Histone methyltransferase NSD modulates gene silencing mechanisms on Drosophila chromosome 4

The nuclear receptor-binding SET domain protein (NSD) gene family encodes histone methyltransferases that mono- and di-methylate lysine 36 on histone H3 (H3K36). Here, we examine the effects of NSD loss-of-function on transcription and heterochromatin formation in Drosophila to elucidate the role of NSD in chromatin structure regulation. Transcriptome analysis showed that NSD deletion activated more genes on chromosome 4, predominantly heterochromatic, than on other chromosomes. We further analyzed the position-effect variegation of fly eyes due to mini-white (mw⁺) transgenes inserted at various chromosomal loci and found that NSD deletion promoted mw⁺ transgene expression on chromosome 4. Additionally, NSD deletion reduced the binding of heterochromatin markers HP1a and H3K9 to chromosome 4. These findings suggest that NSD deletion disrupts chromosome 4 heterochromatin structure by reducing HP1a binding, implying NSD's role as an epigenetic regulator of chromosome 4 silencing.