Somatic clones heterozygous for recessive disease alleles of BMPR1A exhibit unexpected phenotypes in Drosophila.

Takuya Akiyama,Matthew C Gibson,Sırma D User

doi:10.7554/elife.35258

Takuya Akiyama, Matthew C Gibson + Show 1 more

Open Access

https://doi.org/10.7554/elife.35258

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Abstract

The majority of mutations studied in animal models are designated as recessive based on the absence of visible phenotypes in germline heterozygotes. Accordingly, genetic studies primarily rely on homozygous loss-of-function to determine gene requirements, and a conceptually-related 'two-hit model' remains the central paradigm in cancer genetics. Here we investigate pathogenesis due to somatic mutation in epithelial tissues, a process that predominantly generates heterozygous cell clones. To study somatic mutation in Drosophila, we generated inducible alleles that mimic human Juvenile polyposis-associated BMPR1A mutations. Unexpectedly, four of these mutations had no phenotype in heterozygous carriers but exhibited clear tissue-level effects when present in somatic clones of heterozygous cells. We conclude that these alleles are indeed recessive when present in the germline, but nevertheless deleterious when present in heterozygous clones. This unforeseen effect, deleterious heteromosaicism, suggests a 'one-hit' mechanism for disease initiation that may explain some instances of pathogenesis associated with spontaneous mutation.

Highlights

Using human disease-associated alleles of the Drosophila Bone Morphogenetic Protein Receptor 1A (BMPR1A) homologue thickveins as a model, our approach permits the inducible conversion of the wild-type locus to a specific mutant allele (Figure 1A; Figure 1—figure supplement 2A)
While tkvC90R-mCherry homozygosity resulted in weaker phospho-Mothers against dpp (p-Mad) expression in the wing pouch, the same mutation activated BMP signaling in the hinge region in both hetero- and homozygous discs (Figure 2G,L and M; n = 35 and 32 discs, respectively)
Clones of tkvC97R heterozygous cells exhibited strongly reduced BMP/DPP signaling activity and increased p-Mad levels in adjacent wild-type cells (Figure 3I and J). These results demonstrate the unexpectedly detrimental effect of mosaic heterozygosity for a classically recessive allele, an effect we define as deleterious heteromosaicism

Summary

Introduction

An attribute that drives evolution at the population level but simultaneously underlies the incidence of spontaneous disease (Veltman and Brunner, 2012; Frank, 2014; Campbell et al, 2015; Acuna-Hidalgo et al, 2016; Forsberg et al, 2017; Stenson et al, 2017). In order to uniformly induce either hetero- or homozygosity throughout all cells of developing wing discs, larvae of tkv allele switch transgenics carrying hs-flp were subjected to a prolonged 1 hr heat shock at 72 hr after egg laying (AEL) (Figure 2B–L; Figure 2—figure supplement 1A and B). TkvC40Y-mCherry and tkvC97R-mCherry homozygosity resulted in small wing discs with no detectable p-Mad activity, consistent with strong loss-of-function alleles (Figure 2I and J; Figure 2—figure supplement 1B).

Results

Conclusion