Before Christmas, an insightful discussion between SRF's Pete Farley and researchers Heather Mefford and Evan Eichler delved into the complex interplay between genotype (copy number variant status at 1q21.1) and phenotype (psychiatric illness, autism, mental retardation, and congenital abnormalities) (see SRF related news story). The upshot was that although deletions at this locus were statistically associated with pathologies, the severity and nature of those pathologies was extremely variable. This raised questions about whether researchers and clinicians should focus on the disease or the deletion, and what the mechanisms that determine the clinical endpoint might be. This is becoming a clear trend. Another CNV region at 16p11.2 has also been variously associated with both autism and schizophrenia. Deletions of just a single gene, CNTNAP2, as opposed to a gene cluster, have also shown this phenomenon of variable phenotype expression—deletion carriers have been diagnosed with autism, Gilles de la Tourette/obsessive compulsive disorder, schizophrenia/epilepsy, or remain entirely healthy (Bakkaloglu et al., 2008; Friedman et al., 2008; Verkerk et al., 2003; Belloso et al., 2007).

In the same vein, this new paper by Helbig and colleagues describes yet another example of a discrete copy number variant (microdeletion) that was originally linked with psychiatric phenotypes but is now also shown to give rise to idiopathic generalized epilepsy (IGE). The deletion is at 15q13.3, which encompasses the candidate neurotransmitter receptor gene, CHRNA7, among others. In fact, with a frequency of 1 percent in the IGE population and absence in controls, the deletion is the strongest genetic risk factor for this condition and is more prevalent in IGE than in either mental retardation or schizophrenia.

Although the study of CNVs has highlighted this genotype-phenotype issue, it has been observed previously in the context of the overlap of linkage hotspots between schizophrenia and bipolar disorder (Berrettini, 2003), in case-control association studies linking the same gene to multiple disorders (Chubb et al., 2008), and in the case of the Scottish family with the t(1;11) translocation disrupting DISC1, in which carrier phenotypes ranged from healthy to major depression, bipolar disorder, and schizophrenia (Blackwood et al., 2001).

So we are now faced with complex genetic disorders that really live up to their name. As such, two particular issues warrant further discussion.

The first issue is that clinicians seem to observe discrete rather than continuous disorder phenotypes. Despite the current diagnostic manuals leaving little room for diagnostic leeway, it seems that the majority of case phenotypes tend toward a limited number of outcomes such as schizophrenia, bipolar disorder, mental retardation, autism, and epilepsy. Moreover, no psychiatrist can distinguish DISC1 schizophrenia from 1q21.1 schizophrenia or NRG1 schizophrenia without recourse to genetic methodologies, suggesting that there is a positive biological drive towards the endpoint. To borrow what may be a useful analogy from physics, the system is “chaotic” (in terms of its genetic input and its effect on cellular biology) but tends toward “strange attractors” (a limited set of diagnoses) [http://en.wikipedia.org/wiki/Attractor]. Why might this be so? It may be that there are several higher order functional bottlenecks within the brain such as synaptic transmission efficiency, cortical development, astrocyte/oligodendrocyte function, hippocampal neurogenesis, higher order communication between brain regions, etc. These act to “sum” the expected environmental, genetic, and cellular complexity present within an individual and transform it into a limited set of potential outcomes—in essence, these are the strange attractors.

The next issue is how the same mutation can give rise to two (or more) different conditions. It may be useful to think of the Knudson “two-hit” hypothesis of cancer in which environment and other genetic factors act subsequent to a “deep” genetic fault (Knudson, 1971).

The CNV examples above may represent such fundamental disruptions and most probably impinge on neurodevelopmental pathways, priming the brain to be tipped over the threshold into a disease state. In fact, the t(1:11) translocation carriers present evidence for such a phenomenon as both healthy and affected carriers show abnormal P300 brain response activities suggesting this endophenotype highlights an underlying brain dysfunction (Blackwood et al., 2001).

We have to postulate that the additional genetic or environmental influences (modifiers) not only determine entry into the disease state but also dictate the final outcome. Possible candidates for modifiers of the deletions above are the remaining single copy alleles at the CNV locus—exposed recessive mutations, imprinting, or epigenetic modification could all alter expressivity and penetrance of the deletion phenotype. However, limited studies by Eichler’s group seem to discount this possibility (Mefford et al., 2008).

In any case, genomewide association and CNV studies suggest that there is plenty of scope for a sufficient burden of genetic modifiers outside the CNV region. This may also fit in with the seemingly disparate concepts of rare/familial variants exposed by linkage and common/low odds ratio variants revealed by association. Both act causally with the former potentially acting as the “first hit.”

As time progresses, we will move towards the definition of the range of phenotypes potentially resulting from each genotype and the spectrum of genotypes causing each phenotype. CNVs represent a pretty blunt tool to dissect finer relationships between genotype and phenotype, so it is to be expected that rare but penetrant point mutations that emerge from resequencing projects will be of greater use in dissecting function-phenotype links—as has been seen with the connexin gene family, for example (Rabionet et al., 2002).

In summary, it is to be hoped that the clinical and research communities are able to embrace these complexities for what they offer—a deeper understanding of these disorders, one that is intimately linked to the development and function of the brain.

References:

Bakkaloglu B, O'roak BJ, Louvi A, Gupta AR, Abelson JF, Morgan TM, Chawarska K, Klin A, Ercan-Sencicek AG, Stillman AA, Tanriover G, Abrahams BS, Duvall JA, Robbins EM, Geschwind DH, Biederer T, Gunel M, Lifton RP, State MW. Molecular cytogenetic analysis and resequencing of contactin associated protein-like 2 in autism spectrum disorders. Am J Hum Genet. 2008 Jan 1;82(1):165-73. Abstract

Friedman JI, Vrijenhoek T, Markx S, Janssen IM, van der Vliet WA, Faas BH, Knoers NV, Cahn W, Kahn RS, Edelmann L, Davis KL, Silverman JM, Brunner HG, van Kessel AG, Wijmenga C, Ophoff RA, Veltman JA. CNTNAP2 gene dosage variation is associated with schizophrenia and epilepsy. Mol Psychiatry. 2008 Mar 1;13(3):261-6. Abstract

Verkerk AJ, Mathews CA, Joosse M, Eussen BH, Heutink P, Oostra BA, . CNTNAP2 is disrupted in a family with Gilles de la Tourette syndrome and obsessive compulsive disorder. Genomics. 2003 Jul 1;82(1):1-9. Abstract

Belloso JM, Bache I, Guitart M, Caballin MR, Halgren C, Kirchhoff M, Ropers HH, Tommerup N, Tümer Z. Disruption of the CNTNAP2 gene in a t(7;15) translocation family without symptoms of Gilles de la Tourette syndrome. Eur J Hum Genet. 2007 Jun 1;15(6):711-3. Abstract

Berrettini W. Evidence for shared susceptibility in bipolar disorder and schizophrenia. Am J Med Genet C Semin Med Genet. 2003 Nov 15;123C(1):59-64. Abstract

Chubb JE, Bradshaw NJ, Soares DC, Porteous DJ, Millar JK. The DISC locus in psychiatric illness. Mol Psychiatry. 2008 Jan 1;13(1):36-64. Abstract

Blackwood DH, Fordyce A, Walker MT, St Clair DM, Porteous DJ, Muir WJ. Schizophrenia and affective disorders--cosegregation with a translocation at chromosome 1q42 that directly disrupts brain-expressed genes: clinical and P300 findings in a family. Am J Hum Genet. 2001 Aug 1;69(2):428-33. Abstract

Knudson AG. Mutation and cancer: statistical study of retinoblastoma. Proc Natl Acad Sci U S A. 1971 Apr 1;68(4):820-3. Abstract

Mefford HC, Sharp AJ, Baker C, Itsara A, Jiang Z, Buysse K, Huang S, Maloney VK, Crolla JA, Baralle D, Collins A, Mercer C, Norga K, de Ravel T, Devriendt K, Bongers EM, de Leeuw N, Reardon W, Gimelli S, Bena F, Hennekam RC, Male A, Gaunt L, Clayton-Smith J, Simonic I, Park SM, Mehta SG, Nik-Zainal S, Woods CG, Firth HV, Parkin G, Fichera M, Reitano S, Lo Giudice M, Li KE, Casuga I, Broomer A, Conrad B, Schwerzmann M, Räber L, Gallati S, Striano P, Coppola A, Tolmie JL, Tobias ES, Lilley C, Armengol L, Spysschaert Y, Verloo P, De Coene A, Goossens L, Mortier G, Speleman F, van Binsbergen E, Nelen MR, Hochstenbach R, Poot M, Gallagher L, Gill M, McClellan J, King MC, Regan R, Skinner C, Stevenson RE, Antonarakis SE, Chen C, Estivill X, Menten B, Gimelli G, Gribble S, Schwartz S, Sutcliffe JS, Walsh T, Knight SJ, Sebat J, Romano C, Schwartz CE, Veltman JA, de Vries BB, Vermeesch JR, Barber JC, Willatt L, Tassabehji M, Eichler EE. Recurrent rearrangements of chromosome 1q21.1 and variable pediatric phenotypes. N Engl J Med. 2008 Oct 16;359(16):1685-99. Abstract

Rabionet R, López-Bigas N, Arbonès ML, Estivill X. Connexin mutations in hearing loss, dermatological and neurological disorders. Trends Mol Med. 2002 May 1;8(5):205-12. Abstract

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