Mitochondrial Citrate Transporter-dependent Metabolic Signature in the 22q11.2 Deletion Syndrome

The congenital disorder 22q11.2 deletion syndrome (22qDS), characterized by a hemizygous deletion of 1.5–3 Mb on chromosome 22 at locus 11.2, is the most common microdeletion disorder (estimated prevalence of 1 in 4000) and the second risk factor for schizophrenia. Nine of ∼30 genes involved in 22qDS have the potential of disrupting mitochondrial metabolism (COMT, UFD1L, DGCR8, MRPL40, PRODH, SLC25A1, TXNRD2, T10, and ZDHHC8). Deficits in bioenergetics during early postnatal brain development could set the basis for a disrupted neuronal metabolism or synaptic signaling, partly explaining the higher incidence in developmental and behavioral deficits in these individuals. Here, we investigated whether mitochondrial outcomes and metabolites from 22qDS children segregated with the altered dosage of one or several of these mitochondrial genes contributing to 22qDS etiology and/or morbidity. Plasma metabolomics, lymphocytic mitochondrial outcomes, and epigenetics (histone H3 Lys-4 trimethylation and 5-methylcytosine) were evaluated in samples from 11 22qDS children and 13 age- and sex-matched neurotypically developing controls. Metabolite differences between 22qDS children and controls reflected a shift from oxidative phosphorylation to glycolysis (higher lactate/pyruvate ratios) accompanied by an increase in reductive carboxylation of α-ketoglutarate (increased concentrations of 2-hydroxyglutaric acid, cholesterol, and fatty acids). Altered metabolism in 22qDS reflected a critical role for the haploinsufficiency of the mitochondrial citrate transporter SLC25A1, further enhanced by HIF-1α, MYC, and metabolite controls. This comprehensive profiling served to clarify the biochemistry of this disease underlying its broad, complex phenotype.     

Hearing Loss in a Mouse Model of 22q11.2 Deletion Syndrome

22q11.2 Deletion Syndrome (22q11DS) arises from an interstitial chromosomal microdeletion encompassing at least 30 genes. This disorder is one of the most significant known cytogenetic risk factors for schizophrenia, and can also cause heart abnormalities, cognitive deficits, hearing difficulties, and a variety of other medical problems. The Df1/+ hemizygous knockout mouse, a model for human 22q11DS, recapitulates many of the deficits observed in the human syndrome including heart defects, impaired memory, and abnormal auditory sensorimotor gating. Here we show that Df1/+ mice, like human 22q11DS patients, have substantial rates of hearing loss arising from chronic middle ear infection. Auditory brainstem response (ABR) measurements revealed significant elevation of click-response thresholds in 48% of Df1/+ mice, often in only one ear. Anatomical and histological analysis of the middle ear demonstrated no gross structural abnormalities, but frequent signs of otitis media (OM, chronic inflammation of the middle ear), including excessive effusion and thickened mucosa. In mice for which both in vivo ABR thresholds and post mortem middle-ear histology were obtained, the severity of signs of OM correlated directly with the level of hearing impairment. These results suggest that abnormal auditory sensorimotor gating previously reported in mouse models of 22q11DS could arise from abnormalities in auditory processing. Furthermore, the findings indicate that Df1/+ mice are an excellent model for increased risk of OM in human 22q11DS patients. Given the frequently monaural nature of OM in Df1/+ mice, these animals could also be a powerful tool for investigating the interplay between genetic and environmental causes of OM.     

Decreased DGCR8 Expression and miRNA Dysregulation in Individuals with 22q11.2 Deletion Syndrome

Deletion of the 1.5–3 Mb region of chromosome 22 at locus 11.2 gives rise to the chromosome 22q11.2 deletion syndrome (22q11DS), also known as DiGeorge and Velocardiofacial Syndromes. It is the most common micro-deletion disorder in humans and one of the most common multiple malformation syndromes. The syndrome is characterized by a broad phenotype, whose characterization has expanded considerably within the last decade and includes many associated findings such as craniofacial anomalies (40%), conotruncal defects of the heart (CHD; 70–80%), hypocalcemia (20–60%), and a range of neurocognitive anomalies with high risk of schizophrenia, all with a broad phenotypic variability. These phenotypic features are believed to be the result of a change in the copy number or dosage of the genes located in the deleted region. Despite this relatively clear genetic etiology, very little is known about which genes modulate phenotypic variations in humans or if they are due to combinatorial effects of reduced dosage of multiple genes acting in concert. Here, we report on decreased expression levels of genes within the deletion region of chromosome 22, including DGCR8, in peripheral leukocytes derived from individuals with 22q11DS compared to healthy controls. Furthermore, we found dysregulated miRNA expression in individuals with 22q11DS, including miR-150, miR-194 and miR-185. We postulate this to be related to DGCR8 haploinsufficiency as DGCR8 regulates miRNA biogenesis. Importantly we demonstrate that the level of some miRNAs correlates with brain measures, CHD and thyroid abnormalities, suggesting that the dysregulated miRNAs may contribute to these phenotypes and/or represent relevant blood biomarkers of the disease in individuals with 22q11DS.     

22q11.2 distal deletion: a recurrent genomic disorder distinct from DiGeorge syndrome and velocardiofacial syndrome

Microdeletions within chromosome 22q11.2 cause a variable phenotype, including DiGeorge syndrome (DGS) and velocardiofacial syndrome (VCFS). About 97% of patients with DGS/VCFS have either a common recurrent ~3 Mb deletion or a smaller, less common, ~1.5 Mb nested deletion. Both deletions apparently occur as a result of homologous recombination between nonallelic flanking low-copy repeat (LCR) sequences located in 22q11.2. Interestingly, although eight different LCRs are located in proximal 22q, only a few cases of atypical deletions utilizing alternative LCRs have been described. Using array-based comparative genomic hybridization (CGH) analysis, we have detected six unrelated cases of deletions that are within 22q11.2 and are located distal to the ~3 Mb common deletion region. Further analyses revealed that the rearrangements had clustered breakpoints and either a ~1.4 Mb or ~2.1 Mb recurrent deletion flanked proximally by LCR22-4 and distally by either LCR22-5 or LCR22-6, respectively. Parental fluorescence in situ hybridization (FISH) analyses revealed that none of the available parents (11 out of 12 were available) had the deletion, indicating de novo events. All patients presented with characteristic facial dysmorphic features. A history of prematurity, prenatal and postnatal growth delay, developmental delay, and mild skeletal abnormalities was prevalent among the patients. Two patients were found to have a cardiovascular malformation, one had truncus arteriosus, and another had a bicuspid aortic valve. A single patient had a cleft palate. We conclude that distal deletions of chromosome 22q11.2 between LCR22-4 and LCR22-6, although they share some characteristic features with DGS/VCFS, represent a novel genomic disorder distinct genomically and clinically from the well-known DGS/VCF deletion syndromes.     

Sequence-ready physical map of the mouse Chromosome 16 region with conserved synteny to the human Velocardiofacial syndrome region on 22q11.2

Proximal mouse Chromosome (Chr) 16 shows conserved synteny with human Chrs 16, 8, 22, and 3. The mouse Chr 16/human Chr 22 conserved synteny region includes the DiGeorge/Velocardiofacial syndrome region of human Chr 22q11.2. A physical map of the entire mouse Chr 16/human Chr 22 region of conserved synteny has been constructed to provide a substrate for gene discovery, genomic sequencing, and animal model development. A YAC contig was constructed that extends ca. 5.4 Mb from a region of conserved synteny with human Chr 8 at Prkdc through the region conserved with human Chr 3 at DVL3. Sixty-one markers including 37 genes are mapped with average marker spacing of 90 kb. Physical distance was determined across the 2.6-Mb region from D16Mit74 to Hira with YAC fragmentation. The central region from D16Jhu28 to Igl-C1 was converted into BAC and PAC clones, further refining the physical map and providing sequence-ready template. The gene content and borders of three blocks of conserved linkage between human Chr 22q11.2 mouse Chr 16 are refined. Received: 4 November 1998 / Accepted: 21 December 1998     

Familial deletion of 22q11.2.

We present a mother and her son, both carrying a deletion of chromosome 22q.11.2. They manifest clinical heterogeneity. The mother has schizophrenia, an IQ of 70. Tetralogy of Fallot, a hypernasal voice, but does not have the characteristic facies. Her son has mild psychomotor developmental delay. Tetralogy of Fallot and mild facial features characteristic of VCFS.     

Deletion of 7q22 and ectrodactyly.

We report the case of an infant with 3-limb ectrodactyly and a deletion of most of 7q22. This observation, along with 4 similar cases from the literature, suggests the presence of a locus affecting limb differentiation in 7q22 near to the proximal interface.     

Parathyroid gland dysfunction in 22q11.2 deletion syndrome

BACKGROUND: 22q11 deletion syndrome (22q11DS) is characterized by conotruncal cardiac defects and hypoplasia of parathyroid glands and thymus, which result in variable hypoparathyroidism (HPT) and immune deficiency. METHODS: To study the course of HPT and the spectrum of other associated manifestations we evaluated all patients with 22q11DS, confirmed by fluorescence in situ hybridization, and HPT who were under follow-up at the Calcium-bone clinic, The Hospital for Sick Children, Toronto. Patients were clinically assessed and their hospital records were reviewed. RESULTS: Eighteen patients were included. At follow-up assessment at median age of 7.3 years HPT was judged complete in 11 (61%) and partial in 7 patients (39%). Patients with complete HPT presented with hypocalcemia later (median age at diagnosis 2.4 vs. 0.0 years) and more often with a hypocalcemic seizure than patients with partial HPT (73 vs. 29%). The spectrum of other associated manifestations did not differ between the groups. CONCLUSIONS: HPT in patients with 22q11DS is often partial. Many of the patients present with a hypocalcemic seizure which is predictive of complete HPT. Patients with complete and partial HPT do not differ in respect to their other associated features. Patients with features of 22q11DS should be actively screened for hypocalcemia to prevent development of symptomatic hypocalcemia.     

Copy-Number Variation of the Glucose Transporter Gene SLC2A3 and Congenital Heart Defects in the 22q11.2 Deletion Syndrome

The 22q11.2 deletion syndrome (22q11DS; velocardiofacial/DiGeorge syndrome; VCFS/DGS) is the most common microdeletion syndrome and the phenotypic presentation is highly variable. Approximately 65% of individuals with 22q11DS have a congenital heart defect (CHD), mostly of the conotruncal type, and/or an aortic arch defect. The etiology of this phenotypic variability is not currently known. We hypothesized that copy-number variants (CNVs) outside the 22q11.2 deleted region might increase the risk of being born with a CHD in this sensitized population. Genotyping with Affymetrix SNP Array 6.0 was performed on two groups of subjects with 22q11DS separated by time of ascertainment and processing. CNV analysis was completed on a total of 949 subjects (cohort 1, n = 562; cohort 2, n = 387), 603 with CHDs (cohort 1, n = 363; cohort 2, n = 240) and 346 with normal cardiac anatomy (cohort 1, n = 199; cohort 2, n = 147). Our analysis revealed that a duplication of SLC2A3 was the most frequent CNV identified in the first cohort. It was present in 18 subjects with CHDs and 1 subject without (p = 3.12 × 10⁻³, two-tailed Fisher’s exact test). In the second cohort, the SLC2A3 duplication was also significantly enriched in subjects with CHDs (p = 3.30 × 10⁻², two-tailed Fisher’s exact test). The SLC2A3 duplication was the most frequent CNV detected and the only significant finding in our combined analysis (p = 2.68 × 10⁻⁴, two-tailed Fisher’s exact test), indicating that the SLC2A3 duplication might serve as a genetic modifier of CHDs and/or aortic arch anomalies in individuals with 22q11DS.     

Endocrine manifestations of chromosome 22q11.2 microdeletion syndrome

BACKGROUND: Endocrine abnormalities, including hypocalcemia, thyroid dysfunction, and short stature, are associated with chromosome 22q11.2 microdeletion syndrome. This study was undertaken to examine the frequencies and clinical features of endocrine abnormalities in patients with 22q11.2 microdeletion syndrome. METHODS: We analyzed 61 patients with 22q11.2 microdeletion syndrome diagnosed based on the verification of microdeletion by fluorescent in situ hybridization (FISH) using a probe of the DiGeorge syndrome critical region (TUPLE1) at 22q11.2 and a control probe, ARSA at 22q13. Serum total calcium, phosphorus, and intact parathyroid hormone (PTH) levels were measured, thyroid function test was performed, and serum IGF-1 and IGFBP-3 levels were also estimated. Height and weight of patients were compared with individual chronological ages. RESULTS: Hypocalcemia was found in 20 patients (32.8%), and overt hypoparathyroidism in 8 (13.1%). Two patients (3.3%) showed autoimmune thyroid diseases, 1 each with Graves' disease and Hashimoto thyroiditis. Ten patients (16.4%) were below the third percentile in height, but the serum IGF-1 level was normal in 9 out of these 10 patients. CONCLUSION: Our findings show that patients with chromosome 22q11.2 microdeletion syndrome present with variable endocrine manifestations and variable clinical phenotypes. In addition to FISH analysis, careful endocrine evaluations are required in patients with this microdeletion syndrome, particularly for those with hypoparathyroidism or thyroid dysfunction.     

Geneticl and clinical characteristics of 22q11.2 deletion syndrome

In a group of 140 patients with typical phenotype, the 22q11.2 microdeletion was detected in 43 patients (32%) using FISH and MLPA methods. There were no deletions of other chromosomal loci causing to phenotypes similar to the 22q11.2 deletion syndrome (22q11.2DS). Sequencing of the TBX1 gene did not detect any mutations, except for some common neutral polymorphisms. For the first time in the Russian Federation, the diagnostic efficiency of 22q11.2DS appeared to be 32%, as a result of the application of a combination of genetic approaches for a large group of patients with suspected 22q11.2DS.     

Molecular genetic study of the frequency of monosomy 22q11 in DiGeorge syndrome

It is well established that DiGeorge syndrome (DGS) may be associated with monosomy of 22q11-pter. More recently, DNA probes have been used to detect hemizygosity for this region in patients with no visible karyotypic abnormality. However, DGS has also been described in cases where the cytogenetic abnormality does not involve 22q11; for instance, four cases of 10p- have been reported. In this study we have prospectively analyzed patients, by using DNA markers from 22q11, to assess the frequency of 22q11 rearrangements in DGS. Twenty-one of 22 cases had demonstrable hemizygosity for 22q11. Cytogenetic analysis had identified interstitial deletion in 6 of 16 cases tested; in 6 other cases no karyotype was available. When these results are combined with those from our previous studies, 33 of 35 DGS patients had chromosome 22q11 deletions detectable by DNA probes.     

Interstitial deletions in DiGeorge syndrome detected with microclones from 22q11

DiGeorge syndrome in humans is charaterized by immunodeficiency, heart defects, mental retardation and facial dysmorphism; cytogenetic analysis has shown that deletions at 22q11 occur in approximately 25% of cases. To generate DNA markers from this region, we have microdissected and microcloned band q11 of human Chromosome (Chr) 22. Nineteen thousand clones were obtained from material dissected from 20 chromosome fragments. Seventeen of 61 clones analyzed (28%) were repetitive, 27 (44%) gave no signal, and 17 (28%) detected single copy sequences of which ten mapped to Chr 22. Two of these were found to be deleted in patients with DiGeorge syndrome and either monosomy for 22q11-pter or visible interstitial deletions of 22q11. These two markers are also hemizygous in patients with no visible chromosomal abnormality, demonstrating that submicroscopic deletions are common in DiGeorge syndrome patients.     

22q11.2 deletion mosaicism in patients with conotruncal heart defects

BACKGROUND: Some patients with conotruncal heart defects (CTDs) have a chromosome 22q11.2 deletion, but we do not know whether patients with CTDs who are missing the peripheral blood-cell chromosome 22q11.2 deletion are also missing the 22q11.2 deletion in myocardial cells, and whether patients with the 22q11.2 deletion can show a different 22q11.2 deletion in peripheral blood cells and myocardial cells due to a postzygotic mutation during the embryonic period. METHODS: A total of 32 Chinese pediatric nonsyndromic CTD patients (21 with tetralogy of fallot [TOF], 9 with double outlet right ventricle [DORV], 1 with pulmonary artery atresia with ventricular septal defect [PAA/VSD], and 1 with congenitally corrected transposition of the great arteries [CCTGA]), 12 females and 20 males ranging in age from 5 months to 7 years, were included in our study. We used fluorescence in situ hybridization (FISH) to find the chromosome 22q11.2 deletion in peripheral blood cells and compared genotypes of 15 short tandem repeat (STR) markers within 22q11.2 between peripheral blood cells and myocardial cells to search for genetic mosaicism of the chromosome 22q11.2 deletion. RESULTS: Three patients, 2 with TOF and 1 with DORV, were determined to have the peripheral blood cell chromosome 22q11.2 deletion. There was no STR genotypic difference observed between peripheral blood cells and myocardial cells in patients with or without the chromosome 22q11.2 deletion. CONCLUSIONS: Genetic mosaicism may not play a major role in the etiology of isolated CTDs.