Effects of COMT genotype on behavioral symptomatology in the 22q11.2 Deletion Syndrome.

The 22q11.2 Deletion Syndrome (DiGeorge/velocardiofacial syndrome) is associated with elevated rates of psychosis, and is also characterized by severe attentional difficulties and executive dysfunction. Behavioral manifestations of this syndrome could result from haploinsufficiency of the catechol-O-methyltransferase (COMT) gene, located within the 22q11 region. The goal of the present study was to examine COMT genotype in relation to behavioral symptomatology in this syndrome. Val158/108Met was genotyped in 38 patients (16 Met/-, 22 Val/-) with confirmed 22q11.2 deletions who had received the Child Behavior Checklist (CBCL) as part of a comprehensive evaluation. Results indicated that the Val genotype was associated with significantly greater internalizing and externalizing behavioral symptomatology in children with 22q11.2 deletions. Val allele status was associated with a greater-than-four-fold increase in risk for clinically significant behavior problems in children with this syndrome. These data are consistent with previous findings of increased psychopathology associated with the Val genotype in normal individuals and suggest that a functional genetic polymorphism in the 22q11 region may influence behavior in individuals with COMT haploinsufficiency.

Effects of a functional COMT polymorphism on prefrontal cognitive function in patients with 22q11.2 deletion syndrome.

OBJECTIVE: The 22q11.2 deletion syndrome (DiGeorge/velocardiofacial syndrome) is associated with attentional problems and executive dysfunction, and is one of the highest known risk factors for schizophrenia. These behavioral manifestations of 22q11.2 deletion syndrome could result from haploinsufficiency of the catechol O-methyltransferase (COMT) gene, located within the 22q11 region. The goal of the present study was to examine COMT genotype as a predictor of prefrontal cognitive function in patients with 22q11.2 deletion syndrome. METHOD: Patients with confirmed 22q11.2 deletions (N=44) underwent neurocognitive testing following Val(158)Met genotyping (Met hemizygous: N=16; Val hemizygous: N=28). RESULTS: Analyses of covariance revealed that Met-hemizygous patients performed significantly better on a composite measure of executive function (comprising set-shifting, verbal fluency, attention, and working memory) than did Val-hemizygous patients. CONCLUSIONS: These data are consistent with those of previous studies in normal individuals, suggesting that a functional genetic polymorphism in the 22q11 region may influence prefrontal cognition in individuals with COMT haploinsufficiency.

Independent de novo 22q11.2 deletions in first cousins with DiGeorge/velocardiofacial syndrome.

Deletions of chromosome 22q11.2 are found in the vast majority of patients with DiGeorge/velocardiofacial syndrome (DGS/VCFS). This most frequent microdeletion syndrome is estimated to occur in 1 in 4,000 live births. The majority of deletions are de novo, with 10% or less inherited from an affected parent. Here, we report two separate families with recurrence of a 22q11.2 deletion in first cousins. In each family, unaffected siblings (brother and sister) had an affected child. Fluorescence in situ hybridization (FISH) studies of the parents of each affected child were normal and hence, relatives were not considered at an increased risk for recurrence in another pregnancy. We used highly polymorphic microsatellite repeat markers from within 22q11.2 to determine the parental origin of each cousin's deletion and to assess whether parental germline mosaicism for the 22q11.2 deletion might be a factor in these cases. This analysis confirmed that in each case, the deletion occurred on a chromosome 22 derived from unrelated parents, consistent with independent de novo deletion events. Thus, we concluded that germline mosaicism as the underlying mechanism for affected cousins in these families was unlikely. Our findings underscore the high frequency with which the 22q11.2 deletion occurs in the general population and demonstrate the important role that PCR-based parental origin determination can have in recurrence risk counselling. Furthermore, relatives of affected individuals may benefit from genetic counselling and consider prenatal testing for the 22q11.2 deletion in future pregnancies, despite a low recurrence risk.

Aberrant interchromosomal exchanges are the predominant cause of the 22q11.2 deletion.

Chromosome 22q11.2 deletions are found in almost 90% of patients with DiGeorge/velocardiofacial syndrome (DGS/VCFS). Large, chromosome-specific low copy repeats (LCRs), flanking and within the deletion interval, are presumed to lead to misalignment and aberrant recombination in meiosis resulting in this frequent microdeletion syndrome. We traced the grandparental origin of regions flanking de novo 3 Mb deletions in 20 informative three-generation families. Haplotype reconstruction showed an unexpectedly high number of proximal interchromosomal exchanges between homologs, occurring in 19/20 families. Instead, the normal chromosome 22 in these probands showed interchromosomal exchanges in 2/15 informative meioses, a rate consistent with the genetic distance. Meiotic exchanges, visualized as MLH1 foci, localize to the distal long arm of chromosome 22 in 75% of human spermatocytes tested, also reflecting the genetic map. Additionally, we found no effect of proband gender or parental age on the crossover frequency. Parental origin studies in 65 de novo 3 Mb deletions (including these 20 patients) demonstrated no bias. Unlike Williams syndrome, we found no chromosomal inversions flanked by LCRs in 22 sets of parents of 22q11 deleted patients, or in eight non-deleted patients with a DGS/VCFS phenotype using FISH. Our data are consistent with significant aberrant interchromosomal exchange events during meiosis I in the proximal region of the affected chromosome 22 as the likely etiology for the deletion. This type of exchange occurs more often than is described for deletions of chromosomes 7q11, 15q11, 17p11 and 17q11, implying a difference in the meiotic behavior of chromosome 22.