Dual-probe fluorescence in situ hybridization assay for detecting deletions associated with VCFS/DiGeorge syndrome I and DiGeorge syndrome II loci

Over 90% of patients with DiGeorge syndrome (DGS) or velocardiofacial syndrome (VCFS) have a microdeletion at 22q11.2. Given that these deletions are difficult to visualize at the light microscopic level, fluorescence in situ hybridization (FISH) has been instrumental in the diagnosis of this disorder. Deletions on the short arm of chromosome 10 are also associated with a DGS-like phenotype. Since deletions at 22q11.2 and at 10p13p14 result in similar findings, we have developed a dual-probe FISH assay for screening samples referred for DGS or VCFS in the clinical laboratory. This assay includes two test probes for the loci, DGSI at 22q11.2 and DGSII at 10p13p14, and centromeric probes for chromosomes 10 and 22. Of 412 patients tested, 54 were found to be deleted for the DGSI locus on chromosome 22 (13%), and a single patient was found deleted for the DGSII locus on chromosome 10 (0. 24%). The patient with the 10p deletion had facial features consistent with VCFS, plus sensorineural hearing loss, and renal anomalies. Cytogenetic analysis showed a large deletion of 10p [46, XX,del(10)(p12.2p14)] and FISH using a 10p telomere region-specific probe confirmed the interstitial nature of the deletion. Analysis for the DGSI and the DGSII loci suggests that the deletion of the DGSII locus on chromosome 10 may be 50 times less frequent than the deletion of DGSI on chromosome 22. The incidence of deletions at 22q11.2 has been estimated to be 1 in 4000 newborns; therefore, the deletion at 10p13p14 may be estimated to occur in 1 in 200,000 live births.

Isolation and chromosomal localization of two human CDP-diacylglycerol synthase (CDS) genes

Phototransduction in Drosophila is a phosphoinositide-mediated signaling pathway. Phosphatidylinositol 4,5-bisphosphate (PIP2) plays a central role in this process, and its levels are tightly regulated. A photoreceptor-specific form of the enzyme CDP-diacylglycerol synthase (CDS), which catalyzes the formation of CDP-diacylglycerol from phosphatidic acid, is a key regulator of the amount of PIP2 available for signaling. cds mutants develop light-induced retinal degeneration. As part of a search for novel genes that may be involved in eye disease in human, using Drosophila phototransduction genes as a model system, two human CDP-diacylglycerol synthase genes (CDS1 and CDS2) were cloned and sequenced. Radiation hybrid panel mapping and fluorescence in situ hybridization were used to localize the genes to chromosomes 4q21 and 20p13. As yet, no known retinal diseases map to either of these regions.

Cloning and developmental expression analysis of chick Hira (Chira), a candidate gene for DiGeorge syndrome

Deletions within human chromosome 22q11 cause a wide variety of birth defects including the DiGeorge syndrome and velo-cardio-facial (Shprintzen) syndrome. Despite the positional cloning of several genes from the critical region, it is still not possible to state whether the phenotype is secondary to haploinsufficiency of one or more than one gene. In embryological studies phenocopies of these abnormalities are produced by a variety of actions which disrupt the contribution made by the cranial and cardiac neural crest to development. The TUPLE1/HIRA gene is related to WD40 domain transcriptional regulators and maps within the DiGeorge critical region. We have cloned the chick homologue of HIRA and conducted in situ expression analysis in early chick embryos. Hira is expressed in the developing neural plate, the neural tube, neural crest and the mesenchyme of the head and branchial arch structures. HIRA may therefore have a role in the haploinsufficiency syndromes caused by deletion of 22q11.

A common region of 10p deleted in DiGeorge and velocardiofacial syndromes

DiGeorge (DGS, MIM 188400) and velocardiofacial (VCFS, MIM 192430) syndromes may present many clinical problems including cardiac defects, hypoparathyroidism, T-cell immunodeficiency and facial dysmorphism. They are frequently associated with deletions within 22q11.2, but a number of cases have no detectable molecular defect of this region. A number of single case reports with deletions of 10p suggest genetic heterogeneity of DGS. Here we compare the regions of hemizygosity in four patients with terminal deletions of 10p (one patient diagnosed as having hypoparathyroidism and three as DGS) and one patient with a large interstitial deletion (diagnosed as VCFS). Fluorescence in situ hybridization (FISH) analysis demonstrates that these patients have overlapping deletions at the 10p13/10p14 boundary. A YAC contig spanning the shortest region of deletion overlap (SRO) has been assembled, and allows the size of SRO to be approximated to 2 Mb. As with deletions of 22q11, phenotypes vary considerably between affected patients. These results strongly support the hypothesis that haploinsufficiency of a gene or genes within 10p (the DGSII locus) can cause the DGS/VCFS spectrum of malformation.

Isolation of a putative transcriptional regulator from the region of 22q11 deleted in DiGeorge syndrome, Shprintzen syndrome and familial congenital heart disease

A wide spectrum of birth defects are caused by deletions of the DiGeorge syndrome critical region (DGCR) at human chromosome 22q11. Over one hundred such deletions have now been examined and a minimally deleted region of 300kb defined. Within these sequences we have identified a gene expressed during human and murine embryogenesis. The gene, named TUPLE1, and its murine homologue, encodes a protein containing repeated motifs similar to the WD40 domains found in the beta-transducin/enhancer of split (TLE) family. The TUPLE1 product has several features typical of transcriptional control proteins and in particular has homology with the yeast Tup1 transcriptional regulator. We propose that haploinsufficiency for TUPLE1 is at least partly responsible for DiGeorge syndrome and related abnormalities.

Isolation of a gene expressed during early embryogenesis from the region of 22q11 commonly deleted in DiGeorge syndrome

DiGeorge syndrome (DGS) is one of several syndromes associated with deletions within the proximal long-arm of chromosome 22. The region of chromosome 22q11 responsible for the haploinsufficiency syndromes (the DiGeorge Critical Region or DGCR) has been mapped using RFLPs, quantitative Southern blotting and FISH. Similar deletions are seen in the velo-cardio-facial syndrome (VCFS) and familial congenital heart defects. It is not known whether the phenotypic spectrum is the result of the hemizygosity of one gene or whether it is a consequence of contiguous genes being deleted. However, the majority of patients have a large (> = 2Mb deletion). In this paper we report the isolation of a gene, lab name T10, encoding a serine/threonine rich protein of unknown function which maps to the commonly deleted region of chromosome 22q11. Studies in the mouse indicate that it maps to MMU16 and is expressed during early embryogenesis. Although not mapping within the shortest region of overlap for DGS/VCFS, and therefore not the major gene involved in DGS, the expression pattern suggests that this gene may be involved in modifying the haploinsufficient phenotype of hemizygous patients.

Phenotypical features of an unique Irish family with severe autosomal recessive osteogenesis imperfecta

Severe Sillence type II/III Osteogenesis imperfecta (OI) is a lethal or severely crippling disease with either autosomal dominant or recessively inherited type I collagen mutations. Here we describe the detailed clinical features of a thin-ribbed OI variant with deformed limbs. The three consecutively affected children showed no genetic linkage with either of the two type I collagen genes, which implies that a novel mechanism causes this clinical phenotype. It can be prevented using ultrasound to diagnose affected foetuses.

Prenatal diagnosis and prevention of inherited abnormalities of collagen

There is now strong evidence for the implication of collagen alpha 1(I), alpha 2(I) and alpha 1(III) mutations in many forms of osteogenesis imperfecta and inherited arterial aneurysms (Ehlers Danlos syndrome type IV). A sizeable proportion of these disorders have detectable abnormalities by conventional protein chemistry, immunofluorescence, or more sophisticated DNA analysis. Everyone of them with specific defects or with linkage to appropriate gene markers is therefore amenable to prevention using conventional prenatal diagnosis by chorionic villus biopsy (with fibroblast culture), fetoscopic biopsy (with fibroblast culture), ultrasound diagnosis of the severely deformed fetus, or gene linkage studies by chorionic villus biopsy or amniocentesis. Already many collagen alpha 1(I), alpha 2(I) and alpha 1(III) mutations have been characterized including point mutations, small and large deletions and regulatory mutations. Many others are likely to be rapidly studied by exploiting recent advances in DNA technology, and other strong candidate genes include collagen II (some chondrodystrophies), collagen VI (certain arterial and cardiovascular diseases) and collagen VII (dystrophic epidermolysis bullosa). Other important common diseases are likely to include osteoporosis, osteoarthritis and cerebral aneurysms. A detailed review is provided of collagen interstitial genes and proteins, together with a description of the various forms of osteogenesis imperfecta and Ehlers Danlos syndrome in which either collagen alpha 1(I), alpha 2(I) or alpha 1(III) mutations have been identified. Appropriate restriction length polymorphisms (RFLPs) useful in identifying carriers of these mutant genes are also described.