MicroRNA Target Fingered in 22q11.2 Deletion Syndrome
18 January 2013. A report published January 17 in Cell sheds light on the contribution of microRNAs (miRNAs) to 22q11.2 deletion syndrome, a genetic predictor of schizophrenia. The study, led by Maria Karayiorgou and Joseph Gogos, both of Columbia University in New York, finds dramatic reductions in miR-185 expression in a mouse model of the syndrome and shows that a resulting upregulation in the neuronal gene 2310044H10Rik/Mirta22 produces altered dendrite and spine formation.
22q11.2 deletion syndrome, in which a portion of chromosome 22 has been removed, is characterized by birth defects and learning disabilities. Abnormalities in chromosome 22 are also thought to be involved in schizophrenia, since around 30 percent of the deletion carriers go on to develop the illness in adolescence or adulthood (see SRF related news story).
Gogos and Karayiorgou have developed a mouse model of 22q11.2 deletion syndrome, the Df(16)A+/- mouse, which closely mimics the human deletion of 1.5 Mb (see SRF related news story). Previous studies of this mouse line have indicated alterations in hippocampal dendritic spines, as well as alterations in the processing of microRNAs (miRNAs), small, noncoding RNAs that regulate mRNA expression (see SRF related news story; Xu et al., 2010). The altered miRNA expression is thought to be due, at least in part, to loss of one copy of the DGCR8 gene, a critical component of miRNA production (see SRF related news story).
Reduced miR-185 in Df(16)A+/- mice
In the current study, first author Bin Xu and colleagues observed that miR-185 expression is reduced by 70-80 percent in the hippocampus and prefrontal cortex of Df(16)A+/- mice during development and in adulthood. Notably, this reduction is much larger than both what is predicted by the 50 percent reduction in DGCR8 gene dosage and what is observed in DGCR8 mutant mice, suggesting that the effect extends beyond DGCR8. Since the miR-185 gene is also located within the deleted region of Df(16)A+/- mice, it is likely that hemizygosity of the gene itself also contributes to the reduction in miR-185 expression.
A transcriptional profile analysis revealed that the gene 2310044H10Rik is upregulated during postnatal development and adulthood in Df(16)A+/- mice. Two miRNA target site prediction programs indicated that binding sites for miR-185 are within the 3’ untranslated region (UTR) of 2310044H10Rik, suggesting that increased expression of 2310044H10Rik in Df(16)A+/- mice may be a consequence of miR-185 downregulation.
To investigate whether 2310044H10Rik is indeed under the control of miR-185, the authors transfected an miR-185 precursor mimic into Df(16)A+/- primary neuronal cultures. They observed a reduction in 2310044H10Rik in transfected cells, confirming that the gene is repressed by miR-185. Subsequent experiments demonstrated that this repression is dependent on the 3’ UTR of 2310044H10Rik. Two other miRNAs—miR-485 and miR-491—were also demonstrated to play smaller roles in the upregulation of 2310044H10Rik in Df(16)A+/- mice, suggesting that decreases in both miR-185 and DGCR8 contribute to the increase in 2310044H10Rik. Based on their confirmation of repression by miRNAs, the authors renamed the gene Mirta22, which stands for miRNA target of the 22q11.2 microdeletion.
Mirta22 encodes a protein of unknown function that is elevated by approximately 25 percent in Df(16)A+/- mice. Immunocytochemistry revealed that it is distributed throughout the brain, and is found primarily in the Golgi apparatus and dendritic shafts of neurons.
Consistent with a report that miRNAs preferentially target genes in the same functional group (Tsang et al., 2010), the authors used functional annotation clustering analysis to determine that a Golgi-related gene cluster was the major target of miR-185. However, the reduction of miR-185 in Df(16)A+/- mice was associated with only a mild alteration of Golgi-related genes, with only four of 159 Golgi-related probe sets included in the top 100 dysregulated genes in the mouse hippocampus.
Since Mirta22 is localized to the Golgi and dendritic shafts of neurons, Xu and colleagues reasoned that its upregulation by miR-185 may contribute to the impaired dendrite and spine formation observed in Df(16)A+/- mice (Mukai et al., 2008). Consistent with this idea, an increase in Mirta22 produced dendritic abnormalities, including diminished complexity, spine density, and spine width in wild-type neuronal cultures, while a knockdown of Mirta22 reversed these deficits in Df(16)A+/- cultures. Similar results on dendrites were obtained after miR-185 downregulation in wild-type cultures and upregulation in Df(16)A+/- cultures. The effect of Mirta22 also held up in vivo. After crossing Mirta22 mutant and Df(16)A+/- mice, the researchers observed that the addition of the Mirta22 mutation could reverse the deficits in hippocampal dendritic complexity and spine formation in the Df(16)A+/- mice.
In summary, a reduction in miR-185 in a mouse model of 22q11.2 deletion syndrome produces elevated levels of Mirta22 that alter dendrite and spine formation. According to the authors, their study identifies elevated Mirta22 as “the most robust gene change resulting from the 22q11.2 microdeletion, as well as the major downstream transcriptional effect of the 22q11.2-associated miRNA dysregulation.”—Allison A. Curley
Xu B, Hsu P-K, Stark KL, Karayiorgou M, Gogos JA. Derepression of a neuronal inhibitor due to miRNA dysregulation in a schizophrenia-related microdeletion. Cell 2013. 152: 762-275. Abstract
Comments on Related News
Related News: 22q11 and Schizophrenia: New Role for microRNAs and MoreComment by: Linda Brzustowicz
Submitted 21 May 2008
Posted 21 May 2008
While some have expressed frustration over the lack of clear reproducibility of linkage and association findings in schizophrenia, the importance of the chromosome 22q11 deletion syndrome (22q11DS) as a real and significant genetic risk factor for schizophrenia has often been overlooked. While the deletion syndrome is present in a minority of individuals with schizophrenia (estimates of approximately 1 percent), presence of the deletion increases risk of developing schizophrenia some 30-fold, making this one of the clearest known genetic risk factors for a psychiatric illness. As multiple genes are deleted in 22q11DS, it can be a challenge to determine which gene or genes are involved in specific phenotypic elements of this syndrome.
The May 11, 2008, paper by Stark et al. highlights the utility of engineered animals for dissecting the individual effects of multiple genes within a deletion region and provides an important clue into the mechanism likely responsible for at least some of the behavioral aspects of the phenotype. While some may argue about the full validity of animal models of complex human behavior disorders, these systems do have an advantage in manipulability that cannot be achieved in work with human subjects. A key feature of this paper is the comparison of the phenotype of mice engineered to contain a 1.3 Mb deletion of 27 genes with mice engineered to contain a disruption of only one gene in the region, DGCR8. The ability to place both of these alterations on the same genetic background and then do head-to-head comparisons on a number of behavioral, neuropathological, and gene expression assays allows a clear assessment of which components of the mouse phenotype may be attributed specifically to DGCR8 haploinsufficiency. Perhaps not surprisingly, DGCR8 seems to play a role in some, but not all, of the behavioral and neuropathological changes seen in the animals with the 1.3 Mb deletion. The fact that the DGCR8 disruption was able to recapitulate certain elements of the full deletion in the mice does raise its profile as an important candidate gene for some of the neurocognitive elements of 22q11DS, and makes it a potential candidate gene for contributing to schizophrenia risk in individuals without 22q11DS.
Also of great interest is the known function of DGCR8. While the gene name simply stands for DiGeorge syndrome Critical Region gene 8, it is now known that this gene plays an important role in the biogenesis of microRNAs, small non-coding RNAs that regulate gene expression by targeting mRNAs for translational repression or degradation. As miRNAs have been predicted to regulate over 90 percent of genes in the human genome (Miranda et al., 2006), a disruption in a key miRNA processing step could have profound regulatory impacts. Indeed, as reported in the Stark et al. paper and elsewhere (Wang et al., 2007), homozygous deletion of DGCR8 function is lethal in mice. What perhaps seems to be the most surprising result is that haploinsufficiency of DGCR8 function does not induce a more profound phenotype, given the large number of genes that would be expected to be affected if miRNA processing were globally impaired. The Stark et al. paper determined that while the pre-processed form of miRNAs may be elevated in haploinsufficient mice, perhaps only 10-20 percent of all mature miRNAs show altered levels, suggesting that some type of compensatory mechanism may be involved in regulating the final levels of the other miRNAs. Still, the 20-70 percent decrease in the abundance of these altered miRNAs could have a profound effect on multiple cellular processes, given the regulatory nature of miRNAs. In the context of the recent evidence for altered levels of some miRNA in postmortem samples from individuals with schizophrenia (Perkins et al., 2007), the Stark et al. paper adds further support for studying miRNAs as potential candidate genes in all individuals with schizophrenia, not just those with 22q11DS. This paper should serve as an important reminder of how careful analysis of a biological subtype of a disorder can reveal important insights that will be relevant to a much broader set of affected individuals.
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Related News: Are Membrane Molecules Unmoored in 22q11DS Mouse?
Comment by: Doron Gothelf
Submitted 27 October 2008
Posted 27 October 2008
The common theory held until recently regarding the genetic underpinning of neuropsychiatric disorders was based on the “common disease-common variant” model. According to that theory, multiple common alleles in the population contribute small-to-moderate additive or multiplicative effects to the predisposition to neuropsychiatric disorders. With the advances in genetic screening technologies this theory is now being challenged. Recent findings indicate that rare copy number variations (CNVs) may account for a substantial fraction of the overall genetic risk for neuropsychiatric disorders including schizophrenia and autism (Consortium, 2008; Stefansson et al., 2008;
Mefford et al., 2008). The 22q11.2 microdeletion was the most common CNV identified in patients with schizophrenia in a recent large scale study of patients with schizophrenia (Consortium, 2008). The 22q11.2 microdeletion is also the most common microdeletion occurring in humans and up to one third of individuals with 22q11.2 deletion syndrome (22q11.2DS) develop schizophrenia by adulthood. Thus the syndrome serves as an important model from which to learn the path leading from a well defined genetic defect to brain development and eventually to the evolution of schizophrenia.
It is still uncertain whether the neuropsychiatric phenotype associated with 22q11.2DS is a result of a strong effect of haploinsufficiency of one or a few genes from the microdeletion region as some studies suggested (Gothelf et al., 2005; Paterlini et al., 2005; Raux et al., 2007; Vorstman et al., 2008), or the result of cumulative small effects of haploinsufficiency of multiple genes, each contributing a small effect, as other studies suggested (Maynard et al., 2003; Meechan et al., 2006).
The current very elegant study by Mukai and colleagues suggests that haploinsufficiency of a single gene from the 22q11.2 deleted region, Zdhhc8, is responsible for the microscopic neural hippocampal abnormalities present in a mouse model of the disease. Remarkably, these abnormalities were prevented with the reintroduction of enzymatically active ZDHHC8 protein. The works of Gogos and his colleagues (Paterlini et al., 2005; Stark et al., 2008) are consistently and brilliantly getting us closer to revealing the complex association between genes from the 22q11.2 region and the neuropsychiatric phenotype. If indeed haploinsuffiency of single genes like Zdhhc8, COMT, or Dgcr8 have a strong effect on abnormal brain development and the eruption of schizophrenia, it conveys an enormous potential for developing novel pathophysiologically based treatments for this refractory disease. Such treatments will target the enzymatic deficit conveyed by the genetic mutation.
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