Y-chromosome clonal random expression produces mosaics in male mice

Monoallelic expression of X-chromosome inactivation (XCI) genes produces female mosaicism, but it is unknown whether chromosome Y (chrY) mosaicism exists in males. ChrY contains some ubiquitously expressed and evolutionary conserved dosage-sensitive regulator genes, including neighbouring Kdm5d, Ddx3y, Uty, and (in mice) Eif2s3y, that have on chrX counterparts that escape XCI. Here, we have produced a transgenic mice line containing an EGFP gene between Uty and Ddx3y genes. These males have mosaic (tortoiseshell) green fluorescence in the skin, similar to females carrying one copy (hemizygous) of an X-linked EGFP transgene owing to inactivation of one chrX, and show clonal inheritance of the inactive state through cell division. The skin tortoiseshell expression of the Y-GFP indicates that the choice of which chrY remains active is stably maintained and clonally inherited. We have produced 3 Sertoli lines from Y-GFP transgenic male, and we have separated by flow cytometry size homogeneous GFP-positive and GFP-negative cells. To analyse the transcriptomic profile and evaluate whether genes from chrY follow the same EGFP expression pattern and explore the regulatory mechanism of the chrY we performed RNA-seq. The results indicate that GFP-positive and GFP-negative cells have considerable differences in their expression levels at chrY, chrX and autosomal chromosomes. Concretely, 18 chrY genes show under expression in GFP-negative cells, including Ddx3y, Zfy1, Zfy2, Usp9y, Uba1y and 10 genes from the Rbmy family. Moreover, the differences in expression observed brings out the influence of chromosome Y expression on the expression of the rest of the autosomal genes.