In the last century infant mortality has declined precipitously in the Western world, thanks in large part to the development of antibiotics and vaccination. Yet as the suffering and death from infectious disease has reduced, the burden from genetic disease has become proportionately greater: currently around 20% of all infant deaths in developed countries are a result of inherited Mendelian (single-gene) disorders.
What can be done to reduce this burden? Increasingly sophisticated methods for detecting disease in embryos during pregnancy will help, and these have recently taken another step forward with the development of accurate, non-invasive methods based on analysing foetal DNA in the blood of pregnant mothers (an article in the BMJ this week demonstrates the feasibility of this approach for a non-Mendelian disease, Down syndrome; and the same group showed late last year that this approach can also be applied to effectively any known disease-causing mutation). Yet these approaches detect disease after pregnancy has already begun.
Disease mutations can also be detected in embryos prior to implantation, for prospective parents undergoing IVF. But IVF remains an expensive, arduous and invasive procedure, and thus a weapon of last resort for most parents-in-waiting; as Armand Leroi notes drily in an exceptional 2006 article in EMBO Reports: “nature has contrived a cheap, easy and enjoyable way to conceive a child; IVF is none of these things.” (While Leroi goes on to argue that the challenges of IVF are less severe for young couples with no fertility problems, it still seems fairly implausible that this will become the default mode of reproduction in the near future.)
However, for some classes of Mendelian disease it’s possible to move the screening one step back. Recessive diseases are insidious things. The mutations that cause them lurk undetected – each of us carry perhaps 5 to 10 of them – as their carriers are protected by the presence of a healthy second copy of the affected gene. These mutations can thus wait silently for generation after generation, until a carrier is unlucky enough to fall for someone who carries the same mutation, or another mutation in the same gene. The children of such a couple will each have a 25% chance of inheriting one damaged copy of the gene from each parent and thus developing the disease.
The ability of a recessive mutation to pass silently from generation to generation means that many children born with recessive diseases have no family history. And while certain marriage practices (notably serial first-cousin marriage) can dramatically increase the risk of having a child with a recessive disease, these diseases can also explode into appearance in families with no obvious risk factors.
However, the fact that both parents must carry mutations in the same gene to pass a recessive disease to their children raises the possibility of detecting risk before a couple has even conceived children. For instance, one could screen both members of a couple for a panel of known mutations, an approach currently offered by US company Counsyl (disclaimer: my wife and I both accepted free tests from Counsyl in 2009). However, while a panel containing all known Mendelian mutations could detect a substantial fraction of all genetic disease (Leroi again), it can never eliminate the risk, because many Mendelian mutations remain undiscovered. However, one could go one step further: rather than simply look for known mutations, one could examine the entire sequence of all genes known to be associated with Mendelian diseases, and thus identify new mutations lurking in the same gene.
In an article published today in Science Translational Medicine a group of US researchers describe a high-throughput approach for doing precisely that.
Rather than review the technical aspects of the paper in great detail, I’ll just hit the main points I took from it:
- The 448 genes selected for the screening panel are the product of walking a political tightrope. While the authors indicate that the marginal costs of adding extra sequence mean that the optimal cost-benefit ratio comes from including a very broad range of diseases, they have excluded diseases that might trigger controversy (such as deafness and adult-onset disorders).
- The authors do a commendable job of comparing multiple technologies for capturing disease genes and for sequencing the captured DNA; the final product is a combination of Agilent SureSelect for sequence capture and Illumina HiSeq for sequencing.
- This approach allowed them to detect ~95% of the genetic variants in their target genes with very high accuracy. In other words, they might miss around 5% of disease-causing mutations in these genes, but the ones they find are probably real.
- Perhaps the single most important message from the paper, which I hope to expand on in a future post, is that disease mutations reported in the literature are depressingly enriched for false positives. The authors suggest that 27% of mutations in their samples that overlap with entries the largest database, the Human Gene Mutation Database, turned out to be the result of sequencing errors or mistakes in the literature (e.g. common polymorphisms that have been falsely reported to be disease-causing).
- The researchers found that the 104 sequenced samples contained on average 2.8 known disease-causing mutations in the surveyed genes. This fits with expectations: each of us is likely walking around with 5-10 of these severe disease-causing variants in total, which we will only know about if (1) we get our genomes sequenced, or (2) we’re unlucky enough to have children with a partner who carries mutations in the same gene.
- The authors estimate the cost of their test at $378, which they note is “approximating that expended on treatment of severe recessive childhood disorders per U.S. live birth”. In other words, offering this type of screen across the US population as a whole would be roughly cost-neutral from a healthcare stand-point, while simultaneously reducing the number of children dying from genetic diseases.
Given the plummeting costs of sequencing and the economies of scale, the cost-benefit ratio for this type of screening panel will continue to drop. In this context it seems inevitable that some form of sequencing approach will ultimately be implemented as routine for young parents-to-be.
Finally, the media coverage of this study has predictably stirred up the standard ethical and religious objections; these make for interesting dinner party conversation, but are largely irrelevant in any practical sense. Parents, as a group, will simply do whatever it takes to increase the probability that their children will be born healthy. Armand Leroi conveys this point well in the 2006 article mentioned above:
These abortions are eugenic in both intention and effect–that is, their purpose is to eliminate a genetically defective fetus and thus allow for a genetically superior child in a subsequent pregnancy. This is a harsh way of phrasing it; another way is to say that parents just want to have healthy children. Nevertheless, however it is phrased, the conclusion is starkly unavoidable: terminating the pregnancy of a genetically defective fetus is widespread. Moreover, because none of the countries mentioned above coerce parents into aborting deformed fetuses, these abortions–which number many thousands each year–are carried out at the request of the parents, or at least the mothers. This high number of so-called medical abortions shows that many people, in many parts of the world, consider the elimination of a genetically defective fetus to be morally acceptable.
Protests from ethicists and ministers may lead to some entertaining talk radio discussions, but ultimately the desire of parents for healthy children will sweep aside all objections (just as moral objections to tissue transplants and IVF were swept aside for similarly pragmatic reasons). In the face of this implacable tide I find it difficult to get too engaged in the moral debates on these issues; they just seem like a waste of time.