Kdm4dl is expressed in adult mouse testis and encodes a functional KDM4D-like protein

The lysine demethylase 4 (KDM4) family comprises Jumonji C (JmjC) domain-containing histone demethylases that regulate chromatin structure by removing methyl groups from histone H3 lysine residues. Among them, KDM4D preferentially demethylates H3K9me3 and plays important roles in spermatogenesis. However, Kdm4d-deficient mice display only partial reproductive defects, suggesting the presence of additional demethylases with similar activity in the testis. Here, we investigated the Kdm4dl locus, which has been annotated as a pseudogene but shows high sequence similarity to the catalytic JmjC domain of Kdm4d. Reanalysis of publicly available RNA sequencing data together with RT-PCR demonstrated that transcripts from the Kdm4dl locus are expressed in the adult mouse testis. Using 5' and 3' rapid amplification of cDNA ends (RACE), we identified a full-length Kdm4dl transcript encoding a 463-amino acid protein containing conserved JmjN and JmjC domains characteristic of KDM4 family demethylases. Functional analyses showed that ectopically expressed KDM4DL localizes predominantly to the nucleus and significantly reduces cellular H3K9me3 levels in cultured cells, with an effect comparable to KDM4D. Together, these findings demonstrate that the Kdm4dl locus encodes a nuclear protein with H3K9me3 demethylase activity and indicate that KDM4DL represents a previously unrecognized chromatin regulator expressed in the mouse testis.