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STUDY QUESTION: Do testis-specific cells have a normal karyotype in non-mosaic postpubertal Klinefelter syndrome (KS) patients with focal spermatogenesis and in non-mosaic prepubertal KS boys? SUMMARY ANSWER: Spermatogonia have a 46, XY karyotype, and Sertoli cells surrounding these spermatogonia in postpubertal patients also have a 46, XY karyotype, whereas, in prepubertal KS boys, Sertoli cells surrounding the spermatogonia still have a 47, XXY karyotype. WHAT IS KNOWN ALREADY: A significant proportion of patients with non-mosaic KS can have children by using assisted reproductive techniques thanks to focal spermatogenesis. However, the karyotype of the cells that are able to support focal spermatogenesis has not been revealed. STUDY DESIGN, SIZE, DURATION: Testicular biopsy samples from non-mosaic KS patients were included in the study. Karyotyping for sex chromosomes in testis-specific cells was performed by immunohistochemical analysis of inactive X (Xi) chromosome and/or fluorescent in situ hybridization (FISH) analysis of chromosomes 18, X, and Y. PARTICIPANTS/MATERIALS, SETTING, METHODS: A total of 22 KS patients (17 postpubertal and 5 prepubertal) who were non-mosaic according to lymphocyte karyotype analysis, were included in the study. After tissue processing, paraffin embedding, and sectioning, the following primary antibodies were used for cell-specific analysis and Xi detection; one section was stained with MAGE A4 for spermatogonia, SOX9 for Sertoli cells, and H3K27me3 for Xi; the other one was stained with CYP17A1 for Leydig cells, ACTA2 for peritubular myoid cells, and H3K27me3 for Xi. Xi negative (Xi-) somatic cells (i.e. Sertoli cells, Leydig cells, and peritubular myoid cells) were evaluated as having the 46, XY karyotype; Xi positive (Xi+) somatic cells were evaluated as having the 47, XXY. FISH stain for chromosomes 18, X, and Y was performed on the same sections to investigate the karyotype of spermatogonia and to validate the immunohistochemistry results for somatic cells. MAIN RESULTS AND THE ROLE OF CHANCE: According to our data, all spermatogonia in both postpubertal and prepubertal non-mosaic KS patients seem to have 46, XY karyotype. However, while the Sertoli cells surrounding spermatogonia in postpubertal samples also had a 46, XY karyotype, the Sertoli cells surrounding spermatogonia in prepubertal samples had a 47, XXY karyotype. In addition, while the Sertoli cells in some of the Sertoli cell-only tubules had 46, XY karyotype, the Sertoli cells in some of the other Sertoli cell-only tubules had 47, XXY karyotype in postpubertal samples. In contrast to the postpubertal samples, Sertoli cells in all tubules in the prepubertal samples had the 47, XXY karyotype. Our data also suggest that germ cells lose the extra X chromosome during embryonic, fetal, or neonatal life, while Sertoli cells lose it around puberty. Peritubular myoid cells and Leydig cells may also be mosaic in both postpubertal patients and prepubertal boys, but it requires further investigation. LIMITATIONS, REASONS FOR CAUTION: The number of prepubertal testicle samples containing spermatogonia is limited, so more samples are needed for a definitive conclusion. The fact that not all the cell nuclei coincide with the section plane limits the accurate detection of X chromosomes by immunohistochemistry and FISH in some cells. To overcome this limitation, X chromosome analysis could be performed by different techniques on intact cells isolated from fresh tissue. Additionally, there is no evidence that X chromosome inactivation reoccurs after activation of the Xi during germ cell migration during embryogenesis, limiting the prediction of X chromosome content in germ cells by H3K27me3. WIDER IMPLICATIONS OF THE FINDINGS: Our findings will lay the groundwork for new clinically important studies on exactly when and by which mechanism an extra X chromosome is lost in spermatogonia and Sertoli cells. STUDY FUNDING/COMPETING INTEREST(S): This study was funded by The Scientific and Technological Research Council of Türkiye (TUBITAK) (2219 - International Postdoctoral Research Fellowship Program for Turkish Citizens) and the Strategic Research Program (SRP89) from the Vrije Universiteit Brussel. The authors declare no competing interests. TRIAL REGISTRATION NUMBER: N/A.

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While Mek1/2 and Gsk3ß inhibition ("2i") supports the maintenance of murine embryonic stem cells (ESCs) in a homogenous naïve state, prolonged culture in 2i results in aneuploidy and DNA hypomethylation that impairs developmental potential. Additionally, 2i fails to support derivation and culture of fully potent female ESCs. Here we find that mouse ESCs cultured in 2i/LIF supplemented with lipid-rich albumin (AlbuMAX) undergo pluripotency transition yet maintain genomic stability and full potency over long-term culture. Mechanistically, lipids in AlbuMAX impact intracellular metabolism including nucleotide biosynthesis, lipid biogenesis, and TCA cycle intermediates, with enhanced expression of DNMT3s that prevent DNA hypomethylation. Lipids induce a formative-like pluripotent state through direct stimulation of Erk2 phosphorylation, which also alleviates X chromosome loss in female ESCs. Importantly, both male and female "all-ESC" mice can be generated from de novo derived ESCs using AlbuMAX-based media. Our findings underscore the importance of lipids to pluripotency and link nutrient cues to genome integrity in early development.

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RESUMEN

While Mek1/2 and Gsk3β inhibition ("2i") supports the maintenance of murine embryonic stem cells (ESCs) in a homogenous naïve state, prolonged culture in 2i results in aneuploidy and DNA hypomethylation that impairs developmental potential. Additionally, 2i fails to support derivation and culture of fully potent female ESCs. Here we find that mouse ESCs cultured in 2i/LIF supplemented with lipid-rich albumin (AlbuMAX) undergo pluripotency transition yet maintain genomic stability and full potency over long-term culture. Mechanistically, lipids in AlbuMAX impact intracellular metabolism including nucleotide biosynthesis, lipid biogenesis, and TCA cycle intermediates, with enhanced expression of DNMT3s that prevent DNA hypomethylation. Lipids induce a formative-like pluripotent state through direct stimulation of Erk2 phosphorylation, which also alleviates X chromosome loss in female ESCs. Importantly, both male and female "all-ESC" mice can be generated from de novo derived ESCs using AlbuMAX-based media. Our findings underscore the importance of lipids to pluripotency and link nutrient cues to genome integrity in early development.

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