Mutations in ATP6N1B, encoding a new kidney vacuolar proton pump 116-kD subunit, cause recessive distal renal tubular acidosis with preserved hearing.

The multi-subunit H+-ATPase pump is present at particularly high density on the apical (luminal) surface of -intercalated cells of the cortical collecting duct of the distal nephron, where vectorial proton transport is required for urinary acidification. The complete subunit composition of the apical ATPase, however, has not been fully agreed upon. Functional failure of -intercalated cells results in a group of disorders, the distal renal tubular acidoses (dRTA), whose features include metabolic acidosis accompanied by disturbances of potassium balance, urinary calcium solubility, bone physiology and growth. Mutations in the gene encoding the B-subunit of the apical pump (ATP6B1) cause dRTA accompanied by deafness. We previously localized a gene for dRTA with preserved hearing to 7q33-34 (ref. 4). We report here the identification of this gene, ATP6N1B, which encodes an 840 amino acid novel kidney-specific isoform of ATP6N1A, the 116-kD non-catalytic accessory subunit of the proton pump. Northern-blot analysis demonstrated ATP6N1B expression in kidney but not other main organs. Immunofluorescence studies in human kidney cortex revealed that ATP6N1B localizes almost exclusively to the apical surface of -intercalated cells. We screened nine dRTA kindreds with normal audiometry that linked to the ATP6N1B locus, and identified different homozygous mutations in ATP6N1B in eight. These include nonsense, deletion and splice-site changes, all of which will truncate the protein. Our findings identify a new kidney-specific proton pump 116-kD accessory subunit that is highly expressed in proton-secreting cells in the distal nephron, and illustrate its essential role in normal vectorial acid transport into the urine by the kidney.

A gene encoding a putative GTPase regulator is mutated in familial amyotrophic lateral sclerosis 2.

Amyotrophic lateral sclerosis 2 (ALS2) is an autosomal recessive form of juvenile ALS and has been mapped to human chromosome 2q33. Here we report the identification of two independent deletion mutations linked to ALS2 in the coding exons of the new gene ALS2. These deletion mutations result in frameshifts that generate premature stop codons. ALS2 is expressed in various tissues and cells, including neurons throughout the brain and spinal cord, and encodes a protein containing multiple domains that have homology to RanGEF as well as RhoGEF. Deletion mutations are predicted to cause a loss of protein function, providing strong evidence that ALS2 is the causative gene underlying this form of ALS.

Murine phosphatidylserine-specific phospholipase A1 (Ps-pla1) maps to chromosome 16 but is distinct from the lpd (lipid defect) locus.

We have previously generated a mouse transgenic line with an insertional mutation designated lpd that demonstrates a phenotype of hypertriglyceridemia and fatty liver. Since the recently identified phosphatidylserine-specific phospholipase A1 (PS-PLA1) demonstrates significant homology to triglyceride lipases, we reasoned that the mouse Ps-plaI gene may be the disrupted gene within the lpd locus. Using a rat PS-PLA1 cDNA sequence to search the EST database, we identified a mouse EST homolog AA839424. Sequencing analysis of AA839424 revealed a putative Ps-pla1 protein of 456 amino acids with extensive overall structural conservation with human and rat PS-PLA1 and with triglyceride lipases. Conserved sequences in Ps-pla1 include a lipase consensus sequences GxSxG, a catalytic triad, and eight of the ten conserved cysteine residues that are required for tertiary structure. Mouse Ps-plal carries a phosphatidylserine-binding motif that is absent in all triglyceride lipases. Using a mouse whole-genome radiation hybrid (WG-RH) mapping panel (T31), we mapped mouse Ps-pla1 to Chromosome (Chr) 16 between genetic markers D16Mit194 and D16Mit38, which is 17.1 cM centromeric to the lpd locus. On the basis of chromosome location, we conclude that Ps-pla1 and lpd are distinct genes in lipid metabolism.

cDNA cloning, characterization and chromosome mapping of the gene encoding human cartilage associated protein (CRTAP).

We have recently isolated and characterized cDNA clones coding for a novel developmentally regulated avian and mouse embryo protein, CASP for Cartilage Associated Protein. Here we describe the isolation and characterization of the gene coding for the human CASP. The comparison of the putative human and mouse protein sequences with the chick sequence revealed an overall high identity (89% and 51%, respectively). Homology search with known DNA and protein sequences showed that CASPs are related to two mammalian nuclear proteins. Here we demonstrate definitively that CASPs are distinct from these nuclear proteins. However, sequence comparison analyses suggest that all of these proteins belong to a new family. In all human tissues examined two CASP mRNA species were detected, whereas a single mRNA and three mRNAs were found in chick and mouse, respectively. The human CASP gene (CRTAP) was assigned to chromosome 3p22 by fluorescence in situ hybridization.

p200 ARF-GEP1: a Golgi-localized guanine nucleotide exchange protein whose Sec7 domain is targeted by the drug brefeldin A.

The drug brefeldin A (BFA) disrupts protein traffic and Golgi morphology by blocking activation of ADP ribosylation factors (ARFs) through an unknown mechanism. Here, we investigated the cellular localization and BFA sensitivity of human p200 ARF-GEP1 (p200), a ubiquitously expressed guanine nucleotide exchange factor of the Sec7 domain family. Multiple tagged forms of the full-length polypeptide localized to tight ribbon-like perinuclear structures that overlapped with the Golgi marker mannosidase II and were distinct from the pattern observed with ERGIC53/58. Analysis of several truncated forms mapped the Golgi-localization signal to the N-terminal third of p200. BFA treatment of transiently or stably transfected cells resulted in the redistribution of Golgi markers and in loss of cell viability, thereby indicating that overproduction of p200 may not be sufficient to overcome the toxic effect. A 39-kDa fragment spanning the Sec7 domain catalyzed loading of guanosine 5'-[gamma-thio]triphosphate onto class I ARFs and displayed clear sensitivity to BFA. Kinetic analysis established that BFA did not compete with ARF for interaction with p200 but, rather, acted as an uncompetitive inhibitor that only targeted the p200-ARF complex with an inhibition constant of 7 microM. On the basis of these results, we propose that accumulation of an abortive p200-ARF complex in the presence of BFA likely leads to disruption of Golgi morphology. p200 mapped to chromosome 8q13, 3.56 centirays from WI-6151, and database searches revealed the presence of putative isoforms whose inhibition may account for the effects of BFA on various organelles.

Small GTPase Rac1: structure, localization, and expression of the human gene.

Rac1 is a member of the Rho family of small GTPases involved in signal transduction pathways that control proliferation, adhesion, and migration of cells during embryonic development and invasiveness of tumor cells. Here we present the complete structure of the human RAC1 gene and characterize its expression. The gene comprises 7 exons over a length of 29 kb and is localized to chromosome 7p22. The GC-rich gene promoter shows characteristics of a housekeeping gene and Northern blot studies revealed ubiquitous expression of two rac1 transcripts, 1.2 and 2.5 kb in size. The two transcripts are expressed in tissue-specific ratios, reflecting competition between two alternative polyadenylation sites. The RAC1 but not RAC2 gene contains an additional exon 3b that is included by alternative splicing into the variant Rac1b, a constitutively active mutant which induces the formation of lamellipodia in fibroblasts. These data indicate that the RAC1 gene encodes two signaling GTPases. The gene structure reported here will enable studies on the regulation of RAC1 expression during tumorigenesis and development.

Cloning and characterization of three novel genes, ALS2CR1, ALS2CR2, and ALS2CR3, in the juvenile amyotrophic lateral sclerosis (ALS2) critical region at chromosome 2q33-q34: candidate genes for ALS2.

Amyotrophic lateral sclerosis is a progressive neurodegenerative disease that manifests as selective upper and lower motor neuron degeneration. The autosomal recessive form of juvenile amyotrophic lateral sclerosis (ALS2) has previously been mapped to the 1.7-cM interval flanked by D2S116 and D2S2237 on human chromosome 2q33-q34. We identified three novel full-length transcripts encoded by three distinct genes (HGMW-approved symbols ALS2CR1, ALS2CR2, and ALS2CR3) within the ALS2 critical region. The intron-exon organizations of these genes as well as those of CFLAR, CASP10, and CASP8, which were previously mapped to this region, were defined. These genes were evaluated for mutations in ALS2 patients, and no disease-associated sequence alterations in either exons or intron-exon boundaries were observed. Sequence analysis of overlapping RT-PCR products covering the whole coding sequence for each transcript revealed no aberrant mRNA sequences. These data strongly indicate that ALS2CR1, ALS2CR2, ALS2CR3, CFLAR, CASP10, and CASP8 are not causative genes for ALS2.