Barth syndrome may be due to an acyltransferase deficiency

Barth syndrome is an X-linked inherited disorder characterized by short stature, cardioskeletal myopathy, neutropenia, abnormal mitochondria, and respiratory-chain dysfunction [[1]xX-linked cardioskeletal myopathy and neutropenia (Barth syndrome): respiratory-chain abnormalities in cultured fibroblasts. Barth, PG, Van den Bogert, C, Bolhuis, PA, Scholte, HR, van Gennip, AH, Schutgens, RB, and Ketel, AG. J Inherit Metab Dis. 1996; 19: 157–160Crossref | PubMed | Scopus (57)See all References, [2]xAn X-linked mitochondrial disease affecting cardiac muscle, skeletal muscle and neutrophil leucocytes. Barth, PG, Scholte, HR, Berden, JA, Van der Klei-Van Moorsel, JM, Luyt-Houwen, IE, Van’t Veer-Korthof, ET et al. J Neurol Sci. 1983; 62: 327–355Abstract | Full Text PDF | PubMed | Scopus (365)See all References]. It is often fatal in childhood due to cardiac failure or sepsis arising from agranulocytosis. The phenotype associated with this disorder is quite variable, however, and other X-linked cardiomyopathies [[3]xEndocardial fibroelastosis: possible X-linked inheritance. Hodgson, S, Child, A, and Dyson, M. J Med Genet. 1987; 24: 210–214Crossref | PubMedSee all References, [4]xPossible X-linked congenital mitochondrial cardiomyopathy in three families. Orstavik, KH, Skjorten, F, Hellebostad, M, Haga, P, and Langslet, A. J Med Genet. 1993; 30: 269–272Crossref | PubMedSee all References, [5]xX-linked fatal infantile cardiomyopathy maps to Xq28 and is possibly allelic to Barth syndrome. Gedeon, AK, Wilson, MJ, Colley, AC, Sillence, DO, and Mulley, JC. J Med Genet. 1995; 32: 383–388Crossref | PubMedSee all References] may be allelic to Barth syndrome, which maps to a gene-rich region of Xq28 [6xMapping of the locus for X-linked cardioskeletal myopathy with neutropenia and abnormal mitochondria (Barth syndrome) to Xq28. Bolhuis, PA, Hensels, GW, Hulsebos, TJ, Baas, F, and Barth, PG. Am J Hum Genet. 1991; 48: 481–485PubMedSee all References[6]. Recently, a gene mutated in patients afflicted with Barth syndrome (G4.5) was cloned and sequenced [7xA novel X-linked gene, G4.5, is responsible for Barth syndrome. Bione, SP, D’Adamo, P, Maestrini, E, Gedeon, AK, Bolhuis, PA, and Toniolo, D. Nat Genet. 1996; 12: 385–389Crossref | PubMed | Scopus (429)See all References[7]. It encodes several proteins (designated tafazzins) by means of alternate splicing. The biological function of tafazzins is unclear; a BLAST search [8xBasic local alignment search tool. Altschul, SF, Gish, W, Miller, W, Myers, EW, and Lipman, DJ. J Mol Biol. 1990; 215: 403–410PubMed | Scopus (0)See all References[8] finds significant pairwise similarity only to two hypothetical proteins: one from worm (g1130664), and one from yeast (g1066481).Here, I report that human tafazzins belong to a superfamily consisting of acyltransferases involved in phospholipid biosynthesis and other proteins of unknown function. This superfamily was found using PROBE [9xExtracting protein alignment models from the sequence database. Neuwald, AF, Liu, JS, Lipman, DJ, and Lawrence, CE. Nucleic Acids Res. 1997; 25: 1665–1677Crossref | PubMed | Scopus (184)See all References[9], an automated search and multiple alignment program based on iterative database searches. Starting with a plant 1-acylglycerol-3-phosphate acyltransferase (EC 2.3.1.51), g1197334, PROBE returned a superfamily that includes known and putative acyltransferases from bacteria, fungi, plants, and vertebrate and invertebrate metazoans. Characterized enzymes in this superfamily all function in phospholipid biosynthesis and have either glycerolphosphate, 1-acylglycerolphosphate, or 2-acylglycerolphosphoethanolamine acyltransferase activity.The sequence alignment contains five conserved regions that presumably reflect similar structural and functional features shared by these proteins (Figure 1Figure 1). As all of the characterized proteins are acyltransferases involved in phospholipid biosynthesis, the uncharacterized proteins are likely to have similar catalytic activity. Notably, motif A contains a fully conserved residue position that may correspond to a catalytic histidine, as has been found at the active site of other CoA-dependent hydrolases [10xStructure of chloramphenicol acetyltransferase at 1.75-A resolution. Leslie, AG, Moody, PC, and Shaw, WV. Proc Natl Acad Sci USA. 1988; 85: 4133–4137Crossref | PubMedSee all References[10]. Of course, it is possible that tafazzins perform some other hydrolytic function. Indeed, hydrolytic activity was previously predicted for tafazzins based on weak similarity to the Escherichia coli radC gene [11xPositionally cloned human disease genes: patterns of evolutionary conservation and new functional motifs. Mushegian, AR, Bassett, DE, Boguski, MS, Bork, P, and Koonin, EV. Proc Natl Acad Sci USA. 1997; 94: 5831–5836Crossref | PubMed | Scopus (202)See all References[11], which may possess hydrolytic activity needed for DNA repair. Nevertheless, the more extensive similarity of tafazzins to these acyltransferases implies a closer similarity in function.Figure 1Alignment of representative sequences in the tafazzins, or acyltransferase superfamily. A total of 53 proteins in the NCBI non-redundant database were detected by the PROBE search [9xExtracting protein alignment models from the sequence database. Neuwald, AF, Liu, JS, Lipman, DJ, and Lawrence, CE. Nucleic Acids Res. 1997; 25: 1665–1677Crossref | PubMed | Scopus (184)See all References[9], which used default parameter settings. Conserved residues are highlighted in red (for the most conserved positions) or black. Numbers in parentheses are gap lengths. Human tafazzin is detected at the p<0.00001 level of significance; this is based on a database search using an alignment lacking sequences with statistically significant pairwise similarity to tafazzins (that is, lacking sequences g1263110, g1130664, g1066481, g1841552, g1403001 and g1673483). (The database search and the p-value calculation were done as previously described [13xGibbs motif sampling: detection of bacterial outer membrane protein repeats. Neuwald, AF, Liu, JS, and Lawrence, CE. Protein Sci. 1995; 4: 1618–1632Crossref | PubMedSee all References[13].) Protein identifiers are highlighted according to the following color scheme: black, tafazzin and close homologs; red, 1-acylglycerol-phosphate acyltransferases (agpat); blue, glycerolphosphate acyltransferases (gpat); green, 2-acylglycerolphosphoethanolamine acyltransferase (agpeat); unhighlighted, proteins of unknown function.View Large Image | View Hi-Res Image | Download PowerPoint SlideThe potential acyltransferase activity of tafazzins suggests a possible disease mechanism underlying Barth syndrome. Differential splicing of tafazzins [7xA novel X-linked gene, G4.5, is responsible for Barth syndrome. Bione, SP, D’Adamo, P, Maestrini, E, Gedeon, AK, Bolhuis, PA, and Toniolo, D. Nat Genet. 1996; 12: 385–389Crossref | PubMed | Scopus (429)See all References[7] and the existence of at least nine of these putative acyltransferases in roundworm — by contrast with the four detected in the E. coli genome — suggests that a variety of substrate-specific or tissue-and organelle-specific forms of these acyltransferases exist in eukaryotes. If so, then the mitochondrial structural and respiratory-chain abnormalities associated with Barth syndrome may be due to alterations in mitochondrial membrane phospholipid composition. Consistent with this notion, a temperature-sensitive Chinese hamster ovary cell mutant deficient in an enzyme needed for cardiolipin biosynthesis showed alterations in mitochondrial morphology and respiration [12xMitochondrial dysfunction of a cultured Chinese hamster ovary cell mutant deficient in cardiolipin. Ohtsuka, T, Nishijima, M, Suzuki, K, and Akamatsu, Y. J Biol Chem. 1993; 268: 22914–22919PubMedSee all References[12].It is important to note that the roundworm ZK809.2 gene may be an ortholog of the human G4.5 (tafazzins) gene. It shares several splice sites with the human gene and the predicted product is more closely related to tafazzins than to any other protein in the superfamily. Notably, the worm protein is missing exon 5, which appears to be removed from many of the tafazzin splice variants [7xA novel X-linked gene, G4.5, is responsible for Barth syndrome. Bione, SP, D’Adamo, P, Maestrini, E, Gedeon, AK, Bolhuis, PA, and Toniolo, D. Nat Genet. 1996; 12: 385–389Crossref | PubMed | Scopus (429)See all References[7]. Furthermore, the worm homolog shares several conserved regions with human tafazzins that are unconserved in the superfamily as a whole. Thus, ZK809.2 mutants may serve as a useful model to explore the molecular mechanisms underlying Barth syndrome.