (174 mg/kg/da) was begun at age 5 1/2 years and continued to the present (age 6 1/2 years). The cardiac disease has resolved and the muscle strength has returned to normal. Plasma carnitine concentrations have risen to the low-normal range, while urinary carnitine excretion has increased to values which are 30 times normal. The renal clearance of carnitine exceeds normal at all plasma concentrations and plasma carnitine values do not change acutely after an oral carnitine load. These results suggest that there is a distinct form of carnitine deficiency which presents as cardiomyopathy and may be successfully treated with L-carnitine. A defect in renal and possibly gastrointestinal transport of carnitine is a likely cause of this patients disorder.

The peroxisome biogenesis disorder group 4 gene, PXAAA1, encodes a cytoplasmic ATPase required for stability of the PTS1 receptor.

In humans, defects in peroxisome assembly result in the peroxisome biogenesis disorders (PBDs), a group of genetically heterogeneous, lethal recessive diseases. We have identified the human gene PXAAA1 based upon its similarity to PpPAS5, a gene required for peroxisome assembly in the yeast Pichia pastoris. Expression of PXAAA1 restored peroxisomal protein import in fibroblasts from 16 unrelated members of complementation group 4 (CG4) of the PBD. Consistent with this observation, CG4 patients carry mutations in PXAAA1. The product of this gene, Pxaaa1p, belongs to the AAA family of ATPases and appears to be a predominantly cytoplasmic protein. Substitution of an arginine for the conserved lysine residue in the ATPase domain of Pxaaa1p abolished its biological activity, suggesting that Pxaaa1p is an ATPase. Furthermore, Pxaaa1p is required for stability of the predominantly cytoplasmic PTS1 receptor, Pxr1p. We conclude that Pxaaa1p plays a direct role in peroxisomal protein import and is required for PTS1 receptor activity.

PEX11α is required for peroxisome proliferation in response to 4-phenylbutyrate but is dispensable for peroxisome proliferator-activated receptor alpha-mediated peroxisome proliferation

ABSTRACT The PEX11 peroxisomal membrane proteins promote peroxisome division in multiple eukaryotes. As part of our effort to understand the molecular and physiological functions of PEX11 proteins, we disrupted the mouse PEX11α gene. Overexpression of PEX11α is sufficient to promote peroxisome division, and a class of chemicals known as peroxisome proliferating agents (PPAs) induce the expression of PEX11α and promote peroxisome division. These observations led to the hypothesis that PPAs induce peroxisome abundance by enhancing PEX11α expression. The phenotypes of PEX11α−/− mice indicate that this hypothesis remains valid for a novel class of PPAs that act independently of peroxisome proliferator-activated receptor alpha (PPARα) but is not valid for the classical PPAs that act as activators of PPARα. Furthermore, we find that PEX11α−/− mice have normal peroxisome abundance and that cells lacking both PEX11α and PEX11β, a second mammalian PEX11 gene, have no greater defect in peroxisome abundance than do cells lacking only PEX11β. Finally, we report the identification of a third mammalian PEX11 gene, PEX11γ, and show that it too encodes a peroxisomal protein.

PEX11β deficiency is lethal and impairs neuronal migration but does not abrogate peroxisome function

ABSTRACT Zellweger syndrome is a lethal neurological disorder characterized by severe defects in peroxisomal protein import. The resulting defects in peroxisome metabolism and the accumulation of peroxisomal substrates are thought to cause the other Zellweger syndrome phenotypes, including neuronal migration defects, hypotonia, a developmental delay, and neonatal lethality. These phenotypes are also manifested in mouse models of Zellweger syndrome generated by disruption of the PEX5 or PEX2 gene. Here we show that mice lacking peroxisomal membrane protein PEX11β display several pathologic features shared by these mouse models of Zellweger syndrome, including neuronal migration defects, enhanced neuronal apoptosis, a developmental delay, hypotonia, and neonatal lethality. However, PEX11β deficiency differs significantly from Zellweger syndrome and Zellweger syndrome mice in that it is not characterized by a detectable defect in peroxisomal protein import and displays only mild defects in peroxisomal fatty acid β-oxidation and peroxisomal ether lipid biosynthesis. These results demonstrate that the neurological pathologic features of Zellweger syndrome can occur without peroxisomal enzyme mislocalization and challenge current models of Zellweger syndrome pathogenesis.

Genomewide linkage scan for schizophrenia susceptibility loci among Ashkenazi Jewish families shows evidence of linkage on chromosome 10q22.

Previous linkage studies in schizophrenia have been discouraging due to inconsistent findings and weak signals. Genetic heterogeneity has been cited as one of the primary culprits for such inconsistencies. We have performed a 10-cM autosomal genomewide linkage scan for schizophrenia susceptibility regions, using 29 multiplex families of Ashkenazi Jewish descent. Although there is no evidence that the rate of schizophrenia among the Ashkenazim differs from that in other populations, we have focused on this population in hopes of reducing genetic heterogeneity among families and increasing the detectable effects of any particular locus. We pursued both allele-sharing and parametric linkage analyses as implemented in Genehunter, version 2.0. Our strongest signal was achieved at chromosome 10q22.3 (D10S1686), with a nonparametric linkage score (NPL) of 3.35 (genomewide empirical P=.035) and a dominant heterogeneity LOD score (HLOD) of 3.14. Six other regions gave NPL scores >2.00 (on chromosomes 1p32.2, 4q34.3, 6p21.31, 7p15.2, 15q11.2, and 21q21.2). Upon follow-up with an additional 23 markers in the chromosome 10q region, our peak NPL score increased to 4.27 (D10S1774; empirical P=.00002), with a 95% confidence interval of 12.2 Mb for the location of the trait locus (D10S1677 to D10S1753). We find these results encouraging for the study of schizophrenia among Ashkenazi families and suggest further linkage and association studies in this chromosome 10q region.

Mutations in the gene encoding 3 beta-hydroxysteroid-delta 8, delta 7-isomerase cause X-linked dominant Conradi-Hunermann syndrome

X-linked dominant Conradi-Hünermann syndrome (CDPX2; MIM 302960) is one of a group of disorders with aberrant punctate calcification in cartilage, or chondrodysplasia punctata (CDP). This is most prominent around the vertebral column, pelvis and long bones in CPDX2. Additionally, CDPX2 patients may have asymmetric rhizomesomelia, sectorial cataracts, patchy alopecia, ichthyosis and atrophoderma. The phenotype in CDPX2 females ranges from stillborn to mildly affected individuals identified in adulthood. CDPX2 is presumed lethal in males, although a few affected males have been reported,. We found increased 8(9)-cholestenol and 8-dehydrocholesterol in tissue samples from seven female probands with CDPX2 ( ref. 4). This pattern of accumulated cholesterol intermediates suggested a deficiency of 3β-hydroxysteroid-Δ8,Δ 7-isomerase (sterol-Δ8-isomerase), which catalyses an intermediate step in the conversion of lanosterol to cholesterol. A candidate gene encoding a sterol-Δ8-isomerase ( EBP) has been identified and mapped to Xp11.22–p11.23 (Refs 5,6). Using SSCP analysis and sequencing of genomic DNA, we found EBP mutations in all probands. We confirmed the functional significance of two missense alleles by expressing them in a sterol-Δ8-isomerase-deficient yeast strain. Our results indicate that defects in sterol-Δ8-isomerase cause CDPX2 and suggest a role for sterols in bone development.

Genomewide Linkage Scan for Bipolar-Disorder Susceptibility Loci among Ashkenazi Jewish Families

The relatively short history of linkage studies in bipolar disorders (BPs) has produced inconsistent findings. Implicated regions have been large, with reduced levels of significance and modest effect sizes. Both phenotypic and genetic heterogeneity may have contributed to the failure to define risk loci. BP is part of a spectrum of apparently familial affective disorders, which have been organized by severity. Heterogeneity may arise because of insufficient data to define the spectrum boundaries, and, in general, the less-severe disorders are more difficult to diagnose reliably. To address the inherent complexities in detecting BP susceptibility loci, we have used restricted diagnostic classifications and a genetically more homogeneous (Ashkenazi Jewish) family collection to perform a 9-cM autosomal genomewide linkage scan. Although they are genetically more homogeneous, there are no data to suggest that the rate of illness in the Ashkenazim differs from that in other populations. In a genome scan of 41 Ashkenazi pedigrees with a proband affected with bipolar I disorder (BPI) and at least one other member affected with BPI or bipolar II disorder (BPII), we identified four regions suggestive of linkage on chromosomes 1, 3, 11, and 18. Follow-up genotyping showed that the regions on chromosomes 1, 3, and 18 are also suggestive of linkage in a subset of pedigrees limited to relative pairs affected with BPI. Furthermore, our chromosome 18q22 signal (D18S541 and D18S477) overlaps with previous BP findings. This research is being conducted in parallel with our companion study of schizophrenia, in which, by use of an identical approach, we recently reported significant evidence for a schizophrenia susceptibility locus in the Ashkenazim on chromosome 10q22.

The molecular basis of human 3-methylcrotonyl-CoA carboxylase deficiency

Isolated biotin-resistant 3-methylcrotonyl-CoA carboxylase (MCC) deficiency is an autosomal recessive disorder of leucine catabolism that appears to be the most frequent organic aciduria detected in tandem mass spectrometry-based neonatal screening programs. The phenotype is variable, ranging from neonatal onset with severe neurological involvement to asymptomatic adults. MCC is a heteromeric mitochondrial enzyme composed of biotin-containing alpha subunits and smaller beta subunits. Here, we report cloning of MCCA and MCCB cDNAs and the organization of their structural genes. We show that a series of 14 MCC-deficient probands defines two complementation groups, CG1 and 2, resulting from mutations in MCCB and MCCA, respectively. We identify five MCCA and nine MCCB mutant alleles and show that missense mutations in each result in loss of function.