Is a polygenic model of schizophrenia genetics really proven?
Kenneth Kendler’s
article on the nature of genetic variation and the nature of schizophrenia
claims that theory and empirical evidence have proven the polygenic
architecture of this disorder. In fact, both theory and data are entirely
consistent with a very different model of high genetic heterogeneity, where the
disorder is largely caused in individuals by one or a few mutations in any of a
large number of genes, incorporating important and complex effects of genetic
background.
KK provides a scholarly overview of the history of ideas in these intertwined
fields1.
While historically interesting, the early arguments between biometricians and
Mendelians about continuous versus dichotomous traits conflate two distinct
questions: (i) what type of genetic variation contributes to the gradual
evolution of new species?, and (ii) what type of genetic variation causes
disease? There is no reason to expect these to have the same answer and many
reasons not to.
With regard to the
genetic architecture of SZ, KK presents the history of various models, from
those positing a single major locus to those invoking polygenic mechanisms
based on the work of Fisher, Falconer and others. Of course, the single major
locus model has long since been rejected and the current debate is really
between (i) models of extreme genetic heterogeneity, where the disease is
largely caused by one or a small number of rare mutations in each affected
individual (in any of a large number of different genes), and (ii) polygenic
models involving the combined effects of thousands of common variants that
“constitute the gene pool of our species”.
The only reference KK
makes to models of genetic heterogeneity regrettably repeats a commonly held
but mistaken notion, i.e., that the (negative) results of linkage analyses for
SZ refute the theory that the disorder is a “common pathway for a large number
of rare quasi-Mendelian disorders”, based on the idea that multiple linkage
peaks would have been found if that were the case. This is demonstrably false.
Most SZ linkage studies bundled together many small families, as large multiplex
SZ pedigrees are rare. If the disorder shows a high level of genetic
heterogeneity, combining families will necessarily obscure real linkage signals2.
Recent simulations bear this out: in cases where a disorder is associated with
decreased fitness and high genetic heterogeneity, linkage studies are predicted
to fail3.
KK also presents
several lines of positive evidence as supporting – indeed proving – that the
polygenic model of SZ is correct. First, he argues that the existence of a
phenotypic continuum between clinically diagnosable SZ and SZ-like personality
disorders in first-degree relatives proves a polygenic model. It does not. Many
classical single-gene disorders show incomplete penetrance and variable
expressivity. In some cases, these are due to modifier genes in the background,
but – as for SZ itself – phenotypes often vary substantially even between
monozygotic twins. What these observations really highlight is that psychiatric
diagnostic categories do not represent distinct biological phenotypes, but only
one of a range of possible outcomes. The clinical and etiological overlap
between SZ and other neurodevelopmental disorders, including autism, epilepsy
and intellectual disability reinforces this point4.
Second, KK claims
that recent genome-wide association studies and related analyses “have shown
that for schizophrenia, Fisher’s model is largely correct”. This interpretation
is not warranted by the data. A recent, very large-scale GWAS identified 108
loci with common single-nucleotide polymorphisms (SNPs) showing positive
association signals with disease (higher frequency in cases than controls)5.
However, GWAS signals do not identify causal variants or inform as to their
allelic frequency. Numerous examples of synthetic associations caused by rare
mutations have been demonstrated and the fact that rare mutations in many of
the loci implicated are known to confer high risk for neuropsychiatric diseases
supports this possibility5.
But even if the
causal variants are common, this does not imply that the polygenic model is
correct. The GWAS signal is a population-level average statistic and does not
speak to how these variants act in individuals. Rather than acting in purely
polygenic fashion – a hypothetical mechanism never actually demonstrated to
cause disease – common variants may instead act as important modifiers of risk
due to rare variants or environmental perturbations – a perfectly well-established
mechanism (e.g., ref. 6).
Genome-Wide Complex
Trait Analyses also cannot determine the number of contributing loci per
individual, the number of causal variants across the population or the
frequency of causal variants. This is stated clearly by Lee et al: “From the analyses we have performed, we cannot estimate a
distribution of the allele frequency of causal variants”7.
These analyses merely show (or claim) that extremely small statistical
increases in risk can be detected across distant relatedness, presuming the
technical assumptions and methods are valid7,8. In any case, GCTA analyses for SZ show that most genetic risk is NOT associated with common variants.
Genetic epidemiology
at the population level can point to loci of interest but the findings do not
restrict or even really inform on the genetic architecture of the disorder in
individuals. The empirical data are perfectly consistent with a model of high genetic
heterogeneity, where most cases are associated with one or a small number of
high-risk mutations9,
and where the phenotypic expression of these mutations is affected by genetic
background10.
Finally, it seems
strange to draw moral conclusions about how we should think of or treat people
with SZ based on the genetic architecture of the disease. There does not need
to be a continuum of risk across the population for healthy people to feel
sympathy for those affected. It is very clear, from monozygotic twin concordance
rates of ~50%, that those who have SZ were at very high risk of developing it, on average,
with the corollary that the majority of the population had effectively zero
risk. “Liability” may be normally distributed but that is an imaginary
statistical construct – actual risk is clearly not continuous, under any model of genetic architecture. No moral
conclusions derive from that fact.
[Postscript: I haven't gone in to all the positive evidence for an important role for rare mutations of large effect in the etiology of SZ, but see here for many examples and more details: The Genetic Architecture of Neurodevelopmental Disorders.]
References
10.1017/S003329171000070X (2011).
10.1186/gb-2012-13-1-237 (2012).
* I originally wrote this as a letter to the editor at Molecular Psychiatry, in response to the article referenced by Kenneth Kendler, but they didn't like it, so I just decided to post it here instead.
Hi Kevin,
ReplyDeleteEnjoyed the reply to KK (and many of your postings). I have a few related questions. You make a strong case that the common variant hypothesis is not proven and that rare variants could account for a much bigger proportion of schizophrenia that we now understand. Seems reasonable. But aren't there all sorts of other possibilities too? Where to gene-gene interactions fit into all of this? Or gene-environment interactions? What about many gene interactions and other super complex models? I guess there are arguments that many gene effects are additive, but this just seems strange from an intuitive standpoint. Why wouldn't evolution have mixed genetic influences together? I like the idea that modifier genes might act on a phenotype determined by other genes. But this also seems like a slippery slope. Don't modifiers become indistinguishable from interacting genes at some point, when they have a big enough impact on the phenotype? I guess my question comes down to this (possibly a straw man question based on my incomplete understanding): couldn't many additive common variants acting together account for 5% of cases (or some other single digit percentage) and many rare variants acting independently account for another 5%, and much of the remaining genetic influence be hidden by complex, multi-layered interactions that might be pretty near impossible to disentangle?
Thanks.
Thanks Dwight, for your comment. Personally, I think all those mechanisms are in play. I absolutely think gene-gene interactions are crucial in understanding individual risk. I just don't think it's likely that they can all be between thousands of common variants of tiny effect in an individual. (The infinitesimal, death-by-a-tousand-cuts model). That is, I think it is likely to take at least one big insult to push development out of its normally robust path.
ReplyDeleteRather than thinking of some cases being caused by collections of common variants and others being caused by single rare mutations, I think it's much more likely that all cases involved a combination of both. And if there are enough alleles involved then you are right - the designation of some as primary and some as modifiers becomes fuzzy. One way to look at it is whether any particular allele increases risk generally (true for many rare mutations) or only in people with certain other mutations. If the latter, then I'll happily call it a modifier. (The Y chromosome is a good example!)
But really these are all still open questions (which is the main point of my piece).