The xylosyltransferase I gene polymorphism c.343G>T (p.A125S) is a risk factor for diabetic nephropathy in type 1 diabetes

First description of the complete human xylosyltransferase-I promoter region

BACKGROUND

Human xylosyltransferase-I (XT-I) catalyzes the rate-limiting step in proteoglycan glycosylation. An increase in XYLT1 mRNA expression and serum XT activity is associated with diseases characterized by abnormal extracellular matrix accumulation like, for instance, fibrosis. Nevertheless, physiological and pathological mechanisms of transcriptional XT regulation remain elusive.

RESULTS

To elucidate whether promoter variations might affect the naturally occurring variability in serum XT activity, a complete sequence analysis of the XYLT1 promoter was performed in genomic DNA of healthy blood donors. Based on promoter amplification by a specialized PCR technique, sequence analysis revealed a fragment of 238 bp, termed XYLT1 238*, which has never been described in the human XYLT1 reference sequence so far. In silico characterization of this unconsidered fragment depicted an evolutionary conservation between sequences of Homo sapiens and Pan troglodytes (chimpanzee) or Mus musculus (mouse), respectively. Promoter activity studies indicated that XYLT1 238* harbors various transcription factor binding sites affecting basal XYLT1 expression and inducibility by transforming growth factor-β1, the key fibrotic mediator. A microsatellite and two single nucleotide variants (SNV), c.-403C>T and c.-1088C>A, were identified and genotyped in 100 healthy blood donors. Construct associated changes in XYLT1 promoter activity were detected for several sequence variants, whereas serum XT activity was only marginally affected.

CONCLUSIONS

Our findings describe for the first time the entire XYLT1 promoter sequence and provide new insights into transcriptional regulation of XT-I. Future studies should analyze the impact of regulatory XYLT1 promoter variations on XT-associated diseases.

Forward genetics defines Xylt1 as a key, conserved regulator of early chondrocyte maturation and skeletal length

The long bones of the vertebrate body are built by the initial formation of a cartilage template that is later replaced by mineralized bone. The proliferation and maturation of the skeletal precursor cells (chondrocytes) within the cartilage template and their replacement by bone is a highly coordinated process which, if misregulated, can lead to a number of defects including dwarfism and other skeletal deformities. This is exemplified by the fact that abnormal bone development is one of the most common types of human birth defects. Yet, many of the factors that initiate and regulate chondrocyte maturation are not known. We identified a recessive dwarf mouse mutant (pug) from an N-ethyl-N-nitrosourea (ENU) mutagenesis screen. pug mutant skeletal elements are patterned normally during development, but display a ~20% length reduction compared to wild-type embryos. We show that the pug mutation does not lead to changes in chondrocyte proliferation but instead promotes premature maturation and early ossification, which ultimately leads to disproportionate dwarfism. Using sequence capture and high-throughput sequencing, we identified a missense mutation in the Xylosyltransferase 1 (Xylt1) gene in pug mutants. Xylosyltransferases catalyze the initial step in glycosaminoglycan (GAG) chain addition to proteoglycan core proteins, and these modifications are essential for normal proteoglycan function. We show that the pug mutation disrupts Xylt1 activity and subcellular localization, leading to a reduction in GAG chains in pug mutants. The pug mutant serves as a novel model for mammalian dwarfism and identifies a key role for proteoglycan modification in the initiation of chondrocyte maturation.

Mutations in fam20b and xylt1 reveal that cartilage matrix controls timing of endochondral ossification by inhibiting chondrocyte maturation

Differentiating cells interact with their extracellular environment over time. Chondrocytes embed themselves in a proteoglycan (PG)-rich matrix, then undergo a developmental transition, termed "maturation," when they express ihh to induce bone in the overlying tissue, the perichondrium. Here, we ask whether PGs regulate interactions between chondrocytes and perichondrium, using zebrafish mutants to reveal that cartilage PGs inhibit chondrocyte maturation, which ultimately dictates the timing of perichondral bone development. In a mutagenesis screen, we isolated a class of mutants with decreased cartilage matrix and increased perichondral bone. Positional cloning identified lesions in two genes, fam20b and xylosyltransferase1 (xylt1), both of which encode PG synthesis enzymes. Mutants failed to produce wild-type levels of chondroitin sulfate PGs, which are normally abundant in cartilage matrix, and initiated perichondral bone formation earlier than their wild-type siblings. Primary chondrocyte defects might induce the bone phenotype secondarily, because mutant chondrocytes precociously initiated maturation, showing increased and early expression of such markers as runx2b, collagen type 10a1, and ihh co-orthologs, and ihha mutation suppressed early perichondral bone in PG mutants. Ultrastructural analyses demonstrated aberrant matrix organization and also early cellular features of chondrocyte hypertrophy in mutants. Refining previous in vitro reports, which demonstrated that fam20b and xylt1 were involved in PG synthesis, our in vivo analyses reveal that these genes function in cartilage matrix production and ultimately regulate the timing of skeletal development.

Xylosyltransferase II is a significant contributor of circulating xylosyltransferase levels and platelets constitute an important source of xylosyltransferase in serum

Circulating glycosyltransferases including xylosyltransferases I (XylT1) and II (XylT2) are potential serum biomarkers for various diseases. Understanding what influences the serum activity of these enzymes as well as the sources of these enzymes is important to interpreting the significance of alterations in enzyme activity during disease. This article demonstrates that in the mouse and human the predominant XylT in serum is XylT2. Furthermore, that total XylT levels in human serum are approximately 200% higher than those in plasma due in part to XylT released by platelets during blood clotting in vitro. In addition, the data from Xylt2 knock-out mice and mice with liver neoplasia show that liver is a significant source of serum XylT2 activity. The data presented suggest that serum XylT levels may be an informative biomarker in patients who suffer from diseases affecting platelet and/or liver homeostasis.

Xylosyltransferase gene variants and their role in essential hypertension

BACKGROUND

An accumulation of extracellular matrix molecules, such as proteoglycans, is observed in the vascular wall of hypertensive patients. Xylosyltransferases I and II (XT-I and XT-II), the chain-initiating enzymes in the biosynthesis of proteoglycans, catalyze the transfer of D-xylose from UDP-D-xylose to specific serine residues of the core protein. Because associations between XYLT polymorphisms and an altered blood pressure have been observed, genetic variations in the XYLT genes might predispose to essential hypertension. The localization of the XYLT2 gene on chromosome 17q increases its attractiveness as this region has been reported to be a potential candidate locus for essential hypertension.

METHODS

Genotyping of four polymorphisms in the genes XYLT1 and XYLT2 was performed in 150 unrelated essential hypertension patients and 150 age- and sex-matched normotensive controls using restriction fragment length polymorphism analysis.

RESULTS

The allele and genotype frequencies of the XYLT variants investigated did not show any significant differences between patients and controls, among allele-carriers and nonallele-carriers and among recessive and nonrecessive allele-carriers comparing patients and controls. Systolic and diastolic blood pressures did not differ significantly between the genotypes concerning all XYLT variants analyzed. Two XYLT2 variants deviated from Hardy-Weinberg equilibrium (HWE) in the hypertensive group.

CONCLUSIONS

No statistically significant association was found between four XYLT variants and hypertension or blood pressure, suggesting that they do not play a significant role in the development of essential hypertension. The deviation from HWE of two XYLT2 variants might be due to gene-phenotype associations which remain to be explored, as well as the possibility of gene-gene interactions.

Identification of a xylosyltransferase II gene haplotype marker for diabetic nephropathy in type 1 diabetes

BACKGROUND

Proteoglycans are major components of the glomerular basement membrane, being responsible for their permeability properties. Type 1 diabetic patients have an altered proteoglycan metabolism, which contributes to microvascular complications like diabetic nephropathy. Xylosyltransferase II (XT-II) is a chain-initiating enzyme in the biosynthesis of basement membrane proteoglycans and catalyzes the transfer of xylose to selected serine residues in the core protein. Thus, genetic variations in the XT-II coding gene XYLT2 might be implicated in the initiation and progression of late diabetic complications.

METHODS

Genotyping of 6 genetic variations in the XYLT2 gene and haplotype analysis was performed in 697 type 1 diabetic patients (358 with and 338 without diabetic nephropathy).

RESULTS

The haplotype analysis of 6 XYLT2 polymorphisms revealed one haplotype (GATTCG) to be significantly less frequent among type 1 patients with diabetic nephropathy (p=0.002, OR=0.13, 95% CI=0.03-0.59). The haplotype GATTCG consist of the XYLT2 variations c.166G>A, c.177A>G, c.342T>C, IVS6-9T>C, c.1569C>T and c.2402C>G. No genotype-phenotype interactions were revealed.

CONCLUSIONS

Our data show that a XYLT2 haplotype is associated with nephropathy in type 1 diabetic patients.

Xylosyltransferase I variants and their impact on abdominal aortic aneurysms

BACKGROUND

The formation of abdominal aortic aneurysm (AAA) is caused by a destructive remodeling of the extracellular matrix in the vascular wall. Proteoglycan content and biosynthesis have been shown to be altered in AAA. Xylosyltransferase I (XT-I) is the initial and rate-limiting enzyme in the biosynthesis of the proteoglycan-linked glycosaminoglycan chains. A familial predisposition to AAA is well recognized. Thus, variations in the XT-I coding gene XYLT1 might be risk factors for AAA formation.

METHODS

We performed genotyping of two genetic variations in the XYLT1 gene which, have been already linked to proteoglycan-associated diseases, in 129 AAA patients and 129 age- and sex-matched healthy controls.

RESULTS

The T-allele of the polymorphism c.343G>T (p.A115S) was found to be significantly more frequent in AAA patients compared to the healthy control group, demonstrating that carriers of the T-allele have a 5-fold increased risk of developing AAA (odds ratio 4.87, 95%-CI 1.38-17.19; p=0.011).

CONCLUSIONS

Our results show that XT-I polymorphisms potentially confer to the genetic susceptibility of AAA.