Mitochondrial localization and function of a subset of 22q11 deletion syndrome candidate genes.

Six genes in the 1.5 Mb region of chromosome 22 deleted in DiGeorge/22q11 deletion syndrome-Mrpl40, Prodh, Slc25a1, Txnrd2, T10, and Zdhhc8-encode mitochondrial proteins. All six genes are expressed in the brain, and maximal expression coincides with peak forebrain synaptogenesis shortly after birth. Furthermore, their protein products are associated with brain mitochondria, including those in synaptic terminals. Among the six, only Zddhc8 influences mitochondria-regulated apoptosis when overexpressed, and appears to interact biochemically with established mitochondrial proteins. Zdhhc8 has an apparent interaction with Uqcrc1, a component of mitochondrial complex III. The two proteins are coincidently expressed in pre-synaptic processes; however, Zdhhc8 is more frequently seen in glutamatergic terminals. 22q11 deletion may alter metabolic properties of cortical mitochondria during early post-natal life, since expression complex III components, including Uqcrc1, is significantly increased at birth in a mouse model of 22q11 deletion, and declines to normal values in adulthood. Our results suggest that altered dosage of one, or several 22q11 mitochondrial genes, particularly during early post-natal cortical development, may disrupt neuronal metabolism or synaptic signaling.

When half is not enough: gene expression and dosage in the 22q11 deletion syndrome.

The 22q11 Deletion Syndrome (22q11DS, also known as DiGeorge or Velo-Cardio-Facial Syndrome) has a variable constellation of phenotypes including life-threatening cardiac malformations, craniofacial, limb, and digit anomalies, a high incidence of learning, language, and behavioral disorders, and increased vulnerability for psychiatric diseases, including schizophrenia. There is still little clear understanding of how heterozygous microdeletion of approximately 30-50 genes on chromosome 22 leads to this diverse spectrum of phenotypes, especially in the brain. Three possibilities exist: 1) 22q11DS may reflect haploinsufficiency, homozygous loss of function, or heterozygous gain of function of a single gene within the deleted region; 2) 22q11DS may result from haploinsufficiency, homozygous loss of function, or heterozygous gain of function of a few genes in the deleted region acting at distinct phenotypically compromised sites; 3) 22q11DS may reflect combinatorial effects of reduced dosage of multiple genes acting in concert at all phenotypically compromised sites. Here, we consider evidence for each of these possibilities. Our review of the literature, as well as interpretation of work from our laboratory, favors the third possibility: 22q11DS reflects diminished expression of multiple 22q11 genes acting on common cellular processes during brain as well as heart, face, and limb development, and subsequently in the adolescent and adult brain.

Gene dosage in the developing and adult brain in a mouse model of 22q11 deletion syndrome.

We evaluated the consequences of heterozygous chromosome 22q11 deletion - a significant genetic risk for schizophrenia - for expression levels and patterns of a subset of 22q11 genes implicated in schizophrenia and other phenotypes in mouse models of 22q11 deletion syndrome (22q11DS). In deleted embryos, expression levels of at least nine 22q11 orthologues decline by 40-60% in the frontonasal mass/forebrain and other 22q11DS phenotypic sites (branchial and aortic arches, limb buds); however, coincident expression patterns of 22q11 and Snail genes - diagnostic for neural crest-derived mesenchyme - are unchanged, and Snail1 expression levels do not decline. Subsequently, 22q11 mRNA levels are reduced by 40-60% in the brains of developing, adolescent and adult deleted mice without altered expression patterns, dysmorphology or reduced cell density. Apparently, in deleted individuals, 22q11 gene expression declines across otherwise stable cell populations, perhaps disrupting individual cell function via diminished dosage. Such changes might contribute to schizophrenia vulnerability in 22q11DS.