Evolution of attractiveness of Medical Biology to medical students in 2022 / Attractivité du diplôme d'études supérieures de biologie médicale en 2022 chez les étudiants en médecine, quelle évolution ?

Since 2010, the postgraduate training diploma in laboratory medicine has experienced a strong decline in its attractiveness to French medical students. This work aims to objectify and quantify this decline between 2004 and 2021. To do so, we have carried out an analyze of the trend to choose laboratory medicine after the national ranking review performed by all French students. Then, we have also compiled the data on the right to change one's mind and final quits from laboratory medicine postgraduate training. The laboratory medicine reform has led to deep changes in its organization. The accreditation requirements, the laboratories grouping, as well as the financialization of this activity have strongly changed the work of laboratory physician and impacted its attractiveness for the young generations.

Depuis 2010, le diplôme d'étude spécialisé de biologie médicale (DESBM) a connu une forte baisse de son attractivité auprès des étudiants en médecine. Ce travail s'intéresse à objectiver et quantifier cette baisse d'attractivité entre 2004 et 2021. Pour cela, nous avons réalisé une analyse de l'évolution du choix de la biologie médicale à l'issue des épreuves classantes nationales (ECN) et avons compilé les données correspondantes aux droits au remords et abandons définitifs du DES. La réforme de la biologie médicale a entrainé de profondes modifications dans son organisation. L'obligation d'accréditation, la concentration des laboratoires de biologie médicale ainsi que la financiarisation du milieu ont fortement bouleversé la profession et impacté son attractivité pour les jeunes générations.

Adaptation to the dietary sugar D-tagatose via genome instability in polyploid Candida albicans cells.

The opportunistic fungal pathogen Candida albicans undergoes an unusual parasexual cycle wherein diploid cells mate to form tetraploid cells that can generate genetically diverse progeny via a nonmeiotic program of chromosome loss. The genetic diversity afforded by parasex impacts clinically relevant features including drug resistance and virulence, and yet the factors influencing genome instability in C. albicans are not well defined. To understand how environmental cues impact genome instability, we monitored ploidy change following tetraploid cell growth in a panel of different carbon sources. We found that growth in one carbon source, D-tagatose, led to high levels of genomic instability and chromosome loss in tetraploid cells. This sugar is a stereoisomer of L-sorbose which was previously shown to promote karyotypic changes in C. albicans. However, while expression of the SOU1 gene enabled utilization of L-sorbose, overexpression of this gene did not promote growth in D-tagatose, indicating differences in assimilation of the two sugars. In addition, genome sequencing of multiple progenies recovered from D-tagatose cultures revealed increased relative copy numbers of chromosome 4, suggestive of chromosome-level regulation of D-tagatose metabolism. Together, these studies identify a novel environmental cue that induces genome instability in C. albicans, and further implicate chromosomal changes in supporting metabolic adaptation in this species.

Genomic Instability Signature of Palindromic Non-Coding Somatic Mutations in Bladder Cancer.

Numerous pan-genomic studies identified alterations in protein-coding genes and signaling pathways involved in bladder carcinogenesis, while non-coding somatic alterations remain weakly explored. The goal of this study was to identify clinical biomarkers in non-coding regions for bladder cancer patients. We have previously identified in bladder tumors two non-coding mutational hotspots occurring at high frequencies (≥30%). These mutations are located close to the GPR126 and PLEKHS1 genes, at the guanine or the cytosine of a TGAACA core motif flanked, on both sides, by a stretch of palindromic sequences. Here, we hypothesize that such a pattern of recurrent non-coding mutations could be a signature of somatic genomic instability specifically involved in bladder cancer. We analyzed 26 additional mutable non-coding sites with the same core motif in a cohort of 103 bladder cancers composed of 44 NMIBC cases and 59 MIBC cases using high-resolution melting (HRM) and Sanger sequencing. Five bladder cancers were additionally analyzed for protein-coding gene mutations using a targeted NGS panel composed of 571 genes. Expression levels of three members of the APOBEC3 family genes were assessed using real-time quantitative RT-PCR. Non-coding somatic mutations were observed for at least one TGAACA core motif locus in 62.1% (64/103) of bladder tumor samples. These non-coding mutations co-occurred in the bladder tumors but were absent in prostate tumor, HPV-positive Head and Neck Squamous Cell Carcinoma, and high microsatellite instability (MSI-H) colorectal tumor series. This signature of palindromic non-coding somatic mutations, specific to bladder tumors, was not associated with patients' outcome and was more frequent in females. Interestingly, this signature was associated with high tumor mutational burden (TMB) and high expression levels of APOBEC3B and interferon inducible genes. We identified a new type of somatic genomic instability targeting the TGAACA core motif loci flanked by palindromic sequences in bladder cancer. This mutational signature is a promising candidate clinical biomarker for the early detection of relapse and a major low-cost alternative to the TMB to monitor the response to immunotherapy for bladder cancer patients.

A 'parameiosis' drives depolyploidization and homologous recombination in Candida albicans.

Meiosis is a conserved tenet of sexual reproduction in eukaryotes, yet this program is seemingly absent from many extant species. In the human fungal pathogen Candida albicans, mating of diploid cells generates tetraploid products that return to the diploid state via a non-meiotic process of depolyploidization known as concerted chromosome loss (CCL). Here, we report that recombination rates are more than three orders of magnitude higher during CCL than during normal mitotic growth. Furthermore, two conserved 'meiosis-specific' factors play central roles in CCL as SPO11 mediates DNA double-strand break formation while both SPO11 and REC8 regulate chromosome stability and promote inter-homolog recombination. Unexpectedly, SPO11 also promotes DNA repair and recombination during normal mitotic divisions. These results indicate that C. albicans CCL represents a 'parameiosis' that blurs the conventional boundaries between mitosis and meiosis. They also reveal parallels with depolyploidization in mammalian cells and provide potential insights into the evolution of meiosis.

Metabolism-induced oxidative stress and DNA damage selectively trigger genome instability in polyploid fungal cells.

Understanding how cellular activities impact genome stability is critical to multiple biological processes including tumorigenesis and reproductive biology. The fungal pathogen Candida albicans displays striking genome dynamics during its parasexual cycle as tetraploid cells, but not diploid cells, exhibit genome instability and reduce their ploidy when grown on a glucose-rich "pre-sporulation" medium. Here, we reveal that C. albicans tetraploid cells are metabolically hyperactive on this medium with higher rates of fermentation and oxidative respiration relative to diploid cells. This heightened metabolism results in elevated levels of reactive oxygen species (ROS), activation of the ROS-responsive transcription factor Cap1, and the formation of DNA double-strand breaks. Genetic or chemical suppression of ROS levels suppresses each of these phenotypes and also protects against genome instability. These studies reveal how endogenous metabolic processes can generate sufficient ROS to trigger genome instability in polyploid C. albicans cells. We also discuss potential parallels with metabolism-induced instability in cancer cells and speculate that ROS-induced DNA damage could have facilitated ploidy cycling prior to a conventional meiosis in eukaryotes.

Hemizygosity Enables a Mutational Transition Governing Fungal Virulence and Commensalism.

Candida albicans is a commensal fungus of human gastrointestinal and reproductive tracts, but also causes life-threatening systemic infections. The balance between colonization and pathogenesis is associated with phenotypic plasticity, with alternative cell states producing different outcomes in a mammalian host. Here, we reveal that gene dosage of a master transcription factor regulates cell differentiation in diploid C. albicans cells, as EFG1 hemizygous cells undergo a phenotypic transition inaccessible to "wild-type" cells with two functional EFG1 alleles. Notably, clinical isolates are often EFG1 hemizygous and thus licensed to undergo this transition. Phenotypic change corresponds to high-frequency loss of the functional EFG1 allele via de novo mutation or gene conversion events. This phenomenon also occurs during passaging in the gastrointestinal tract with the resulting cell type being hypercompetitive for commensal and systemic infections. A "two-hit" genetic model therefore underlies a key phenotypic transition in C. albicans that enables adaptation to host niches.

Epigenetic control of pheromone MAPK signaling determines sexual fecundity in Candida albicans.

Several pathogenic Candida species are capable of heritable and reversible switching between two epigenetic states, "white" and "opaque." In Candida albicans, white cells are essentially sterile, whereas opaque cells are mating-proficient. Here, we interrogate the mechanism by which the white-opaque switch regulates sexual fecundity and identify four genes in the pheromone MAPK pathway that are expressed at significantly higher levels in opaque cells than in white cells. These genes encode the ß subunit of the G-protein complex (STE4), the pheromone MAPK scaffold (CST5), and the two terminal MAP kinases (CEK1/CEK2). To define the contribution of each factor to mating, C. albicans white cells were reverse-engineered to express elevated, opaque-like levels of these factors, either singly or in combination. We show that white cells co-overexpressing STE4, CST5, and CEK2 undergo mating four orders of magnitude more efficiently than control white cells and at a frequency approaching that of opaque cells. Moreover, engineered white cells recapitulate the transcriptional and morphological responses of opaque cells to pheromone. These results therefore reveal multiple bottlenecks in pheromone MAPK signaling in white cells and that alleviation of these bottlenecks enables efficient mating by these "sterile" cell types. Taken together, our findings establish that differential expression of several MAPK factors underlies the epigenetic control of mating in C. albicans We also discuss how fitness advantages could have driven the evolution of a toggle switch to regulate sexual reproduction in pathogenic Candida species.