A novel, wave-shaped profile of germline selection of pathogenic mtDNA mutations is discovered by bypassing a classical statistical bias.

Abstract

The shift of the level of disease-causing mtDNA mutations (heteroplasmy) from mother to child is typically negatively correlated with the mothers heteroplasmy (Hm). In other words, mothers with low Hm tend to have children with a higher mutation level (Hch) than their own. In contrast, mothers with high Hm typically see a decrease in heteroplasmy in their children. This trend has been commonly interpreted as a result of a descending germline selection profile, i.e., positive selection at low Hm, gradually turning negative at high Hm. Here we demonstrate, however, that the negative correlation is mostly driven by RTM, or Regression To the Mean, a classical statistical bias. We further show that RTM can be nullified by using the average between the mothers and childs heteroplasmy, as a new variable, instead of the commonly used mothers heteroplasmy in blood. Additionally, we demonstrate that mother/child average is a better approximation of the actual germline heteroplasmy. Moreover, the elimination of RTM revealed a previously hidden wave-shaped HS-profile (positive mother-to-child shift at intermediate average mother-child heteroplasmy, decreasing towards high and low average heteroplasmy). In confirmation of this finding, we show that simulations that involve both wave-shaped HS-profile and RTM, reproduce the observed patterns of inheritance of mtDNA mutations in unprecedented detail. From the health care perspective, the uncovering of the wave-shaped HS-profile (and the removal of the RTM bias) are crucial for families affected by mtDNA disease. From the fundamental perspective, the wave- shaped profile offers a novel understanding of the dynamics of mtDNA in the germline and a novel potential mechanism that prevents the spread of detrimental mtDNA mutations in the population. SignificanceFrom the clinical perspective, the existence of wave-shaped selection may improve predictions and decisions for families affected by mtDNA diseases. From the fundamental perspective, it provides insight into the dynamics of general mtDNA mutations in the germline and in the population, as long as they follow wave-shaped selection profile. In Fig. 1, blue and red arrows represent the direction of expected changes of the heteroplasmy in a lineage with time/generations. With wave- shaped selection (Fig. 1B), a great majority of nascent low- fraction mutations are expected to converge back to zero and vanish. However, due to random intracellular genetic drift, some mutations will, occasionally, expand and enter the range of positive selection. Then they will be expanded by the selection to higher, detrimental levels, and become prone to downstream removal via death of highly mutated germ cells or inability of highly sick individuals to continue their lineage. In this way, the wave-shaped selection may help to prevent the spread of detrimental mutations in the population and in the species. In contrast, if the descending selection profile (Fig. 1A) was in effect, the nascent low heteroplasmy detrimental mutations would have been pushed to intermediate heteroplasmy levels where they will stay longer in hidden disease carriers enabling effective spread of mutation in the population. O_FIG O_LINKSMALLFIG WIDTH=192 HEIGHT=200 SRC="FIGDIR/small/568140v2_fig1.gif" ALT="Figure 1"> View larger version (41K): org.highwire.dtl.DTLVardef@1142cc0org.highwire.dtl.DTLVardef@186002aorg.highwire.dtl.DTLVardef@74d176org.highwire.dtl.DTLVardef@163ca6c_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOFigure 1.C_FLOATNO (Graphic Summary) Pathogenic mtDNA mutations that cause a host of devastating inherited diseases are usually thought to follow an intriguing inheritance trend: Mothers with low levels of mutation (called mother heteroplasmy, Hm) tend to bear children with higher child heteroplasmy (Hch) then their own which constitutes positive Heteroplasmy Shift (HS=Hch-Hm). In contrast, mothers with high heteroplasmy Hm bear children with lower heteroplasmy Hc (negative HS). C_FIG