Lethal Weapon: How Many Lethal Mutations Do We Carry?

 

By John McLaughlin

Many human genetic disorders, such as cystic fibrosis and sickle cell anemia, are caused by recessive mutations with a predictable pattern of inheritance. Tracking hereditary disorders such as these is an important part of genetic counseling, for example when planning a family. In fact, there exists an online database dedicated to medical genetics, Mendelian Inheritance in Man, which contains information on most human genetic disorders and their associated phenotypes.

 

The authors of a new paper in Genetics set out to estimate the number of recessive lethal mutations carried in the average human’s genome. The researchers’ rationale for specifically focusing on recessive mutations is their higher potential impact on human health; because deleterious mutations that are recessive are less likely to be purged by selection, they can be maintained in heterozygotes with little impact on fitness, and therefore occur in greater frequency. For the purposes of their analysis, recessive lethal disorders (i.e. caused by a recessive lethal mutation) were defined by two main criteria: first, when homozygous for its causative mutation, the disease leads to the death or effective sterility of its carrier before reproductive age, and second, mutant heterozygotes do not display any disease symptoms.

 

For this study, the researchers had access to an excellent sample population, a religious community known as the Hutterian Brethren. This South Dakotan community of ~1600 individuals is one of three closely related groups that migrated from Europe to North America in the 19th century. Importantly, the community has maintained a detailed genealogical record tracing back to the original 64 founders, which also contains information on individuals affected by genetic disorders since 1950. An additional bonus is that the Hutterites practice a communal lifestyle in which there is no private property; this helps to reduce the impact of confounding socioeconomic factors on the analysis.

 

Four recessive lethal genetic disorders have been identified in the Hutterite pedigree since their more detailed records began: cystic fibrosis, nonsyndromic mental retardation, restrictive dermopathy, and myopathy. To estimate the number of recessive lethal mutations carried by the original founders, the team used both the Hutterite pedigree and a type of computational simulation known as “gene dropping”. In a typical gene dropping simulation, alleles are assigned to a founder population, the Mendelian segregation and inheritance of these alleles across generations is simulated, and the output is compared with the known pedigree. One simplifying assumption made during the analysis is that no de novo lethal mutations had arisen in the population since its founding; therefore, any disorders arising in the pedigree are attributed to mutations carried by the original founder population.

 

After combining the results from many thousands of such simulations with the Hutterite pedigree, the authors make a final estimate of roughly one or two recessive lethal mutations carried per human genome (the exact figure is ~0.58). What are the implications of this estimate for human health? Although mating between more closely related individuals has been long known to increase the probability of recessive mutations homozygosing in offspring, a more precise risk factor was generated from this study’s mutation estimate. In the discussion section it is noted that mating between first cousins, although fairly rare today in the United States, is expected to increase the chance of a recessive lethal disorder in offspring by ~1.8%.

 

Perhaps the most interesting finding from this paper was the consistency of the predicted lethal mutation load across the genomes of different animal species. The authors compared their estimates for human recessive lethal mutation number to those from previous studies examining this same question in fruit fly and zebrafish genomes, and observed a similar value of one or two mutations per genome. Of course, the many simplifying assumptions made during their analyses should be kept in mind; the estimates are considered tentative and will most likely be followed up with similar future work in other human populations. It will certainly be interesting to see how large-scale studies such as this one will impact human medical genetics in the future.