Gene-dosage issues: a recurrent theme in whole genome duplication events.

Two recent studies have addressed the long-term consequences of whole genome duplications (WGD). Specifically, they analyzed transcriptomes of the plant Arabidopsis thaliana and of four salmonids to assess the impact of WGD on gene expression. These studies point to commonalities in gene expression adjustments after polyploidization that we outline and discuss below.

Dominant negative variants and cotranslational assembly of macromolecular complexes.

Pathogenic variants occurring in protein-coding regions underlie human genetic disease through various mechanisms. They can lead to a loss of function (LOF) such as in recessive conditions or in dominant conditions due to haploinsufficiency. Dominant-negative (DN) effects, counteracting the activity of the normal gene-product, and gain of function (GOF) are also mechanisms driving dominance. Here, I discuss a few papers on these specific mechanisms. In short, there is accumulating evidence pointing to differences between LOF versus non-LOF variants (DN and GOF). The latter are thought to have milder effects on protein structure and, as expected, DN variants are enriched at protein interfaces. This tendency to cluster in 3D space can help improve the ability of computational tools to predict the pathogenicity of DN variants, which is currently a challenging issue. More recent results support the hypothesis whereby cotranslational assembly of macromolecular complexes can buffer deleterious consequences of variants that would otherwise lead to DN effects (DNEs). Indeed, subunits the variants of which are responsible for DNEs tend to elude cotranslational assembly, thus poisoning complexes involving wild-type subunits. The constraints explaining why the buffering of DNEs is not universal require further investigation.

The forkhead DNA-binding domain binds specific G2-rich RNA sequences.

Transcription factors contain a DNA-binding domain ensuring specific recognition of DNA target sequences. The family of forkhead (FOX) transcription factors is composed of dozens of paralogs in mammals. The forkhead domain (FHD) is a segment of about 100 amino acids that binds an A-rich DNA sequence. Using DNA and RNA PCR-SELEX, we show that recombinant FOXL2 proteins, either wild-type or carrying the oncogenic variant C134W, recognize similar DNA-binding sites. This suggests that the oncogenic variant does not alter the intrinsic sequence-specificity of FOXL2. Most importantly, we show that FOXL2 binds G2-rich RNA sequences whereas it virtually fails to bind similar sequences in DNA chemistry. Interestingly, a statistically significant subset of genes responding to the knock-down of FOXL2/Foxl2 harbor such G2-rich sequences and are involved in crucial signaling pathways and cellular processes. In addition, we show that FOXA1, FOXO3a and chimeric FOXL2 proteins containing the FHD of the former are also able to interact with some of the preferred FOXL2-binding sequences. Our results point to an unexpected and novel characteristic of the forkhead domain, the biological relevance of which remains to be explored.

Forkhead Transcription Factors in Health and Disease.

Forkhead box (FOX) proteins belong to an evolutionarily conserved family of transcription factors that has evolved by gene/genome duplication. FOX family members have undergone sequence and regulatory diversification. However, they have retained some degree of functional redundancy, in addition to playing specific roles, both during development and in the adult. Genetic alterations or misregulation of FOX genes underlie human genetic diseases, cancer, and/or aging. In this review, we provide an updated overview of the main characteristics of the members of this family, in terms of breadth of expression, protein domain composition, evolution, and function.

A cellular model provides insights into the pathogenicity of the oncogenic FOXL2 somatic variant p.Cys134Trp.

BACKGROUND: FOXL2 is a transcription factor expressed in ovarian granulosa cells. A somatic variant of FOXL2 (c.402 C > G, p.Cys134Trp) is the hallmark of adult-type granulosa cell tumours. METHODS: We generated KGN cell clones either heterozygous for this variant (MUT) or homozygous for the wild-type (WT) allele by CRISPR/Cas9 editing. They underwent RNA-Seq and bioinformatics analyses to uncover pathways impacted by deregulated genes. Cell morphology and migration were studied. RESULTS: The differentially expressed genes (DEGs) between WT/MUT and WT/WT KGN cells (DEGs-WT/MUT), pointed to several dysregulated pathways, like TGF-beta pathway, cell adhesion and migration. Consistently, WT/MUT cells were rounder than WT/WT cells and displayed a different distribution of stress fibres and paxillin staining. A comparison of the DEGs-WT/MUT with those found when FOXL2 was knocked down (KD) in WT/WT KGN cells showed that most DEGs-WT/MUT cells were not so in the KD experiment, supporting a gain-of-function (GOF) scenario. MUT-FOXL2 also displayed a stronger interaction with SMAD3. CONCLUSIONS: Our work, aiming at better understanding the GOF scenario, shows that the dysregulated genes and pathways are consistent with this idea. Besides, we propose that GOF might result from an enhanced interaction with SMAD3 that could underlie an ectopic capacity of mutated FOXL2 to bind SMAD4.

DNA Content, Cell Size, and Cell Senescence.

A recent study assessed the impact of cell size on various cell properties. Oversized yeast cells display slow cell division, cytoplasmic dilution, and transcriptomic and proteomic alterations. It highlights commonalities between aging yeast and mammalian cells, suggesting the existence of a range of DNA content: cell volume ratio ensuring optimal function.

The Muller's Ratchet and Aging.

Aging entails an irreversible deceleration of physiological processes, altered metabolic activities, and a decline of the integrity of tissues, organs, and organ systems. The accumulation of alterations in the genetic and epigenetic spaces has been proposed as an explanation for aging. They result, at least in part, from DNA replication and chromosome segregation errors due to cell division during development, growth, renewal, and repair. Such deleterious alterations, including epigenetic drift, irreversibly accumulate in a stepwise, ratchet-like manner and reduce cellular fitness, similar to the process known as Muller's ratchet. Here, we revisit the Muller's ratchet principle applied to the aging of somatic cell populations and discuss the implications for understanding the origins of senescence, frailty, and morbidity.

Recurrent missense variants in clonal hematopoiesis-related genes present in the general population.

Clonal hematopoiesis (CH) consists in an abnormal expansion of a hematopoietic stem cell bearing an advantageous somatic variant. A survey of known recurrent somatic missense variants in DNMT3A, SF3B1, SRSF2, and TP53, some of the most prominent genes underlying CH of indeterminate potential (CHIP), in gnomAD noncancer database shows the presence of 73 variants. Many of them reach frequencies higher than 0.01% in various populations and, in many cases, are enriched in specific populations. Consistent with a potential involvement in CHIP, we found that the age distribution of the carriers is shifted towards old ages. Moreover, the variant allele frequencies are on average lower than 50%, expected for germline heterozygous variants. The pervasive presence of some of such variants in blood DNA from elder individuals is compatible with CHIP of somatic origin. On practical grounds, CHIP can lead to misclassification of somatic variants in cancer-predisposition genes as inherited, which bear consequences for the affected individuals and their families.

Predictable increase in female reproductive window: A simple model connecting age of reproduction, menopause, and longevity.

With the ever-increasing lifespan along with societal changes, women can marry and procreate later than in previous centuries. However, pathogenic genetic variants segregating in the population can lead to female subfertility or infertility well before the average age of normal menopause, leading to counter-selection of such deleterious alleles. In reviewing this field, we speculate that a logical consequence would be the later occurrence of menopause and the extension of women's reproductive lifespan. We illustrate this point with a simple model that applies to other variants that contribute to female infertility, including epigenetic variation. We also consider the effect of medical interventions and lifestyle.

Pathogenic "germline" variants associated with myeloproliferative disorders in apparently normal individuals: Inherited or acquired genetic alterations?

Myeloproliferative syndromes (MPS) are hematologic malignancies due to the expansion of an abnormal hematopoietic stem cell. They include chronic myeloid leukemia (CML) and non-CML MPS such as polycythemia vera, essential thrombocythemia and primary myelofibrosis. The latter are distinguished by somatic pathogenic variants affecting JAK2, CALR, or MPL genes. Apparent germline pathogenic variants have been reported in the general population. Here, we found that two gnomAD data-sets report more homozygotes than expected for the JAK2 c.1849G > T(Val617Phe) variant. We propose that somatic gene conversion can explain the presence of those unexpected homozygotes in normal populations. Consistently, homozygous individuals are older than 65 years. We also found a lower-than-expected frequency of the JAK2 variant in younger individuals suggesting that somatic mutation can underlie its presence in (at least some) heterozygotes. Regarding pathogenic variants in MPL and CALR, they are also present in the gnomAD data-sets explored. However, we cannot conclude that such seemingly germline variants are in fact somatic alterations. These results suggest that apparently normal individuals bearing MPS-related variants can be subclinical/undiagnosed MPS cases of somatic origin. It would be interesting to assess the hematologic phenotype of such individuals and the presence of the relevant variants in other tissues.