August 5, 2013. A 22q11.22 deletion that spans the gene TOP3β is associated with schizophrenia and cognitive impairments, reports a new study published online August 4 in Nature Neuroscience. Led by Utz Fischer of the University of Würzburg in Germany and Nelson Freimer of the University of California, Los Angeles, the researchers also report that TOP3β forms a complex with FMRP, the culprit behind Fragile X syndrome, a form of intellectual disability that shares some cognitive deficits with schizophrenia.
Accompanying this paper is an interesting elaboration of TOP3β’s function, also published online in Nature Neuroscience the same day. Lead study authors Sige Zou and Weidong Wang, both of the National Institute on Aging in Baltimore, Maryland, provide additional evidence that TOP3β interacts with FMRP and that its DNA topoisomerase abilities extend to RNA.
The 22q11.22—schizophrenia link
The existence of multiple population bottlenecks in Finland has created several regions within the country, known as sub-isolates, that have elevated frequencies of genetic variants and corresponding increases in a variety of disorders. For example, northeastern Finland has rates of schizophrenia that are nearly three times higher than the rest of the country (Perälä et al., 2008). In the first study, first authors Georg Stoll, Olli Pietiläinen, and Bastian Linder capitalized on this unique situation to look for schizophrenia-associated deletions enriched in the northeastern Finnish population (n = 173) compared to the rest of Finland (n = 1,586).
They found that deletions at 4q12 and 22q11.22 were more frequent in the sub-isolate population. A comparison of 185 sub-isolate schizophrenia subjects with 747 sub-isolate controls confirmed that the 22q11.22 deletion was also present more often in the schizophrenia subjects, with an odds ratio of 1.84, and was nominally associated with schizophrenia in a sample of the whole Finnish population, but not in two non-Finnish samples. The 4q12 deletion, on the other hand, was present in equal frequencies in both groups.
The 22q11.22 deletion was also associated with cognitive impairment in a cohort of nearly 5,000 members of the Northern Finland 1966 Birth Cohort. Excluding members of the cohort with a diagnosis of schizophrenia, carriers of the 22q11.22 deletion had a higher frequency of intellectual disability and mild learning disabilities. In addition, in a subset of the sub-isolate and whole Finnish schizophrenia samples, deletion carriers with a diagnosis of schizophrenia (n = 19) performed worse on verbal memory tests than the schizophrenia subjects without the deletion (n = 547).
TOP3β takes center stage
The 240 kb 22q11.22 deletion spans two genes: IGLV2-14 and TOP3β. In addition, this variant also lies within the deletion region that causes 22q11.2 microdeletion syndrome, a developmental disorder strongly associated with schizophrenia (see SRF related news story). Given the limited amount of information available about IGLV2-14, Stoll and colleagues elected to focus on TOP3β. Based on prior studies implicating TOP3β in neurodevelopmental disorders, they hypothesized that the gene may be contributing to the association of the 22q11.22 deletion with schizophrenia and cognitive abnormalities (Xu et al., 2012).
The TOP3β gene encodes DNA topoisomerase III, an enzyme responsible for fixing winding irregularities (over- or under-winding) of DNA. After confirming that TOP3β expression is disrupted in 22q11.22 deletion carriers, the researchers analyzed its function and found that it was bound to a surprising partner: fragile X mental retardation protein (FMRP). FMRP is an RNA binding protein that represses the translation of neuronal mRNAs and whose absence leads to Fragile X syndrome, one of the most common forms of mental retardation (intellectual disability).
Further biochemical analyses revealed that the interaction of TOP3β and FMRP was mediated by Tudor domain-containing protein 3 (TDRD3). Together, the three proteins formed a heterotrimeric TOP3β-TDRD3-FMRP (TTF) complex that was present on cytosolic messenger ribonucleoproteins (mRNPs), complexes of mRNA and RNA-binding proteins that properly package mRNA. In addition, Stoll and colleagues report that the DNA topoisomerase TOP3β could bind directly to RNA and was found to be catalytically active on RNA substrates, providing further evidence of its role in mRNA metabolism.
“The identification of TTF uncovered an unexpected biochemical link between Fragile X syndrome patients and TOP3β deletion,” note the authors. Fragile X syndrome and schizophrenia share similar cognitive deficits such as impairment in working memory and other executive functions, so “the dysregulation of TTF-containing mRNPs may underlie the overlapping neurocognitive impairment phenotypes observed in Fragile X syndrome and schizophrenia,” they conclude.
The life and times of TOP3β
In the second study, first authors Dongyi Xu and Weiping Shen shed more light on the TTF complex and found that the transfection into HEK293 cells of a missense FMRP mutation, originally identified in a Fragile X syndrome patient, markedly reduced the association of FMRP with TOP3β-TDRD3. In addition, the researchers found that TOP3β and FMRP share many of the same mRNA targets, many of which were related to synapse function.
In fact, a null mutation of the FMRP gene, FMR1, in Drosophila produced abnormal neuromuscular junctions characterized by excessive synaptic branching and axon terminals. Although a similar effect was observed in TOP3β mutant flies, double mutants lacking both FMR1 and TOP3β had fewer synaptic branches and terminals than either of the single mutants, suggesting that the two proteins interact in an antagonistic manner.
Xu and colleagues also provided the first demonstration that the DNA topoisomerase TOP3β can also act as an RNA topoisomerase and that both reactions require the same amino acid residue. Although single-stranded and linear RNA are not generally thought to have topological problems, “our discovery of TOP3β as an RNA topoisomerase necessitates reconsideration of this assumption,” write the authors.—Allison A. Curley.
Stoll G, Pietiläinen OP, Linder B, Suvisaari J, Brosi C, Hennah W, Leppä V, Torniainen M, Ripatti S, Ala-Mello S, Plöttner O, Rehnström K, Tuulio-Henriksson A, Varilo T, Tallila J, Kristiansson K, Isohanni M, Kaprio J, Eriksson JG, Raitakari OT, Lehtimäki T, Jarvelin MR, Salomaa V, Hurles M, Stefansson H, Peltonen L, Sullivan PF, Paunio T, Lönnqvist J, Daly MJ, Fischer U, Freimer NB, Palotie A. Deletion of TOP3ß, a component of FMRP-containing mRNPs, contributes to neurodevelopmental disorders. Nat Neurosci . 2013 Aug 4. Abstract
Xu D, Shen W, Guo R, Xue Y, Peng W, Sima J, Yang J, Sharov A, Srikantan S, Yang J, Fox D, Qian Y, Martindale JL, Piao Y, Machamer J, Joshi SR, Mohanty S, Shaw AC, Lloyd TE, Brown GW, Ko MS, Gorospe M, Zou S, Wang W. Top3ß is an RNA topoisomerase that works with fragile X syndrome protein to promote synapse formation. Nat Neurosci . 2013 Aug 4. Abstract