The role of gene dosage in budding yeast centrosome scaling and spontaneous diploidization

Ploidy is the number of whole sets of chromosomes in a species. Ploidy is typically a stable cellular feature that is critical for survival. Polyploidization is a route recognized to increase gene dosage, improve fitness under stressful conditions and promote evolutionary diversity. However, the mechanism of regulation and maintenance of ploidy is not well characterized. Here, we examine the spontaneous diploidization associated with mutations in components of the Saccharomyces cerevisiae centrosome, known as the spindle pole body (SPB). Although SPB mutants are associated with defects in spindle formation, we show that two copies of the mutant in a haploid yeast favors diploidization in some cases, leading us to speculate that the increased gene dosage in diploids ‘rescues’ SPB duplication defects, allowing cells to successfully propagate with a stable diploid karyotype. This copy number-based rescue is linked to SPB scaling: certain SPB subcomplexes do not scale or only minimally scale with ploidy. We hypothesize that lesions in structures with incompatible allometries such as the centrosome may drive changes such as whole genome duplication, which have shaped the evolutionary landscape of many eukaryotes.

The role of gene dosage in budding yeast centrosome scaling and spontaneous diploidization

Ploidy is the number of whole sets of chromosomes in a species. Ploidy is typically a stable cellular feature that is critical for survival. Polyploidization is a route recognized to increase gene dosage, improve fitness under stressful conditions and promote evolutionary diversity. However, the mechanism of regulation and maintenance of ploidy is not well characterized. Here, we examine the spontaneous diploidization associated with mutations in components of the Saccharomyces cerevisiae centrosome, known as the spindle pole body (SPB). Although SPB mutants are associated with defects in spindle formation, we show that two copies of the mutant in a haploid yeast favors diploidization in some cases, leading us to speculate that the increased gene dosage in diploids ‘rescues’ SPB duplication defects, allowing cells to successfully propagate with a stable diploid karyotype. This copy number-based rescue is linked to SPB scaling: certain SPB subcomplexes do not scale or only minimally scale with ploidy. We hypothesize that acquisition of lesions in structures with incompatible allometries such as the centrosome may drive changes such as whole genome duplication, which have shaped the evolutionary landscape of many eukaryotes.Author SummaryPloidy is the number of whole sets of chromosomes in a species. Most eukaryotes alternate between a diploid (two copy) and haploid (one copy) state during their life and sexual cycle. However, as part of normal human development, specific tissues increase their DNA content. This gain of entire sets of chromosomes is known as polyploidization, and it is observed in invertebrates, plants and fungi, as well. Polyploidy is thought to improve fitness under stressful conditions and promote evolutionary diversity, but how ploidy is determined is poorly understood. Here, we use budding yeast to investigate mechanisms underlying the ploidy of wild-type cells and specific mutants that affect the centrosome, a conserved structure involved in chromosome segregation during cell division. Our work suggests that different scaling relationships (allometry) between the genome and cellular structures underlies alterations in ploidy. Furthermore, mutations in cellular structures with incompatible allometric relationships with the genome may drive genomic changes such duplications, which are underly the evolution of many species including both yeasts and humans.

Disruption of the topological associated domain at Xp21.2 is related to gonadal dysgenesis: A general mechanism of pathogenesis

Duplications of dosage sensitive sex-locus Xp21.2 including NR0B1 have been linked to 46,XY gonadal dysgenesis (GD) and their effects are attributed merely to increase gene dosage of NR0B1 (DAX1). Here we present a general mechanism how deletions, duplications, triplications or inversions with or without NR0B1 at Xp21.2 can lead to partial or complete GD by disrupting the cognate topological associated domain (TAD) in the vincinity of NR0B1. Our model is supported by three unrelated patients: two showing a 287kb overlapping duplication at the Xp21.2 locus upstream of NR0B1 containing CXorf21 and GK and one patient having a large new triplication of Xp21.2 as the most likely cause of GD. Whole Genome sequencing uncovered the exact structural rearrangements of the duplications and the triplication. Comparison with a previously published deletion upstream of NR0B1 revealed a common 35kb overlap between the deletion, our newly reported NR0B1 upstream duplications and the triplication as well as all other copy number variations (CNVs) at Xp21.2 reported so far. This overlap contains a strong CCCTC-binding factor (CTCF) binding site representing one boundary of the NR0B1 TAD. All three CNVs at Xp21.2 most likely disrupt this TAD boundary, which isolates NR0B1 from CXorf21 and GK and putatively results in GK and CXorf21 enhancer adoption and ensuing ectopic NR0B1 expression. As a result, the patients’ transcriptomes developed an intermediate expression pattern with both ovarian and testicular features and greatly reduced expression of spermatogenesis-related genes. This model not only allows better diagnosis of GD displaying CNVs at Xp21.2, but also gives deeper insight how spatiotemporal activation of developmental genes can be disrupted by reorganized TADs also in other rare diseases.