Effective cancer treatment has been precluded by the presence of various forms of intratumoral complexity that drive treatment resistance and metastasis. Recent single-cell sequencing technologies are significantly facilitating the characterization of tumor internal architecture during disease progression. New applications and advances occurring at a fast pace predict an imminent broad application of these technologies in many research areas. As occurred with next-generation sequencing (NGS) technologies, once applied to clinical samples across tumor types, single-cell sequencing technologies could trigger an exponential increase in knowledge of the molecular pathways involved in cancer progression and contribute to the improvement of cancer treatment. Multifaceted Heterogeneity and Its Impact on Cancer ProgressionTumors comprise various cell populations in constant evolution. Some of this complex heterogeneity derives from genetic diversification and Darwinian selection of tumor cells as they adapt to variable environments. Next-generation sequencing (NGS; see Glossary) used for the past decade had enough sensitivity to detect mutations present in minor cell populations and, combined with multisampling of human tumors (multisampling sequencing), fostered many studies that characterized intratumor heterogeneity in various cancers [1]. The level of intratumor heterogeneity is considered a main driver of therapy resistance and metastasis and is associated with poor prognosis [2].In addition, human cancers frequently have tumor cell populations with different transcriptional programs. This functional diversity is likely associated with the genetic heterogeneity described above but is also the result of many other factors. First, the presence of a hierarchical structure, where a group of quiescent stem-like cells fosters the growth of a tumor comprising cells in different differentiation states, was demonstrated in various tumor types [3]. Additionally, different transcriptional programs can be activated in tumor cells as a response to stochastic factors or to a variable tumor microenvironment. This functional diversity provides tumors with a plasticity that grants a high capacity for adaptation [4].Finally, human tumors comprise not only malignant/transformed cells but also a plethora of different cell types recruited from the surrounding tissue and the immune system. The tumor microenvironment shows also genetic and transcriptional diversity and plays important roles in tumor progression, metastasis, and treatment resistance [1,5].Fine characterization of these levels of tumor heterogeneity is essential to the successful treatment of cancer patients. The recent development of technologies based on sequencing individual cells (single-cell sequencing technologies) opens new ways for the characterization of tumor heterogeneity. At the genetic level, single-cell DNA-seq technologies offer higher sensitivity in the detection of minority clones, the reconstruction of clone structure, and the identification of concurrent ...
In humans and most mammals, differentiation of the embryonic gonad into ovaries or testes is controlled by the Y-linked gene SRY. Here we show a role for the Gadd45g protein in this primary sex differentiation. We characterized mice deficient in Gadd45a, Gadd45b and Gadd45g, as well as double-knockout mice for Gadd45ab, Gadd45ag and Gadd45bg, and found a specific role for Gadd45g in male fertility and testis development. Gadd45g-deficient XY mice on a mixed 129/C57BL/6 background showed varying degrees of disorders of sexual development (DSD), ranging from male infertility to an intersex phenotype or complete gonadal dysgenesis (CGD). On a pure C57BL/6 (B6) background, all Gadd45g−/− XY mice were born as completely sex-reversed XY-females, whereas lack of Gadd45a and/or Gadd45b did not affect primary sex determination or testis development. Gadd45g expression was similar in female and male embryonic gonads, and peaked around the time of sex differentiation at 11.5 days post-coitum (dpc). The molecular cause of the sex reversal was the failure of Gadd45g−/− XY gonads to achieve the SRY expression threshold necessary for testes differentiation, resulting in ovary and Müllerian duct development. These results identify Gadd45g as a candidate gene for male infertility and 46,XY sex reversal in humans.
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