Saturday, 20 October 2018

Mutated islands of brain matter from development might be common in the human population

You are made up of a lot of cells, and so is your brain. You were also derived from a single cell: the union of a sperm and an egg. In order for your body to grow from a single cell into an adult human, a massive amount of cell division must occur, which means that the DNA inside your cells must also be replicated intensively. In copying all of this DNA, “spelling mistakes” can sometimes be made. If that mistake occurs early enough in development, all of the subsequent cells which are copied from the mutant parent also receive this mistake, which potentially gives rise to large islands of mutated cells (called “somatic mosaicism”). If a copying error occurs at a particularly important base of DNA, this could potentially cause disease in the tissue once you have fully developed into an adult. 

Inherited mutations in certain genes are known, in rare cases, to cause neurodegenerative disease (such as Alzheimer's and Parkinson's disease). We wondered whether non-heritable “spelling mistakes” in these disease-causing genes is common enough in the human population to potentially explain the more common forms of neurodegenerative disease. 

Our experimental collaborators at the University of Cambridge went searching for mutated chunks of brain matter in post-mortem samples of brains from 54 human individuals. Using genetic sequencing technology, they found evidence for these mutated islands of grey matter. However, none of these samples were pathological themselves, since only a small fraction of the brain per individual was sampled. This provided an opportunity for mathematical modelling of how the brain develops, so that we may predict the prevalence of pathological mutations in human brains, given the experimental data. 

Our mathematical model is incredibly simple (and crude -- others have developed much more sophisticated approaches): we assume that, in order to grow a brain, you take the initial cell from which you were derived, and double it repeatedly until the mass of cells corresponds to the number of cells in the brain (this is called a binary tree). Each copying event is called a “generation”, and corresponds to a row of the tree below. If a mutation occurs whilst copying the DNA of a particular cell at a particular generation, then a fixed fraction of all the subsequent daughters will also be mutated, generating a mutant region. Repeatedly simulating brain development using a computer allows us to gather statistics about the probability of an individual harbouring pathological islands of brain matter. 

Mathematical modelling of brain development reveals that islands of pathologically mutated cells are potentially common in the population. Left: We modelled neurodevelopment as a simple binary tree, where an initial cell doubles repeatedly until the final adult brain is created. DNA copying errors are carried forward into daughter cells. Right: A typical simulated individual. Coloured circles represent islands of pathologically mutated cells in the adult human brain. Whole brain area (black circle) is not to scale with the mutated regions (coloured circles). The mutated regions are really tiny proportionately. 

Neurodevelopment is, of course, much more complicated than a series of doubling events. Amongst other effects, regions spatially re-arrange themselves, cells die, and cell division isn't always symmetric (i.e. daughter cells may not always be capable of dividing themselves). We explored several modifications to the simple model above, and found that our extrapolations were surprisingly robust. We argue that, once the developing human brain consists of about 1 million cells, as long as each daughter cell gives rise to roughly the same number of daughter cells in subsequent divisions, and that spatial mixing of the brain isn't too strong, every individual is expected to harbour about 1 pathologically mutated island of cells consisting of about 10,000 to 100,000 cells. The basic idea is that if those 1 million cells replicate once then they are really likely to have a pathological mutation crop up in one of those 1 million divisions. Larger regions may also occur, but are rarer, and conversely, smaller regions are more common (see the right panel of the figure above). This kind of argument suggests that for a whole range of possible ways in which our brain develops we’re likely to have islands of mutation. 

We also discuss an observation which emerges from the tree-structure of neurodevelopment which may allow us to directly estimate the mutation rate from a simple back-of-the-envelope calculation. Any particular experiment will have a certain detection sensitivity, in that it will be able to detect mutations common to a minimum number of cells in a sample, and no fewer (in our case this was ~0.5% of cells in a sample). Because of the tree structure of neurodevelopment, the most common mutations observed will occur at exactly the detection sensitivity: larger mutated islands become exponentially rarer, whereas smaller mutations are too small to be measurable. 

Now consider cutting a whole brain up into a number of equally-sized chunks. As the size of the chunks increases (where we are able to detect mutations affecting 0.5% of each chunk), each chunk is tuning into a mutation event which affects more cells, and therefore higher up in the neurodevelopmental tree. But, the number of mutated cells from any particular generation in the tree is a constant: mutations high up in the tree are larger, but also rarer, and these two effects precisely balance each other. Therefore, regardless of how large each chunk is, the total number of cells which you expect to be able to detect is independent of chunk size. The total number of detectably mutated cells does, of course, depend on the mutation rate and the total number of bases that are sequenced. Furthermore, we may say that the total number of detectably mutated cells equals: (number of mutated chunks) x (number of mutated cells per mutated chunk): this argument itself is also independent of the size of each chunk, only depending on the detection sensitivity and the fraction of detectably mutated chunks across the whole experiment. Therefore, we may equate the total number of mutations from any given generation, and the total number of detectably mutated, to write down the mutation rate entirely independently of the size of each brain chunk. Another way of putting the above argument is that (for very simple models of the brain we describe) we expect that the quantity: (the fraction of chunks containing mutation) x (sensitivity of the detection technique), is an invariant directly linked to the mutation rate (specifically to the number of mutations expected in a single replication of the sequences studied). This doesn't depend on the size of the chunks or the size of the brain. As such, if our experiment had half the sensitivity to mutated cells per brain chunk (so 1% instead of 0.5%), we’d have had to measure twice as many bits of brain to obtain a similar number of detectably mutated brain chunks. It's obviously crude but helpful for insight -- and we're order-of-magnitude enthusiasts (see this and this and this

Overall, our results suggest that pathologically mutated islands of brain matter are potentially possessed by all of us. These islands may potentially be sources of protein aggregates, which could spread in the brain and cause neurodegeneration; perhaps they’re regions which could be thought of as randomly triggering pathology sometime over our lives with the rate of triggering proportional to the size of the region. Future work is required to verify this, by direct observation of pathologically mutated islands, and mechanistic studies to quantify how large an island is “large enough” to have a high chance of inducing disease within a human lifespan. 

You can freely access our work, which has recently been published in Nature Communications, as "High prevalence of focal and multi-focal somatic genetic variants in the human brain" Juvid, Nick and our friends in the Department of Clinical Neurosciences at the University of Cambridge especially Mike, Wei and Patrick.

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