DNA fluorescent probes for diagnosis of velocardiofacial and related syndromes

Cardiac Registry screening for DiGeorge Critical Region deletion using loss of heterozygosity analysis

DiGeorge (DGS), velocardiofacial, and conotruncal anomaly face syndromes comprise a phenotypic spectrum that is associated with a submicroscopic 22q11.2 deletion in the majority of cases. These syndromes variably express complex congenital heart disease, cellular immune deficits, hypocalcemia, craniofacial anomalies, and learning disabilities. This retrospective study correlates the presence of a deletion in this region with autopsy and clinical findings in a cohort of patients selected from the Cardiac Registry at Boston Children's Hospital. DNA was extracted from formalin-fixed paraffin-embedded cardiac tissue sampled from 189 patients with conotruncal anomalies. Polymerase chain reaction (PCR) was performed using 4 fluorescently labeled oligonucleotide primer pairs for unique short tandem repeat polymorphisms in the DGS critical region. The PCR products were analyzed for loss of heterozygosity (LOH), and a deletion was assumed when at least 3 consecutive loci demonstrated homozygosity. Of the 189 cases, 16 (8%) met our criteria for LOH and were assumed to have a deletion. These patients included 6 (35%) of 17 patients diagnosed clinically with DGS prior to death. Of the 10 non-DGS patients with LOH, 4 had aortic atresia and 3 had tetralogy of Fallot, both frequently seen in DGS. Polymerase chain reaction is a useful screening alternative to fluorescence in situ hydridization for detecting 22q11.2 deletions in archived tissue samples. This study identified a probable deletion in a subset of cases from a cardiac registry with cardiac defects associated with the DGS phenotype.

Differential detection of deletion 22q11.2 syndrome by specialty and indication

This study identified cytogenetic abnormalities in a population screened for deletion 22q11.2 syndrome (D22S) by fluorescence in situ hybridization (FISH) and G-banding and correlated these abnormalities to referring specialty and submitted indications. Requests for the D22S FISH assay were retrospectively reviewed over a 29-month period in our institution. Positive test results for D22S FISH and other abnormalities found by other FISH assays and G-banding were correlated to submitting specialist and indication. Thirteen medical services ordered D22S FISH testing on 297 patients over 29 months. The detection rate for all cytogenetic aberrations was 9.4% (28 of 297) including 5.4% (16 of 297) for D22S detection by FISH and 2.7% (8 of 297) for detection of additional cytogenetic anomalies by G-banding cytogenetics. Sixty-six of 297 patients negative by D22S FISH and G-banding were screened using other FISH assays and 3 of 47 (6.4%) patients screened using subtelomeric probes were positive for deletion and 1 of 3 (33%) patients screened for Prader-Willi syndrome was positive for deletion. Pediatric geneticists requested 53.9% (160 of 297) of the tests, yielding 60.7% (17 of 28) of positive test results. Tetralogy of Fallot and developmental delay were the indications associated with the most positive test results. In our institution, pediatric geneticists identify the largest spectrum of indications with D22S and appear most aware of the association of developmental delay with D22S. Performing conventional cytogenetics and other FISH assays, in addition to FISH for D22S, is important because there is considerable overlap between D22S and the phenotype of several other syndromes.

Prevalence and clinical manifestations of 22q11.2 microdeletion in adults with selected conotruncal anomalies

OBJECTIVES

This study was designed to determine the prevalence and clinical manifestations of 22q11.2 microdeletion in adults with selected conotruncal anomalies and to assess the clinician's ability to predict the presence or absence of 22q11.2 microdeletion on the basis of clinical features.

BACKGROUND

It is known that 22q11.2 microdeletion is a chromosomal anomaly with cardiac and extracardiac manifestations. The prevalence and manifestations in adults have not been well characterized.

METHODS

A total of 103 consecutive adults with either tetralogy of Fallot (TOF), pulmonary atresia/ventricular septal defect (PA/VSD), or truncus arteriosus (TA) were prospectively screened for 22q11.2 microdeletion using a fluorescence in situ hybridization (FISH) assay. Clinicians were asked to predict 22q11.2 microdeletion status on the basis of clinical features. A geneticist blinded to FISH assay results reviewed photographs of the patients for typical dysmorphic features of 22q11.2 microdeletion.

RESULTS

Six patients (prevalence 5.8%, 95% confidence interval 1.3 to 10.3) had 22q11.2 microdeletion (3 with TOF, 2 with PA/VSD, 1 with TA). In two of these patients, the clinician incorrectly predicted absence of the deletion. In three, typical dysmorphic features of 22q11.2 microdeletion were absent.

CONCLUSIONS

Our work showed that 22q11.2 microdeletion is under-recognized in adults with congenital heart disease. The absence of typical phenotypic features makes it difficult to correctly predict if the deletion is present. Screening for 22q11.2 microdeletion should be considered in adults with high-risk cardiac lesions, as it has important implications in reproductive counseling and surveillance for associated extracardiac manifestations.

Primer on medical genomics. Part XI: Visualizing human chromosomes

In the past century, various methods to visualize human chromosomes were discovered. Chromosome analyses provide an overall view of the human genome that cannot be achieved with any other approach. The methods to visualize chromosomes include various techniques to produce bands along chromosomes, specialized procedures for specific disorders, and fluorescent-labeled DNA for targeted loci. Cytogenetic methods guide the study of the relationship between chromosome structure and gene function. They also aid in mapping locations of genes and identifying chromosome anomalies associated with medical disorders. The clinical diagnosis, prognosis, and response to treatment can be established for many malignant diseases. Cytogenetic methods provide an important diagnostic tool for clinical practice.

Microduplication 22q11.2, an emerging syndrome: clinical, cytogenetic, and molecular analysis of thirteen patients

Chromosome 22, particularly band 22q11.2, is predisposed to rearrangements due to misalignments of low-copy repeats (LCRs). DiGeorge/velocardiofacial syndrome (DG/VCFS) is a common disorder resulting from microdeletion within the same band. Although both deletion and duplication are expected to occur in equal proportions as reciprocal events caused by LCR-mediated rearrangements, very few microduplications have been identified. We have identified 13 cases of microduplication 22q11.2, primarily by interphase fluorescence in situ hybridization (FISH). The size of the duplications, determined by FISH probes from bacterial artificial chromosomes and P(1) artificial chromosomes, range from 3-4 Mb to 6 Mb, and the exchange points seem to involve an LCR. Molecular analysis based on 15 short tandem repeats confirmed the size of the duplications and indicated that at least 1 of 15 loci has three alleles present. The patients' phenotypes ranged from mild to severe, sharing a tendency for velopharyngeal insufficiency with DG/VCFS but having other distinctive characteristics, as well. Although the present series of patients was ascertained because of some overlapping features with DG/VCF syndromes, the microduplication of 22q11.2 appears to be a new syndrome.

Voltage-gated calcium channel gamma 2 subunit gene is not deleted in velo-cardio-facial syndrome

Velo-cardio-facial syndrome (VCFS) has been associated with schizophrenic symptoms in some patients and is caused by a deletion of 22q11.21--q11.23. The voltage-gated calcium channel (VGCC) gamma 2 subunit is located on chromosome 22 and is telemeric to the most commonly observed VCFS deletion region but is near a putative marker for schizophrenia (D22S278). Metaphase spreads of four controls, four patients with VCFS, and one patient with VCFS and schizophrenia were evaluated for the VCFS deletion using the VCFS-diagnostic probe, TUPLE 1, and for deletion of VGCC gamma 2 subunit gene using probes for that gene's exon 1 and exons 3 and 4. All of the VCFS patients had deletion of the TUPLE 1 probe on one chromosome of the chromosome 22 pair. None showed deletion of the gamma 2 subunit exons studied. The location of the gamma 2 subunit gene at 22q13.1 was confirmed by FISH in all cases. This study did not show a deletion of the gamma 2 subunit gene as a distinguishing feature of our patient with VCFS and schizophrenia.

Pulmonary atresia with ventricular septal defect and persistent airway hyperresponsiveness

OBJECTIVE

We and others have observed significant hyperinflation and airflow obstruction after the surgical repair of pulmonary atresia and ventricular septal defect. This study sought to objectively characterize this problem and determine the prevalence of airway hyperresponsiveness in these patients.

METHODS

We performed a prospective study of children and young adults with pulmonary atresia and ventricular septal defect between June 1996 and December 1998. The participants were stratified into 2 distinct molecular genotypes on the basis of chromosome 22q11.2 microdeletion. A clinical diagnosis of asthma and an objective assessment of airway hyperresponsiveness were determined by means of an asthma inventory scale and methacholine challenge testing, respectively. Thirty-three patients were enrolled. Thirteen had velocardiofacial syndrome, each with chromosome 22q11.2 microdeletion.

RESULTS

None of the nonsyndromic patients had evidence for haploinsufficiency. Overall, 66.7% (22/33) met criteria for a clinical diagnosis of airway hyperresponsiveness: 62% (8/13) from the microdeletion genotype and 70% (14/20) from the nonsyndromic group.

CONCLUSIONS

We have identified an extremely strong association between pulmonary atresia and ventricular septal defect and persistent airway hyperresponsiveness. Haploinsufficiency at chromosome 22q11.2 did not contribute to this predilection for airway hyperresponsiveness. These results provide a basis to anticipate persistent respiratory difficulties after operations in patients with pulmonary atresia and ventricular septal defect. Moreover, this at-risk patient population may yield unique insights into fundamental mechanisms involved in the pathogenesis of airway hyperresponsiveness.

Surface expression of glycoprotein Ibα is dependent on glycoprotein Ibβ: evidence from a novel mutation causing Bernard-Soulier syndrome

Bernard-Soulier syndrome is a rare bleeding disorder caused by a quantitative or qualitative defect in the platelet glycoprotein (GP) Ib-IX-V complex. The complex, which serves as a platelet receptor for von Willebrand factor, is composed of 4 subunits: GPIb, GPIbβ, GPIX, and GPV. We here describe the molecular basis of a novel form of Bernard-Soulier syndrome in a patient in whom the components of the GPIb-IX-V complex were undetectable on the platelet surface. Although confocal imaging confirmed that GPIb was not present on the platelet surface, GPIb was readily detectable in the patient's platelets. Moreover, immunoprecipitation of plasma with specific monoclonal antibodies identified circulating, soluble GPIb. DNA-sequence analysis revealed normal sequences for GPIb and GPIX. There was a G to A substitution at position 159 of the gene encoding GPIbβ, resulting in a premature termination of translation at amino acid 21. Studies of transient coexpression of this mutant, W21stop-GPIbβ, together with wild-type GPIb and GPIX, demonstrated a failure of GPIX expression on the surface of HEK 293T cells. Similar results were obtained with Chinese hamster ovary  IX cells, a stable cell line expressing GPIb that retains the capacity to re-express GPIX. Thus, we found that GPIbβ affects the surface expression of the GPIb-IX complex by failing to support the insertion of GPIb and GPIX into the platelet membrane.

Surface expression of glycoprotein Ibα is dependent on glycoprotein Ibβ: evidence from a novel mutation causing Bernard-Soulier syndrome

Bernard-Soulier syndrome is a rare bleeding disorder caused by a quantitative or qualitative defect in the platelet glycoprotein (GP) Ib-IX-V complex. The complex, which serves as a platelet receptor for von Willebrand factor, is composed of 4 subunits: GPIb, GPIbβ, GPIX, and GPV. We here describe the molecular basis of a novel form of Bernard-Soulier syndrome in a patient in whom the components of the GPIb-IX-V complex were undetectable on the platelet surface. Although confocal imaging confirmed that GPIb was not present on the platelet surface, GPIb was readily detectable in the patient's platelets. Moreover, immunoprecipitation of plasma with specific monoclonal antibodies identified circulating, soluble GPIb. DNA-sequence analysis revealed normal sequences for GPIb and GPIX. There was a G to A substitution at position 159 of the gene encoding GPIbβ, resulting in a premature termination of translation at amino acid 21. Studies of transient coexpression of this mutant, W21stop-GPIbβ, together with wild-type GPIb and GPIX, demonstrated a failure of GPIX expression on the surface of HEK 293T cells. Similar results were obtained with Chinese hamster ovary  IX cells, a stable cell line expressing GPIb that retains the capacity to re-express GPIX. Thus, we found that GPIbβ affects the surface expression of the GPIb-IX complex by failing to support the insertion of GPIb and GPIX into the platelet membrane.

Childhood-onset schizophrenia associated with parkinsonism in a patient with a microdeletion of chromosome 22

The deletion of a gene or genes on chromosome 22q11 is responsible for the velocardiofacial syndrome (VCFS), which is associated with cardiac anomalies, short stature, palate abnormalities, learning disabilities, and developmental delay. Herein we describe a 30-year-old man with VCFS in whom a chronic psychotic disorder originated during childhood. A 10% rate of psychotic disorders has been reported in association with this genetic syndrome. In our patient, the clinical manifestation was complicated by extrapyramidal symptoms that predated the onset of psychotic symptoms. To our knowledge, extrapyramidal symptoms have not previously been reported in a patient with VCFS. The diagnosis of VCFS was confirmed with the fluorescence in situ hybridization probe for VCFS. The role of the atypical antipsychotic drug clozapine is discussed with respect to treating this patient who has severe psychotic symptoms coexisting with extrapyramidal symptoms and seizures. In light of the observation that patients with VCFS have an unexpectedly high rate of psychotic disorders, issues concerning the genetics of schizophrenia are intriguing.

Human endplate acetylcholinesterase deficiency caused by mutations in the collagen-like tail subunit (ColQ) of the asymmetric enzyme

In skeletal muscle, acetylcholinesterase (AChE) exists in homomeric globular forms of type T catalytic subunits (ACHET) and heteromeric asymmetric forms composed of 1, 2, or 3 tetrameric ACHET attached to a collagenic tail (ColQ). Asymmetric AChE is concentrated at the endplate (EP), where its collagenic tail anchors it into the basal lamina. The ACHET gene has been cloned in humans; COLQ cDNA has been cloned in Torpedo and rodents but not in humans. In a disabling congenital myasthenic syndrome, EP AChE deficiency (EAD), the normal asymmetric species of AChE are absent from muscle. EAD could stem from a defect that prevents binding of ColQ to ACHET or the insertion of ColQ into the basal lamina. In six EAD patients, we found no mutations in ACHET. We therefore cloned human COLQ cDNA, determined the genomic structure and chromosomal localization of COLQ, and then searched for mutations in this gene. We identified six recessive truncation mutations of COLQ in six patients. Coexpression of each COLQ mutant with wild-type ACHET in SV40-transformed monkey kidney fibroblast (COS) cells reveals that a mutation proximal to the ColQ attachment domain for ACHET prevents association of ColQ with ACHET; mutations distal to the attachment domain generate a mutant approximately 10.5S species of AChE composed of one ACHET tetramer and a truncated ColQ strand. The approximately 10.5S species lack part of the collagen domain and the entire C-terminal domain of ColQ, or they lack only the C-terminal domain, which is required for formation of the triple collagen helix, and this likely prevents their insertion into the basal lamina.

Moyamoya syndrome associated with congenital heart disease

OBJECTIVE

To describe the association between moyamoya syndrome and congenital heart disease and to discuss its clinical implications. Study Design. Retrospective analysis of a case series from two institutions.

RESULTS

Five patients with moyamoya syndrome and structural congenital heart disease were identified. Coarctation of the aorta was present in 3 patients, in association with a ventricular septal defect (1 patient), aortic and mitral valve stenoses (1 patient), and tetralogy of Fallot (1 patient). Tetralogy of Fallot and a large paramembranous ventricular septal defect were found in the other 2 patients. Four patients underwent surgical repair of their congenital heart disease during the first year of life and 1 patient had balloon dilation of aortic coarctation at 5 years of age. In all patients, moyamoya syndrome was diagnosed after surgical intervention for congenital heart disease-at 6 months of age in 1 patient, at 2 years of age in 3 patients, and at 6 years in 1 patient. Strokes were the most common presenting sign (3 patients) followed by seizures (2 patients). By the age of 33 months, 4 of 5 patients had undergone cerebral revascularization surgery to halt the clinical progression of moyamoya syndrome.

CONCLUSIONS

Moyamoya syndrome should be considered in the differential diagnosis of seizures and stroke in patients with structural congenital heart disease. Prompt diagnosis and surgical management of the occlusive cerebral angiopathy should lead to improved neurological outcome in these patients.