Human Deletion Syndrome Research Articles

Abstract P3192: Prdm16 Regulates Cardiac Metabolic Genes Through Perm1

The heart is a metabolically demanding organ that requires significant amounts of energy to function properly, and even minor metabolic deficiencies during development can lead to heart failure. Cardiomyopathies are a group of conditions that affect the heart's contractile function, and PRDM16 deficiency is a primary cause of cardiomyopathies associated with 1p36 deletion syndrome in humans. Conditional deletion of Prmd16 in the heart causes a spectrum of cardiac phenotypes including left ventricular non-compaction, late-onset heart failure, and dilated cardiomyopathy. Still, the mechanisms underlying these cardiac phenotypes with loss of Prdm16 are not fully understood. We and others demonstrated that lack of Prdm16 in the heart causes alterations in fatty acid (FA) oxidation and oxidative phosphorylation (OXPHOS) gene expression. Further, we now show that Prdm16 cardiac deletion reduces the expression of key regulators of FA oxidation and OXPHOS genes including Perm1. Loss and gain of Prdm16 function in vitro confirmed a cell-autonomous regulation of Perm1 by Prdm16 in H9c2 rat cardiomyoblasts and neonatal rat ventricular myocytes (NRVMs). Prdm16 ChIP-sequencing analysis of embryonic hearts revealed direct binding of Prdm16 to the promoter of Perm1. Prdm16 binding to Perm1 promoter was further confirmed by ChIP-qPCR in H9c2 cells. Knockdown of Prdm16 in NRVMs reduced maximal mitochondrial respiration, which was rescued by over-expressing Perm1. Further, PERM1 mRNA was diminished in human induced pluripotent stem cells-derived cardiomyocytes (iPSC-CMs) derived from a patient with a PRDM16 mutation (Q187X). These results are evidence that PRDM16 is an upstream regulator of PERM1 in the heart.

Further evidence for lack of association of PRDM9 polymorphisms and 22q11.2 deletion syndrome

The 22q11.2 Deletion Syndrome (22q11.2DS) represents the most common recurrent deletion syndrome in humans. It results from Non-Allelic Homologous Recombination (NAHR) between sister chromatids or homologous chromosomes, favored by the presence of sequences of low copy repeats (LCRs) in the region. Additional events leading to the clustering in this region might be associated to proteins of the meiotic recombination mechanism acting as predisposing factors, as suggested for PRDM9. The aim of this study was to investigate polymorphisms in the PRDM9 gene in parents of individuals with the 22q11.2DS. The casuistry was composed of 31 family trios, including the index case and both parents, for which the parental origin of the 22q11 deletion was defined through microsatellite DNA markers analyses. The results were informative for 28 of the 31 families, for which 17 (61%) had maternal and 11 (39%) paternal origin. The genotyping of the PRDM9 for the 56 parents showed that 43 (77%) of them were homozygous, while the other 13 (23%) were heterozygous. Sequencing of the PRDM9 polymorphic region showed a genotypic distribution of 26 (60%) individuals A/A and 17 (40%) individuals N/N. Among the control group, represented by the non-transmitting parents, the frequency of allele A was 65%, while among the parents-of-origin it represented 55%, a difference with no statistical significance (p = 0.3811 Fisher's exact test). This study reinforces the lack of evidence for the association of the PRDM9 genotype as a susceptibility factor to the 22q11.2DS.

A rare cardiac phenotype of dextrocardia observed in a fetus with 1p36 deletion syndrome and a balanced translocation: a prenatal case report

BackgroundChromosome 1p36 deletion syndrome is a contiguous genetic disorder with multiple congenital anomalies and mental retardation. It has been emerging as one of the most common terminal deletion syndromes in humans with the rapid utility of microarray analysis. However, the prenatal findings of 1p36 deletion syndrome are still limited. We report a fetus with 1p36 deletion and cardiac phenotype of dextrocardia, combined with a balanced translocation between chromosome 5 and 6. The phenotype of dextrocardia is rarely reported in prenatal 1p36 deletion cases.Case presentationWe present a prenatal 1p36 deletion case with congenital heart diseases and single umbilical artery. Fetal echocardiography showed dextrocardia, ventricular septal defect and pericardial effusion. Fetal karyotype revealed a de novo balanced translocation of 46,XY,t(5;6)(q11.2;q23.3). Chromosomal microarray analysis detected a pathogenic deletion in 1p36.21p36.12, with the size of 6.38 Mb. Further whole genome sequencing revealed that the balanced translocation disrupted the EYA4 and ITGA1 genes.ConclusionsAlthough congenital heart diseases are very common clinical manifestations among patients with 1p36 deletion, dextrocardia is a quite rare cardiac phenotype. This is the second case with 1p36 deletion and dextrocardia, and the first prenatally diagnosed 1p36 deletion case with dextrocardia. Our case indicates that genes in 1p36 are associated with not only heart structural anomalies, but also cardiac laterality development. Our results also imply that the EYA4 gene disrupted by the balanced translocation might be related with the cardiac development.

Deletions and duplications of the 22q11.2 region in spermatozoa from DiGeorge/velocardiofacial fathers.

BackgroundDiGeorge/velocardiofacial syndrome (DGS/VCFS) is the most common deletion syndrome in humans. Low copy repeats flanking the 22q11.2 region confer a substrate for non-allelic homologous recombination (NAHR) events leading to rearrangements. This study sought to identify DGS/VCFS fathers with increased susceptibility to deletions and duplications at the 22q11.2 region in spermatozoa and to assess the particular contribution of intra-chromatid and/or inter-chromatid NAHR. Semen samples from nine DGS/VCFS fathers were analyzed by triple-color FISH using a probe combination that discriminated between normal, deleted and duplicated genotypes. Microsatellite analysis were performed in the parents and the affected children to determine the parental origin of the deleted chromosome 22.ResultsA significant increase in 22q11.2 deletions was observed in the sperm of two out of nine DGS/VCFS fathers (odds ratio 2.03-fold, P < 0.01), and in both cases the deletion in the offspring was transmitted by the father. Patients with significant increases in sperm anomalies presented a disturbed deletion:duplication 1:1 ratio (P < 0.01).ConclusionsAltogether, results support that intra-chromatid NAHR is the mechanism responsible for the higher rate of sperm deletions, which is directly related to the transmission of the deleted chromosome 22 to offspring. Accordingly, the screening of sperm anomalies in the 22q11.2 region should be taken into account in the genetic counseling of DGS/VCFS families.

The evaluation of DiGeorge syndrome gene deletion using molecular cytogenetic techniques

Background Di-George Syndrome (DGS) is known as 22q11.2 deletion syndrome. It is a genetic disorder that is being recognized with increasing frequency with a documented incidence of approximately 1 in 4000 and is the most common human deletion syndrome, typically present early in life and is rarely appearing in adult patients (1). Micro-deletion of chromosome 22q11.2 is one of the most clinically variable syndromes, with more than 180 features associated with the deletion. The syndrome is caused by genetic deletions (loss of a small part of the genetic material) found on one of the two 22nd chromosomes (2). Very rarely, patients with similar clinical features may have deletions on the chromosome 10. An accurate diagnosis is needed for the proper management of affected cases. Diagnosis relies on conventional cytogenetic and Fluorescent In Situ Hybridization (FISH) techniques. The newly developed technique, array Comparative Genomic Hybridization (aCGH), allows

Accurate, fast and cost-effective diagnostic test for monosomy 1p36 using real-time quantitative PCR.

Monosomy 1p36 is considered the most common subtelomeric deletion syndrome in humans and it accounts for 0.5–0.7% of all the cases of idiopathic intellectual disability. The molecular diagnosis is often made by microarray-based comparative genomic hybridization (aCGH), which has the drawback of being a high-cost technique. However, patients with classic monosomy 1p36 share some typical clinical characteristics that, together with its common prevalence, justify the development of a less expensive, targeted diagnostic method. In this study, we developed a simple, rapid, and inexpensive real-time quantitative PCR (qPCR) assay for targeted diagnosis of monosomy 1p36, easily accessible for low-budget laboratories in developing countries. For this, we have chosen two target genes which are deleted in the majority of patients with monosomy 1p36: PRKCZ and SKI. In total, 39 patients previously diagnosed with monosomy 1p36 by aCGH, fluorescent in situ hybridization (FISH), and/or multiplex ligation-dependent probe amplification (MLPA) all tested positive on our qPCR assay. By simultaneously using these two genes we have been able to detect 1p36 deletions with 100% sensitivity and 100% specificity. We conclude that qPCR of PRKCZ and SKI is a fast and accurate diagnostic test for monosomy 1p36, costing less than 10 US dollars in reagent costs.

Deletion of the Mouse Homolog ofKCNAB2, a Gene Linked to Monosomy 1p36, Results in Associative Memory Impairments and Amygdala Hyperexcitability

Ablation of the distal end of the short arm of chromosome 1 [1p36 deletion syndrome (1p36DS)] is one of the most commonly occurring terminal deletion syndromes in humans, occurring in ∼1 in 5000 newborns. Subjects with 1p36DS manifest a wide range of clinical features including growth delay, congenital heart defects, and craniofacial dysmorphism. In addition, individuals with 1p36DS often exhibit some form of neurological abnormality and are typically cognitively impaired. Although there is significant variability with regard to the extent of the deletion, several genes have been mapped to region 1p36 that are known to regulate neuronal function. One such gene--KCNAB2--encodes the potassium channel auxiliary subunit Kvβ2, which has been previously shown to modulate voltage-gated potassium currents in heterologous expression systems. Here, we present experiments characterizing mice in which the ortholog of KCNAB2 was deleted. We find that deletion of Kcnab2 in mice leads to deficits in associative learning and memory. In addition, using whole-cell current-clamp, we find that deletion of Kcnab2 leads to a reduction in the slow afterhyperpolarization following a burst of action potentials and a concomitant increase in neuronal excitability in projection neurons in the lateral nucleus of the amygdala. Our results suggest that loss of Kvβ2 likely contributes to the cognitive and neurological impairments observed in 1p36DS patients.

Calmodulin methyltransferase is an evolutionarily conserved enzyme that trimethylates Lys-115 in calmodulin

Calmodulin (CaM) is a key mediator of calcium-dependent signalling and is subject to regulatory post-translational modifications, including trimethylation of Lys-115. In this paper, we identify a class I, non-SET domain protein methyltransferase, calmodulin-lysine N-methyltransferase (EC 2.1.1.60). A polypeptide chosen from a fraction enriched in calmodulin methyltransferase activity was trypsinized and analysed by tandem mass spectrometry. The amino-acid sequence obtained identified conserved, homologous proteins of unknown function across a wide range of species, thus implicating a broad role for lysine methylation in calcium-dependent signalling. Encoded by c2orf34, the human homologue is a component of two related multigene deletion syndromes in humans. Human, rat, frog, insect and plant homologues were cloned and Escherichia coli-recombinant proteins catalysed the formation of a trimethyllysyl residue at position 115 in CaM, as verified by product analyses and mass spectrometry.

Prenatal and postnatal diagnosis of 22q11.2 deletion syndrome

Microdeletion of chromosome 22q11.2, the most common human deletion syndrome encompasses a wide spectrum of abnormalities. Many clinical or ultrasonographic findings may support deletion studies, either in utero or in the post-natal period. The objective of our study was to evaluate the circumstances of 22q11.2 deletion diagnosis in a single centre of genetics during a 12 years period. Testing for 22q11.2 deletion was performed in 883 cases. Congenital heart defect was the most common reason for referral. An antenatal 22q11.2 microdeletion was detected in 8 fetuses (4.7%) among 169 pregnancies, all presenting conotruncal anomalies. In one case prenatal diagnosis led to the identification of the deletion in the mildly affected father and had negative impact on the family. During the same period, postnatal 22q11.2 DS was diagnosed in 81 out of 714 patients aged from birth to 42 years (11.3%) ( p = 0.02). A CHD was present in 37 (45.7%). This figure is significantly lower than the 75% commonly reported. These results suggest that deletion studies could be justifiable in fetuses with non-cardiac prenatal sonographic findings that have been reported in association with 22q11.2 DS. However, as most of these malformations are rather common and non specific, systematic 22q11.2 testing is not justifiable. In such cases, careful cardiac and thymus examination could provide additional clues for 22q11.2 testing. In addition parents should be given accurate information before antenatal or postnatal testing, including the wide variability of the clinical phenotype, the impossibility to establish a precise prognosis concerning psychomotor development and psychiatric risks.

Cri du chat syndrome: A critical review

A new syndrome was identified in 1963, when Lejeune et al. reported a genetic disease resulting from a partial or total deletion on the short arm of chromosome 5 (5p-) and named it the cri du chat syndrome (CdCS). This term makes reference to the main clinical feature of the syndrome, a high-pitched monochromatic cat-like crying, that usually disappears in the first years of life. CdCS is one of the most common chromosomal deletion syndromes in humans, with an incidence of 1:15.000-1:50.000 live-births. Our purpose was to review different aspects of this syndrome (concept, epidemiology, aetiology, clinical features, diagnostic methods and prognosis) emphasizing both: the breakthrough in this field introduced by new cytogenetic and molecular techniques, and the orofacial manifestations most frequently reported. The main orofacial anomalies registered were mandibular microretrognathia, high palate but rarely cleft, variable malocclusion (frequently anterior open-bite), enamel hypoplasia, poor oral hygiene, generalized chronic periodontitis, and retardation of tooth eruption, although there is not enough evidence to support any clear association between these pathologies and the CdCS.

Seizures as the first manifestation of chromosome 22q11.2 deletion syndrome in a 40-year old man: a case report

BackgroundThe microdeletion of chromosome 22q11.2 is the most common human deletion syndrome. It typically presents early in life and is rarely considered in adult patients. As part of the manifestations of this condition, patients can have parathyroid glandular involvement ranging from hypocalcemic hypoparathyroidism to normocalcemia with normal parathryroid hormone levels. The first manifestation of the syndrome might be seizures due to profound hypocalcemia.Case presentationA 40-year-old man without significant past medical history presented with a new-onset generalized tonic-clonic seizure. He had no personal history of hypocalcemia or seizures. Physical examination was remarkable for short stature, hypertelorism, prominent forehead and nasal voice. His initial laboratory examination showed hypocalcemia (Calcium 5.2 mg/dl and Calcium ionized 0.69 mmol/l) with hypoparathyroidism (Parathyroid hormone intact < 2.5 pg/ml. NV: 14–72 pg/ml). Urine Calcium was 3 mg/dl on a spot and 88 mg in a 24-hour urine collection (NV: 100–300 mg/24 hs). The electrocardiogram showed a prolonged corrected QT interval. Echocardiogram, abdominal ultrasound and electroencephalogram were normal. A computer tomography of the brain showed basal ganglia calcification. The subtle physical findings and the presence of idiopathic hypoparathyroidism motivated the performance of fluorescent in situ hybridization which demonstrated a microdeletion on one of the homologs 22q11.2. The patient was treated with calcium citrate and calcitriol with good response.ConclusionMicrodeletion of chromosome 22q11.2 is among the most clinically variable syndromes, with more than 180 features associated with the deletion. It has a variable phenotypical expression, requiring a high level of awareness for its early diagnosis. Seizures, related to marked hypocalcemia due to idiopathic hypoparathyroidism, might be the presenting feature in an adult patient with this syndrome.

Association between a common haplotype in the COMT gene region and psychiatric disorders in individuals with 22q11.2DS

The 22q11.2 deletion syndrome (22q11.2DS) is the most common hemizygous deletion syndrome in humans. In addition to a wide range of physical abnormalities 22q11.2DS subjects show high prevalence of several psychiatric disorders. In our previous study we showed that the low-activity allele (158Met) of the COMT gene is a risk factor for attention deficit hyperactivity disorder (ADHD) and obsessive-compulsive disorder (OCD) in 22q11.2DS individuals. In the present study we have genotyped fifty-five 22q11.2DS individuals and 95 of their parents for eight SNPs in and around the COMT gene. A haplotype composed of three SNPs [rs2097603; rs4680 (158Val/Met); rs165599] representing the major linkage disequilibrium blocks in COMT and previously implicated in functional variation, was found to be associated with ADHD and OCD in 22q11.2DS individuals. A common risk haplotype (G-A-A) was significantly associated with both ADHD (OR 3.13, chi2=4.38, p=0.036) and OCD (OR 4.00, chi2=6.41, p=0.011) in 22q11.2DS individuals. Interestingly, the same haplotype was recently found to be associated with efficient prefrontal performance in the general population. The risk haplotype was not found to be associated with IQ scores in our 22q11.2DS sample. Parental origin of the deletion did not affect the susceptibility to ADHD and OCD in the 22q11.2DS subjects. This study demonstrated the association of a particular COMT haplotype with susceptibility to both ADHD and OCD in 22q11.2DS and supports the hypothesis that COMT gene variations contribute to genetic predisposition to psychiatric disorders in the general population.

Representational oligonucleotide microarray analysis (ROMA) and comparison of binning and change-point methods of analysis: application to detection of del22q11.2 (DiGeorge) Syndrome

DiGeorge (del22q11.2) syndrome is estimated to occur in 1:4,000 births, is the most common contiguous-gene deletion syndrome in humans, and is caused by autosomal dominant deletions in the 22q11.2 DiGeorge syndrome critical region (DGCR). Multiple microarray methods have been developed recently for analyzing such copy number changes, but data analysis and accurate deletion detection remains challenging. Clinical use of these microarray methods would have many advantages, particularly when the possibility of a chromosomal disorder cannot be determined simply on the basis of history and physical examination data alone. We investigated the use of the microarray technique, representational oligonucleotide microarray analysis (ROMA), in the detection of del22q11.2 syndrome. Genomic DNA was isolated from three well-characterized cell lines with 22q11.2 DGCR deletions and from the blood of a patient suspected of having del22q11.2 syndrome, and analyzed using both the binning and change-point model algorithms. Though the 22q11.2 deletion was easily identified with either method, change-point models provide clearer identification of deleted regions, with the potential for fewer false-positive results. For circumstances in which a clear, a priori, copy-number change hypothesis is not present, such as in many clinical samples, change-point methods of analysis may be easier to interpret.

22q11.2 deletion syndrome as a natural model for COMT haploinsufficiency-related dopaminergic dysfunction in ADHD

In the current issue of IJNP Gothelf and co-workers report an important study which sheds light on the association between chromosome 22q11.2 deletion syndrome (22q11.2 DS; velocardiofacial syndrome, VCFS), dopaminergic transmission, and psychiatric disorders. 22q11.2 DS occurs in at least 1/5000 live births and thus is the most common chromosomal deletion syndrome in humans (Botto et al., 2003). The deletion in the long arm of chromosome 22, usually comprises up to 3 Mb, and is transmitted dominantly, although most patients suffer from de-novo mutations. The clinical phenotype ranges from immune deficiency with thymal aplasia, congenital heart abnormalities, velopharyngeal insufficiency with hypernasal speech, slight dysmorphy with characteristic facies, to severe syndromal developmental abnormalities with mental retardation (Shprintzen, 2000). Most interestingly for the field of neuropsychiatry, 22q11.2 DS is associated with an increased rate of variety of psychiatric disorders. In childhood and adolescence, 22q11.2 DS is associated with attention deficit hyperactivity disorder (ADHD; Antshel et al., 2005; Niklasson et al., 2002), affecting up to 55% of the patients (Gothelf et al., 2004). Another, less frequent comorbidity in adolescence seems to be obsessive–compulsive disorder (OCD; Gothelf et al., 2006). Finally, there is an increased prevalence for comorbidity with schizophrenic and bipolar affective psychoses in adulthood (Murphy, 2005).

Tbx1 is regulated by forkhead proteins in the secondary heart field†

Transcriptional regulation in a tissue-specific and quantitative manner is essential for developmental events, including those involved in cardiovascular morphogenesis. Tbx1 is a T-box-containing transcription factor that is responsible for many of the defects observed in 22q11 deletion syndrome in humans. Tbx1 is expressed in the secondary heart field (SHF) and is essential for cardiac outflow tract (OFT) development. We previously reported that Tbx1 is regulated by sonic hedgehog by means of forkhead (Fox) transcription factors in the head mesenchyme and pharyngeal endoderm, but how it is regulated in the SHF is unknown. Here, we show that Tbx1 expression in the SHF is regulated by Fox proteins through a combination of two evolutionarily conserved Fox binding sites in a dose-dependent manner. Cell fate analysis using the Tbx1 enhancer suggests that SHF-derived Tbx1-expressing cells contribute extensively to the right ventricular myocardium as well as the OFT during early development and ultimately give rise to the right ventricular infundibulum, pulmonary trunk, and pulmonary valves. These results suggest that Fox proteins are involved in most, if not all, Tbx1 expression domains and that Tbx1 marks a subset of SHF-derived cells, particularly those that uniquely contribute to the right-sided outflow tract and proximal pulmonary artery.

Velo-cardio-facial syndrome

Velo-cardio-facial syndrome (VCFS) is the most common contiguous gene deletion syndrome in humans, caused by a microdeletion from chromosome 22 at the q11.2 locus. Moreover, it is one of the most common multiple anomaly syndromes associated with congenital heart disease and is certainly the most common syndrome causing conotruncal heart anomalies. The population prevalence of this syndrome is probably increasing because of the advances in the diagnosis and treatment of congenital heart malformations. Presenting symptoms are highly variable because VCFS is often the trigger for a number of developmental malformation sequences, including DiGeorge sequence, Robin sequence, and Potter sequence. Because of the variability of the phenotypic expression of VCFS, the disorder has been delineated in several sets of literature from different areas of specialization, and from publications in the U.S., Eastern Europe, and Japan. The result has been nosologic differences with various names for the same disorder caused by a 22q11.2 deletion, including VCFS, Shprintzen syndrome, DiGeorge syndrome, Sedlačková syndrome, Cayler syndrome, and conotruncal anomalies face syndrome. All of these labels represent exactly the same syndrome. Understanding the cause of the various phenotypes associated with VCFS is dependent on careful studies of genotype to phenoype matching. The deleted region in most cases of VCFS encompasses 3 million base pairs of DNA and 30 genes. The challenge is therefore to determine if the broad spectrum of anomalies are caused by one major gene, most or all of the deleted genes, or the interaction between the genes in the deleted region and other genes elsewhere in the genome. There is some evidence for each of these hypotheses. The ability to complete phenotype to genotype matching is enhanced by the recognition of more cases and the careful study of phenotypic expression. Towards this end, screening of cases with congenital heart disease is imperative and guidelines for screening are discussed.

Tbx1 regulates fibroblast growth factors in the anterior heart field through a reinforcing autoregulatory loop involving forkhead transcription factors.

Birth defects, which occur in one out of 20 live births, often affect multiple organs that have common developmental origins. Human and mouse studies indicate that haploinsufficiency of the transcription factor TBX1 disrupts pharyngeal arch development, resulting in the cardiac and craniofacial features associated with microdeletion of 22q11 (del22q11), the most frequent human deletion syndrome. Here, we have generated an allelic series of Tbx1 deficiency that reveals a lower critical threshold for Tbx1 activity in the cardiac outflow tract compared with other pharyngeal arch derivatives, including the palatal bones. Mice hypomorphic for Tbx1 failed to activate expression of the forkhead transcription factor Foxa2 in the pharyngeal mesoderm, which contains cardiac outflow precursors derived from the anterior heart field. We identified a Fox-binding site upstream of Tbx1 that interacted with Foxa2 and was necessary for pharyngeal mesoderm expression of Tbx1, revealing an autoregulatory loop that may explain the increased cardiac sensitivity to Tbx1 dose. Downstream of Tbx1, we found a fibroblast growth factor 8 (Fgf8) enhancer that was dependent on Tbx1 in vivo for regulating expression in the cardiac outflow tract, but not in pharyngeal arches. Consistent with its role in regulating cardiac outflow tract cells Tbx1 gain of function resulted in expansion of the cardiac outflow tract segment derived from the anterior heart field as marked by Fgf10. These findings reveal a Tbx1-dependent transcriptional and signaling network in the cardiac outflow tract that renders mouse cardiovascular development more susceptible than craniofacial development to a reduction in Tbx1 dose, similar to humans with del22q11.

Monosomy 1p36 breakpoints indicate repetitive DNA sequence elements may be involved in generating and/or stabilizing some terminal deletions.

Monosomy 1p36 is the most commonly observed terminal deletion syndrome in humans. Our previous molecular studies on a large cohort of subjects suggest that monosomy 1p36 can result from a variety of chromosomal rearrangements including terminal truncations, interstitial deletions, derivative chromosomes, inverted duplications, and complex rearrangements. However, the mechanism(s) by which rearrangements of 1p36 are generated and/or stabilized is not understood. Sequence analysis of breakpoint junctions may provide valuable clues to the underlying mechanisms of many chromosomal aberrations. In this report, we analyze the breakpoints at the DNA-sequence level in four subjects with variable-sized deletions of 1p36. All four breakpoints fall within repetitive DNA-sequence elements (LINEs, SINEs, etc). This suggests that repetitive DNA-sequence elements may play an important role in generating and/or stabilizing terminal deletions of 1p36. Mechanisms by which repetitive elements may be involved in the process of terminal deletion formation and stabilization are discussed.

The zebrafish van gogh mutation disrupts tbx1, which is involved in the DiGeorge deletion syndrome in humans.

The van gogh (vgo) mutant in zebrafish is characterized by defects in the ear, pharyngeal arches and associated structures such as the thymus. We show that vgo is caused by a mutation in tbx1, a member of the large family of T-box genes. tbx1 has been recently suggested to be a major contributor to the cardiovascular defects in DiGeorge deletion syndrome (DGS) in humans, a syndrome in which several neural crest derivatives are affected in the pharyngeal arches. Using cell transplantation studies, we demonstrate that vgo/tbx1 acts cell autonomously in the pharyngeal mesendoderm and influences the development of neural crest-derived cartilages secondarily. Furthermore, we provide evidence for regulatory interactions between vgo/tbx1 and edn1 and hand2, genes that are implicated in the control of pharyngeal arch development and in the etiology of DGS.

Monosomy 1p36 breakpoint junctions suggest pre-meiotic breakage-fusion-bridge cycles are involved in generating terminal deletions.

Terminal deletions of 1p36 result in a mental retardation syndrome that is presumably caused by haploinsufficiency of a number of genes. Although monosomy 1p36 is the most commonly observed terminal deletion syndrome in humans, the molecular mechanism(s) that generates and stabilizes terminal deletions of 1p36 is not completely understood. Our previous molecular analysis of a large cohort of monosomy 1p36 subjects demonstrated that deletion sizes vary widely from approximately 1 Mb to >10.5 Mb in the most distal portion of 1p36 with no single common breakpoint. In this report, we have identified the precise breakpoint junctions in three subjects with apparently pure terminal deletions of 1p36 ranging from 2.5 to 4.25 Mb. These junctions revealed one deletion to be stabilized by telomeric repeat sequences and two to have terminal deletions associated with cryptic interrupted inverted duplications at the ends of the chromosomes. These interrupted inverted duplication/deletion breakpoints are reminiscent of those seen in tumor cell lines that have undergone breakage-fusion-bridge (BFB) cycles leading to gene amplification. We propose a pre-meiotic model for the formation of these deletions in which a terminally deleted chromosome is generated in the germ line and passes through at least one BFB cycle to produce gametes with terminal deletions associated with interrupted inverted duplications. These data suggest that, on a molecular level, seemingly pure terminal deletions visualized cytogenetically may be more complex, and BFB cycles may play an important role in generating terminal deletions associated with genetic disease in humans.