It is clear that human evolution over the last few millenia has been particularly rapid for mental/cognitive traits. This seems to have created the hazard of unintended consequences in the form of mental illness, when this high-strung and finely tuned system goes haywire. There has been a steady stream of genome variants discovered to be associated with prominent mental illnesses like schizophrenia, autism, and bipolar disorder. These variants are inherited (not sporadic, like the mutations responsible for most cancers) and tend to be either rare or have very slight effects, for obvious reasons.
"The majority of disease-associated genetic variation lies in noncoding regions enriched for noncoding RNAs (ncRNAs) and cis-regulatory elements that regulate gene expression and splicing of their cognate coding gene targets."
One feature that stands out among the genetic variants that have been found to date are that they are rarely in coding regions, thus do not affect the sequence of the protein encoded by the affected gene. Instead, they affect regulation- the delicate mechanisms by which genes are turned on and off in particular places and times. It is now common knowledge that humans have hardly more genes than fish or potatoes. It is gene regulation that makes us what we are, and recent decades have revealed whole new mechanisms and participants in this regulation, such as long non-coding RNAs.
A recent paper conducted a fishing expedition to find how gene regulation varies among people with mental illness, in a quest to find causal changes and molecular patterns characteristic of these syndromes. They had access to thousands of brain samples, and put them through a modern analysis of transcripts, including from 16,541 protein-coding genes and 9,233 genes expressing short or long regulatory RNAs. One theme that came up is that they found many differences among gene splice forms. Most eukaryotic genes are spliced after they are transcribed, being composed on the genome of a series of separate coding exons. This splicing (removing the intervening intronic RNA and joining the coding exons) is highly regulated and for many genes, fundamentally influential on what protein they ultimately produce. Some genes have dozens of different splice forms, many with significantly different roles. For example, a key functional domain like an inhibitor or a DNA-binding region may be left out of one form, converting its encoded protein into one that performs functions precisely opposite to those of the full-length form.
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| Screen shot from the NIH gene resource, showing the human gene titin, which encodes the largest known protein, which structurally spans the muscle sarcomere. Each green segment is an exon, totalling 363. Each horizontal line is a distinct splice form, varying in which exons are included. Skeletal and cardiac muscle express different splice forms of this protein, and sequence variations in this gene are responsible for some syndromes such as dilated cardiomyopathy. |
After putting their brain tissue samples through purification and sequencing of all the RNAs, and alignment with known sequences, the authors came up with a set of differences of gene expression between affected and control subjects. They claim that a quarter of all the genes they looked at were noticeably affected by one of the three disorders they looked at- autism, schizophrenia, and bipolar disorder. Naturally, this finding may be a consequence of wide-spread structural and functional changes in affected brains that may be well downstream from any causative factor. There was relatively little overlap between the three. Schizophrenia showed the most differentially expressed genes/splice forms by far (several-fold more than the others), and shared about half of those seen differentially expressed in bipolar disorder, and one-fifth of those in autism. One might speculate that on the whole, schizophrenia is a more severe disorder which would lead to more dramatic differences in gene expression in an absolute sense.
"Notably, 48 DS [differentially spliced] genes (10%; FDR = 8.8 × 10−4) encode RNA binding proteins or splicing factors, with at least six splicing factors also showing DTE [differential overall transcript expression] in ASD [autism spectrum disorders] (MATR3), SCZ [schizophrenia] (QKI, RBM3, SRRM2, U2AF1), or both (SRSF11)."
Interestingly, the authors also tested (in animals) whether drugs used to treat these major disorders could generate the differences seen above. The answer was no- differential expression was somewhat affected by lithium, but not significantly by the others tested. Secondly, the authors wheeled in a separate method, sifting through genomic variations to find other genes with variations known to be causally associated with these diseases, and guess whether these variations (aka mutations) might affect transcriptional expression. The results did not overlap very well. For bipolar disorder, none of the 17 genes identified by this latter method overlapped with the differentially expressed genes identified by the first method.
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| Part of the general scheme of the experiment, and schematic results. Isoforms of some genes show differences in disease vs control tissues, and patterns from many such genes and differences can be assembled to diagnose particular cells or processes that are being notably affected. But note in the middle panel that the quantitative difference in the alternative splicing pattern between disease and control for this particular example is minuscule- in the 1 to 4% range. It may be statistically significant, but could be minor in effect. The last panel illustrates fold-change ranges for non-coding long RNAs among the different syndromes and known patterns of cell-type expression, in a schematic sense. Genes known to be expressed in microglia showed particularly significant changes. |
So what did they get? Mainly a lot of little fish, and quite a few that had already been suggested to be important for brain function by other methods. One prominent theme was the involvement of immune-related genes. Genes characteristic of astrocyte and microglia cell expression, and of interferon responses, among other signatures, were significantly up-regulated. This agrees with other work indicating that inflammatory mechanisms are used to prune synapses and cells generally in the brain, and this process is over-active in schizophrenia. An example gene is complement C4A, which encodes part of a key immune system that identifies and clears foreign material with the help of phagocytes. It used to be thought that the central nervous system was immune-privileged, i.e. not surveiled by it or affected by it. But that turns out to be very far from the truth, and this gene's overexpression is genetically identified as a causal factor for schizophrenia.
Another big fish they caught was a gene called RBFOX1. Certain spliced forms were significantly less abundant in the affected tissues, supporting a long line of work identifying variations in this gene as candidate causal factors for autism, and its function as an RNA binding protein that regulates the alternative splicing of other genes as well as their later transcript stability and lifetime. Reduced function of this protein is known to increase neuronal excitability and increase seizures. It seems thus to be a key node in neuronal development and functional regulation, through its control of the expression of other genes.
Did these authors find anything new? That is naturally hard to say at this point, since such conclusions would require quite a bit of followup work to analyze the function of novel genes that were found. The expedition showed that it was technically capable of hauling in not only a lot of fish, but many known already to be significant in these syndromes, either causally or as markers and downstream effects. Choosing which to track down to their actual biology is a difficult question- one that must come next. It is a catch that may provide sustenance for many students and post-docs to come.
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