Transmission Ratio Distortion in Genetic Prion Diseases: Clarifying Methodological Considerations

We are grateful for the thoughtful engagement with our work on transmission ratio distortion in genetic prion diseases. As clinicians and researchers working alongside affected families for decades, we share your commitment to rigorous, cautiously applied science. However, we must respectfully but firmly disagree that ascertainment bias substantially undermines our findings. We believe there is a fundamental misunderstanding of our study design. The central issue revolves around two distinct analytical levels: (1) how families enter our study (population-level ascertainment) and (2) how we calculate transmission within families once identified. We acknowledge that families reaching clinical attention may not represent every family with PRNP mutations. However, our analytical approach systematically examines complete nuclear families—one confirmed/obligate carrier parent and all biological offspring with known genetic status—regardless of which members initially led to clinical recognition. Large pedigrees were subdivided into individual nuclear family units following established methodology, with each unit contributing based on the complete count of both carrier and non-carrier offspring. Consider a hypothetical family identified because Branch A showed 80% transmission. Our methodology would equally analyze Branch B (perhaps 33% transmission) and Branch C (50% transmission). If these three nuclear families contributed seven carriers among 12 offspring total, we record 58% transmission—not the 80% that led to identification. This fundamentally differs from analyzing only high-transmission branches. For ascertainment bias to inflate our estimates as suggested, two conditions must be met simultaneously: (1) distinct “types” of families with inherently different transmission propensities for the same mutation and (2) high-transmission families reaching clinical attention disproportionately. Both conditions are biologically implausible and empirically unsupported. If transmission ratio distortion is genuine—whether through meiotic drive, gametic selection, or embryonic mechanisms documented in other genetic disorders—it should be mutation-specific, not family-specific. There is no biological mechanism whereby Family Smith with p.E200K would exhibit 80% transmission while Family López with identical p.E200K shows 50%. Indeed, both variants in our study (p.E200K and p.D178N) demonstrated remarkably similar transmission rates (70.5% and 67.1%) despite different geographic distributions, clinical phenotypes, and ascertainment contexts. Even accepting differential family propensities, the second condition fails. For families with three offspring (near our median) and 80% penetrance by age 80, a family with 50% transmission has ~78% probability of ≥ 1 affected individual versus ~90% for 67% transmission. This 12-percentage-point difference cannot create a detection threshold systematically excluding lower-transmission families, particularly given high penetrance and long observation periods. Most compellingly, our data directly contradict systematic ascertainment bias. We consistently observed nuclear families with transmission rates below 50%—including 25%–33%—within pedigrees also containing families above 67%. If our sample were biased toward high-transmission families, we would not capture low-transmission nuclear families within identified pedigrees; yet, we demonstrably do. The 35.4% of nuclear families at/below 50% transmission indicates successful capture of the full spectrum once families enter our study. To inflate a true 50% mean to observed 67% while retaining 35% of observations below 50% would require an extremely right-skewed distribution with a heavy tail of very high transmission rates—which we do not observe. This still requires the biologically implausible assumption of inherent family differences for identical mutations. Regarding “obligate carriers,” these were not identified solely because they transmitted (which would be circular), but through complete genotype reconstruction where all offspring were systematically genotyped. When analyzing these parents' nuclear families, we count all offspring—both carriers and non-carriers. The designation “obligate” refers to parental genotype certainty based on offspring data, not selection based on transmission. Concerning ~30% excluded families: exclusions were determined a priori based on methodological criteria before transmission analysis—no genotyped offspring, ambiguous parental sex for sex-specific analyses, or insufficient genetic information. Crucially, no exclusions were based on observed transmission rates. Our analytical rigor includes generalized estimating equations accounting for within-family clustering, > 80% power for primary analyses, and consistency of statistically significant results across both variants (p < 0.001 overall) and most sex-specific analyses (p < 0.05). Effect sizes of 17–21 percentage points above expected 50% argue strongly for genuine biological phenomena rather than methodological artifact. We recognize your appropriately conservative stance on genetic counseling practice. While population-based prospective studies would be ideal, they may be extraordinarily difficult for diseases this rare, particularly given the ethical complexities of predictive testing and long mutation-to-manifestation latency. Our intention has never been to advocate immediate counseling changes from 50% to 67%–70% risk. Rather, these data warrant nuanced discussion acknowledging both theoretical Mendelian expectations and observed transmission patterns, with clear explanation of uncertainties and ongoing debate. Families making profound reproductive decisions deserve access to the best available evidence, appropriately contextualized, rather than adherence to assumptions that may not fully reflect empirical reality. The 50% Mendelian ratio remains a valid starting point, but our data suggest it may not tell the complete story for these specific mutations. Presenting these findings was not undertaken lightly. Working alongside prion disease families for many years, we understand the profound weight genetic information carries for individuals making life-altering reproductive decisions. Yet scientific integrity requires reporting systematic observations even when they challenge established frameworks, provided we do so with appropriate methodological rigor and caution. We remain hopeful that larger studies will corroborate or refute our findings. Until then, the responsible path is acknowledging the evidence, engaging in the critical scientific discourse your letter exemplifies, and ensuring families are fully informed of both established models and emerging questions. This balanced approach ultimately best serves those whose lives are most affected by our conclusions. All authors contributed to the conceptualization and writing of this response. H.E. is employed by ATLAS Molecular Pharma SL. The remaining authors declare no competing interests. Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.