The study proposes an alternative mechanism of how the chromosomal translocation in the Scottish pedigree reported by Millar et al. (2000) could cause a behavioral phenotype. I think this is exactly the point: In the Scottish family, can such transcripts and fusion proteins be identified? Do they really exist? If they do, this paper certainly would help to identify them. I understand that it is very difficult to obtain material that could prove this, and I would like to hear Kirsty Millar's or David Porteous's comment on this.
Regarding the data on the solubility assays of the fusion transcripts, I would have liked to see more controls, like those of the DISC1 fragments only (i.e., without Boymaw domains) and whole lysate controls, since the paper reads as if insolubility would be a specific phenotype of the DISC1-Boymaw fusion protein. From our experience, I guess it isn't; we see insolubility with N-terminal, as well as C-terminal, DISC1 fragments, with expression time being a critical variable to consider.
Altogether, I believe the paper by Zhou et al. opens a novel and very interesting perspective on a possible scenario of how behavioral phenotypes could be caused by the DISC1 translocation in the Scottish family. It is also a nice synthesis of genetic and protein data.
Millar JK, Wilson-Annan JC, Anderson S, Christie S, Taylor MS, Semple CA, Devon RS, St. Clair DM, Muir WJ, Blackwood DH, Porteous DJ. Disruption of two novel genes by a translocation co-segregating with schizophrenia. Hum Mol Genet. 2000 May 22;9(9):1415-23. Abstract
View all comments by Carsten Korth
I would like to explain a little more on the protein expression from the two fusion transcripts. After we identified the brain-specific Boymaw mRNA, we generated the two fusion transcripts between the Boymaw and DISC1 genes. Because the DISC1 gene is expressed in human brain, it is likely that the ORF (Open Reading Frame) of DISC1-Boymaw fusion transcripts would be recognized for translation. However, it is unclear whether the Boymaw-DISC1 fusion transcripts will be translated because there are several small ORFs upstream of the long internal ORF of the DISC1 gene. These small ORFs in the long 5’ UTR (about 520 bp) could interfere with the translation of the C-terminal DISC1.
To examine whether the C-terminal DISC1 protein could be produced from the Boymaw-DISC1 fusion transcripts, we conducted the transfection experiments by using the DISC1-Boymaw fusion transcript as a positive control. Abundant C-terminal DISC1 proteins were produced from the Boymaw-DISC1 fusion transcripts. Surprisingly, the expression of DISC1-Boymaw fusion proteins is very low and can only be detected in pellet. Overexpression of proteins could cause some to be insoluble, but this does not seem to be the case for the DISC1-Boymaw fusion proteins, as their expression is much lower than that of the C-terminal DISC1 proteins. In contrast, there is little detection of the C-terminal DISC1 in pellet.
Because of the low expression of the DISC1-Boymaw proteins, we harvested the cells only when the expression was at its peak (about two days after transfection). In the future, it will be more informative to include both full-length and truncated DISC1 genes to compare their expression as suggested by Carsten Korth. It will be interesting to know whether the fusion of Boymaw to DISC1 makes the DISC1 less soluble, which may link the Scottish family to the finding of insoluble DISC1 proteins in postmortem brains of sporadically collected patients with schizophrenia, major depression, and bipolar disorder (Leliveld et al., 2008).
To examine the potential biological effects of the two fusion transcripts, we have conducted gene-targeting experiments to generate humanized DISC1-Boymaw mice. We have obtained positive ES (embryonic stem) cell colonies, and the generation of chimeric mice is in progress.
Leliveld SR, Bader V, Hendriks P, Prikulis I, Sajnani G, Requena JR, Korth C. Insolubility of disrupted-in-schizophrenia 1 disrupts oligomer-dependent interactions with nuclear distribution element 1 and is associated with sporadic mental disease. J Neurosci. 2008 Apr 9;28(15):3839-45. Abstract
View all comments by Xianjin Zhou
The paper by Dr. Zhou and colleagues, describing insoluble DISC1-Boymaw fusion proteins generated by DISC1 translocation, may be one of the most important accomplishments in the field of DISC1 research since the pioneer of this field, Dr. St. Clair, identified the Scottish pedigree associated with major mental illnesses (St. Clair et al., 1990). Although the gene coding for DISC1 is disrupted in the Scottish pedigree, it has been totally unclear how the disruption may lead to psychiatric conditions in some (but not all) of the family members with this disruption.
If a putative truncated protein is generated, it seems to function as a dominant-negative protein (Kamiya et al., 2005). In lymphoblasts from some patients with this chromosomal abnormality in the pedigree, immunoreactivity of DISC1 fails to be observed (Millar et al., 2005). These two observations may not be contradictory, as both scenarios can result in an overall loss of DISC1 function (Sawa and Snyder, 2005). Therefore, many groups have tried knockdown expression of DISC1 via RNAi to address possible roles for DISC1 in psychiatric conditions (Duan et al., 2007; Mao et al., 2009; Kim et al., 2009; Enomoto et al., 2009; Niwa et al., 2010; Hayashi-Takagi et al., 2010). These results have suggested that a loss of DISC1 function can result in the deficits of neuronal connectivity in various mechanisms, which may be relevant to the pathophysiology of major mental illnesses. However, many investigators have wondered whether the biological impacts of the balanced translocation found in the Scottish pedigree might be more complicated than that.
In this outstanding paper, Dr. Zhou and colleagues have very carefully characterized DISC1 fusion protein generated by this translocation. They obtained evidence that the DISC1 fusion protein can aggregate in cells—that is, the translocation may actually result in a gain of DISC1 toxic function. Since Dr. Korth pioneered the concept that DISC1 can form aggregates (Leliveld et al., 2008), several groups have obtained evidence that DISC1 can be amyloidogenic, and that such amyloidogenecity can be modified. Thus, the current report by Dr. Zhou et al. consistently extends this concept and suggests that the translocation can augment such amyloidogenecity by generating the fusion protein.
The limitation of this study is that all the experiments were conducted in cells. Information from the brain in vivo is awaited. Nonetheless, I strongly recommend this brief communication, one of the most important papers in the history of DISC1 research in the past 20 years.
St. Clair D, Blackwood D, Muir W, Carothers A, Walker M, Spowart G, Gosden C, Evans HJ. Association within a family of a balanced autosomal translocation with major mental illness. Lancet. 1990 Jul 7;336(8706):13-6. Abstract
Kamiya A, Kubo K, Tomoda T, Takaki M, Youn R, Ozeki Y, Sawamura N, Park U, Kudo C, Okawa M, Ross CA, Hatten ME, Nakajima K, Sawa A. A schizophrenia-associated mutation of DISC1 perturbs cerebral cortex development. Nat Cell Biol. 2005 Dec;7(12):1167-78. Abstract
Millar JK, Pickard BS, Mackie S, James R, Christie S, Buchanan SR, Malloy MP, Chubb JE, Huston E, Baillie GS, Thomson PA, Hill EV, Brandon NJ, Rain JC, Camargo LM, Whiting PJ, Houslay MD, Blackwood DH, Muir WJ, Porteous DJ. DISC1 and PDE4B are interacting genetic factors in schizophrenia that regulate cAMP signaling. Science. 2005 Nov 18;310(5751):1187-91. Abstract
Sawa A, Snyder SH. Genetics. Two genes link two distinct psychoses. Science. 2005 Nov 18;310(5751):1128-9. Abstract
Duan X, Chang JH, Ge S, Faulkner RL, Kim JY, Kitabatake Y, Liu XB, Yang CH, Jordan JD, Ma DK, Liu CY, Ganesan S, Cheng HJ, Ming GL, Lu B, Song H. Disrupted-In-Schizophrenia 1 regulates integration of newly generated neurons in the adult brain. Cell. 2007 Sep 21;130(6):1146-58. Abstract
Mao Y, Ge X, Frank CL, Madison JM, Koehler AN, Doud MK, Tassa C, Berry EM, Soda T, Singh KK, Biechele T, Petryshen TL, Moon RT, Haggarty SJ, Tsai LH. Disrupted in schizophrenia 1 regulates neuronal progenitor proliferation via modulation of GSK3beta/beta-catenin signaling. Cell. 2009 Mar 20;136(6):1017-31. Abstract
Kim JY, Duan X, Liu CY, Jang MH, Guo JU, Pow-anpongkul N, Kang E, Song H, Ming GL. DISC1 regulates new neuron development in the adult brain via modulation of AKT-mTOR signaling through KIAA1212. Neuron. 2009 Sep 24;63(6):761-73. Abstract
Enomoto A, Asai N, Namba T, Wang Y, Kato T, Tanaka M, Tatsumi H, Taya S, Tsuboi D, Kuroda K, Kaneko N, Sawamoto K, Miyamoto R, Jijiwa M, Murakumo Y, Sokabe M, Seki T, Kaibuchi K, Takahashi M. Roles of disrupted-in-schizophrenia 1-interacting protein girdin in postnatal development of the dentate gyrus. Neuron. 2009 Sep 24;63(6):774-87. Abstract
Niwa M, Kamiya A, Murai R, Kubo K, Gruber AJ, Tomita K, Lu L, Tomisato S, Jaaro-Peled H, Seshadri S, Hiyama H, Huang B, Kohda K, Noda Y, O'Donnell P, Nakajima K, Sawa A, Nabeshima T. Knockdown of DISC1 by in utero gene transfer disturbs postnatal dopaminergic maturation in the frontal cortex and leads to adult behavioral deficits. Neuron. 2010 Feb 25;65(4):480-9. Abstract
Hayashi-Takagi A, Takaki M, Graziane N, Seshadri S, Murdoch H, Dunlop AJ, Makino Y, Seshadri AJ, Ishizuka K, Srivastava DP, Xie Z, Baraban JM, Houslay MD, Tomoda T, Brandon NJ, Kamiya A, Yan Z, Penzes P, Sawa A. Disrupted-in-Schizophrenia 1 (DISC1) regulates spines of the glutamate synapse via Rac1. Nat Neurosci. 2010 Mar;13(3):327-32. Abstract
Leliveld SR, Bader V, Hendriks P, Prikulis I, Sajnani G, Requena JR, Korth C. Insolubility of disrupted-in-schizophrenia 1 disrupts oligomer-dependent interactions with nuclear distribution element 1 and is associated with sporadic mental disease.
J Neurosci. 2008 Apr 9;28(15):3839-45. Abstract
View all comments by Akira Sawa
Since the translocation event deletes portions of the CHORDC1 and DISC1 genes, is it possible that CHORDC1 and not DISC1 is a causative factor in the psychiatric phenotype?View all comments by Aaron Marley