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[No authors listed]. Rare chromosomal deletions and duplications increase risk of schizophrenia. Nature. 2008 Sep 11 ; 455(7210):237-41. Pubmed Abstract

Comments on News and Primary Papers
Comment by:  Christopher RossRussell L. Margolis
Submitted 1 August 2008
Posted 1 August 2008

The two recent papers in Nature, from the Icelandic group (Stefansson et al., 2008), and the International Schizophrenia Consortium (2008) led by Pamela Sklar, represent a landmark in psychiatric genetics. For the first time two large studies have yielded highly significant consistent results using multiple population samples. Furthermore, they arrived at these results using quite different methods. The Icelandic group used transmission screening and focused on de novo events, using the Illumina platform in both a discovery population and a replication population. By contrast, the ISC study was a large population-based case-control study using the Affymetrix platform, which did not specifically search for de novo events.

Both identified the same two regions on chromosome 1 and chromosome 15, as well as replicating the previously well studied VCFS region on chromosome 22. Thus, we now have three copy number variants which are replicated and consistent across studies. This provides data on rare highly penetrant variants complementary to the family based study of DISC1 (Porteous et al., 2006), in which the chromosomal translocation clearly segregates with disease, but in only one family. In addition, they are in general congruent with three other studies (Walsh et al., 2008; Kirov et al., 2008; Xu et al., 2008) which also demonstrate a role for copy number variation in schizophrenia. These studies together should put to rest many of the arguments about the value of genetics in psychiatry, so that future studies can now begin from a firmer base.

However, these studies also raise at least as many questions as they answer. One is the role of copy number variation in schizophrenia in the general population. The number of cases accounted for by the deletions on chromosome 1 and 15 in the ISC and Icelandic studies is extremely small--on the order of 1% or less. The extent to which copy number variation, including very rare or even private de novo variants, will account for the genetic risk for schizophrenia in the general population is still unknown. The ISC study indicated that there is a higher overall load of copy number variations in schizophrenia, broadly consistent with Walsh et al and Xu et al but backed up by a much larger sample size, allowing the results to achieve high statistical significance. The implications of these findings are still undeveloped,

Another issue is the relationship to the phenotype of schizophrenia in the general population. Many more genotype-phenotype studies will need to be done. It will be important to determine whether there is a higher rate of mental retardation in the schizophrenia in these studies than in other populations.

Another question is the relationship between these copy number variations (and other rare events) and the more common variants accounting for smaller increases in risk, as in the recent O’Donovan et al. (2008) association study in Nature Genetics. It is far too early to know, but there may well be some combination of rare mutations plus risk alleles that account for cases in the general population. This would then be highly reminiscent of Alzheimer’s disease, Parkinson’s disease, and other diseases which have been studied for a longer period of time.

For instance, in Alzheimer’s disease there are rare mutations in APP and presenilin, as well as copy number variation in APP, with duplications causing the accelerated Alzheimer’s disease seen in Down syndrome. These appear to interact with the risk allele in APOE, and possibly other risk alleles, and are part of a pathogenic pathway (Tanzi and Bertram, 2005). Similarly in Parkinson’s disease, rare mutations in α-synuclein, LRRK2 and other genes can be causative of PD, though notably the G2019S mutation in LRRK2 has incomplete penetrance. In addition, duplications or triplications of α-synuclein can cause familial PD, and altered expression due to promoter variants may contribute to risk. By contrast, deletions in Parkin cause an early onset Parkinsonian syndrome (Hardy et al., 2006). Finally, much of PD may be due to genetic risk factors or environmental causes that have not yet been identified. Further studies will likely lead to the elucidation of pathogenic pathways. These diseases can provide a paradigm for the study of schizophrenia and other psychiatric diseases. One difference is that the copy number variations in the neurodegenerative diseases are often increases in copies (as in APP and α-synuclein), consistent with gain of function mechanisms, while the schizophrenia associations were predominantly with deletions, suggesting loss of function mechanisms. The hope is that as genes are identified, they can be linked together in pathways, leading to understanding of the neurobiology of schizophrenia (Ross et al., 2006).

The key unanswered questions, of course, are what genes or other functional domains are deleted at the chromosome 1, 15, and 22 loci, whether the deletions at these loci are sufficient in themselves to cause schizophrenia, and, if sufficient, the extent to which the deletions are penetrant. Both of the current studies identified deletions large enough to include several genes. The hope is that at least a subset of copy number variations (unlike SNP associations identified in schizophrenia to date) may be causative, making the identification of the relevant genes or other functional domains—at least in principle—more feasible.

Another tantalizing observation is that the copy number variations associated with schizophrenia were defined by flanking repeat regions. This raises the question of the extent to which undetected smaller insertions, deletions or other copy number variations related to other repetitive motifs, such as long tandem repeats, may also be associated with schizophrenia. Identification and testing of these loci may prove a fruitful approach to finding additional genetic risk factors for schizophrenia.

References:

Hardy J, Cai H, Cookson MR, Gwinn-Hardy K, Singleton A. Genetics of Parkinson's disease and parkinsonism. Ann Neurol. 2006 Oct;60(4):389-98. Abstract

Kirov G, Gumus D, Chen W, Norton N, Georgieva L, Sari M, O'Donovan MC, Erdogan F, Owen MJ, Ropers HH, Ullmann R. Comparative genome hybridization suggests a role for NRXN1 and APBA2 in schizophrenia. Hum Mol Genet . 2008 Feb 1 ; 17(3):458-65. Abstract

Porteous DJ, Thomson P, Brandon NJ, Millar JK. The genetics and biology of DISC1—an emerging role in psychosis and cognition. Biol Psychiatry. 2006 Jul 15;60(2):123-31. Abstract

Ross CA, Margolis RL, Reading SA, Pletnikov M, Coyle JT. Neurobiology of schizophrenia. Neuron. 2006 Oct 5;52(1):139-53. Abstract

Singleton A, Myers A, Hardy J. The law of mass action applied to neurodegenerative disease: a hypothesis concerning the etiology and pathogenesis of complex diseases. Hum Mol Genet. 2004 Apr 1;13 Spec No 1:R123-6. Abstract

Tanzi RE, Bertram L. Twenty years of the Alzheimer's disease amyloid hypothesis: a genetic perspective. Cell. 2005 Feb 25;120(4):545-55. Abstract

Walsh T, McClellan JM, McCarthy SE, Addington AM, Pierce SB, Cooper GM, Nord AS, Kusenda M, Malhotra D, Bhandari A, Stray SM, Rippey CF, Roccanova P, Makarov V, Lakshmi B, Findling RL, Sikich L, Stromberg T, Merriman B, Gogtay N, Butler P, Eckstrand K, Noory L, Gochman P, Long R, Chen Z, Davis S, Baker C, Eichler EE, Meltzer PS, Nelson SF, Singleton AB, Lee MK, Rapoport JL, King MC, Sebat J. Rare structural variants disrupt multiple genes in neurodevelopmental pathways in schizophrenia. Science. 2008 Apr 25;320(5875):539-43. Abstract

Xu B, Roos JL, Levy S, van Rensburg EJ, Gogos JA, Karayiorgou M. Strong association of de novo copy number mutations with sporadic schizophrenia. Nat Genet. 2008 Jul;40(7):880-5. Abstract

View all comments by Christopher Ross
View all comments by Russell L. MargolisComment by:  Daniel Weinberger, SRF Advisor
Submitted 3 August 2008
Posted 3 August 2008

Several recent reports have suggested that rare CNVs may be highly penetrant genetic factors in the pathogenesis of schizophrenia, perhaps even singular etiologic events in those cases of schizophrenia who have them. This is potentially of enormous importance, as the definitive identification of such a “causative” factor may be a major step in unraveling the biologic mystery of the condition. I would stress several issues that need to be considered in putting these recent findings into a broader perspective.

It is very difficult to attribute illness to a private CNV, i.e., one found only in a single individual. This point has been potently illustrated by a study of clinically discordant MZ twins who share CNVs (Bruder et al., AJHG, 2008). Inherited CNVs, such as those that made up almost all of the CNVs described in the childhood onset cases of the study by Walsh et al. (Science, 2008), are by definition not highly penetrant (since they are inherited from unaffected parents). The finding by Xu et al. (Nat Gen, 2008) that de novo (i.e., non-inherited) CNVs are much more likely to be associated with cases lacking a family history is provocative but difficult to interpret as no data are given about the size of the families having a family history and those not having such a history. Unless these family samples are of comparable size and obtained by a comparable ascertainment strategy, it is hard to know how conclusive the finding is. Indeed, in the study of Walsh et al., rare CNVs were just as likely to be found in patients with a positive family history. Finally, in contrast to private CNVs, recurrent (but still rare) CNVs, such as those identified on 1q and 15q in the studies of the International Schizophrenia Consortium (Nature, 2008) and Stefansson et al. (Nature, 2008), are strongly implicated as being associated with the diagnosis of schizophrenia and therefore likely involved in the causation of the illnesses in the cases having these CNVs. In all, these new CNV regions, combined with the VCFS region on 22q, suggest that approximately five to 10 patients out of 1,000 who carry the diagnosis of schizophrenia may have a well-defined genetic lesion (i.e., a substantial deletion or duplication).

The overarching question now is how relevant these findings are to the other 99 percent of individuals with this diagnosis who do not have these recurrent CNVs. Before we had the capability to perform high-density DNA hybridization and SNP array analyses, chromosomal anomalies associated with the diagnosis of schizophrenia were identified using cytogenetic techniques. Indeed, VCFS, XXX, XXY (Kleinfelter’s syndrome), and XO (Turner syndrome) have been found with similarly increased frequency in cases with this diagnosis in a number of studies. Now that we have greater resolution to identify smaller structural anomalies, the list of congenital syndromes that increase the possibility that people will manifest symptoms that earn them this diagnosis appears to be growing rapidly. Are we finding causes for the form of schizophrenia that most psychiatrists see in their offices, or are we instead carving out a new set of rare congenital syndromes that share some clinical characteristics, as syphilis was carved out from the diagnosis of schizophrenia at the turn of the twentieth century? Is schizophrenia a primary expression of these anomalies or a secondary manifestation? VCFS is associated with schizophrenia-like phenomena but even more often with mild mental retardation, autism spectrum, and other psychiatric manifestations. The same is true of the aneuploidies that increase the probability of manifesting schizophrenia symptoms. The two new papers in Nature allude to the possibility that epilepsy and intellectual limitations may also be associated with these CNVs. The diagnostic potential of any of these new findings cannot be determined until the full spectrum of their clinical manifestations is clarified.

One of the important insights that might emerge from identification of these new CNV syndromes is the identification of candidate genes that may show association with schizophrenia based on SNPs in these regions. VCFS has been an important source of promising candidate genes with broader clinical relevance (e.g., PRODH, COMT). Stefansson et al. report, however, that none of the 319 SNPs in the CNV regions showed significant association with schizophrenia in quite a large sample of individuals not having deletions in these regions. The Consortium report also presumably has the results of SNP association testing in these regions in their large sample but did not report them. It is very important to explore in greater genetic detail these regions of the genome showing association with the diagnosis of schizophrenia in samples lacking these lesions and to fully characterize the clinical picture of individuals who have them. It is hoped that insights into the pathogenesis of symptoms related to this diagnosis will emerge from these additional studies.

Anyone who has worked in a public state hospital or chronic schizophrenia care facility (where I spent over 20 years) is not surprised to find an occasional patient with a rare congenital or acquired syndrome who expresses symptoms similar to those individuals also diagnosed with schizophrenia who do not have such rare syndromes. Our diagnostic procedures are not precise, and the symptoms that earn someone this diagnosis are not specific. Schizophrenia is not something someone has; it is a diagnosis someone is given. In an earlier comment for SRF on structural variations in the genome related to autism, I suggested that, “From a genetic point of view, autism is a syndrome that can be reached from many directions, along many paths. It is not likely that autism is any more of a discrete disease entity than say, blindness or mental retardation.” These new CNV syndromes manifesting schizophrenia phenomena are probably a reminder that the same is true of what we call schizophrenia.

View all comments by Daniel Weinberger