The Drosophila cancer-germline, head-to-head gene pairTrxTanddhdis dispensable for normal brain development but plays a major role inl(3)malignant brain tumourgrowth

Drosophila thioredoxins Deadhead (Dhd) and Thioredoxin-T (TrxT) are encoded by a head-to-head gene pair in the X chromosome. Akin to human cancer-germline (CG) genes,dhdandTrxTexpression is normally sex and germline specific, but becomes upregulated in brain tumours of either sex caused by mutation inl(3)malignant brain tumour(l(3)mbt).

Using CRISPR/Cas9-mediated knock-out alleles and RNA-seq, we show that while bothTrxTanddhdare dispensable for normal brain development, each of them has a significant but partial effect onl(3)mbtbrain tumour development as well as on tumour-linked transcriptomic signatures. Moreover, this effect is enhanced by the concomitant removal of both thioredoxins that strongly inhibitsl(3)mbtlarval brain tumour traits, reduces gene expression differences between tumour and wild-type tissue, and essentially erases all traces of gene expression differences between tumours from larvae of different sex. These results show thatTrxTanddhdplay a major synergistic role in the emergence ofl(3)mbttumour-linked transcriptomic signatures and tumour development, which is remarkable taking into account that these two genes are never expressed together under normal conditions. We have also found thatTrxTis crucial butdhdhas no effect on the growth of male-derivedl(3)mbtallografts, hence suggesting that the initial stages of tumour development and sustained, long-term tumour growth may depend upon different molecular pathways.

In humans, head-to-head inverted gene pairs are abundant among the CG genes that map to the X chromosome, some of which have been shown to work as a unit cancer. Our results identify the first instance of an X-linked, head-to-head CG gene pair in Drosophila and underpin the potential of such CG genes that are dispensable for normal development and homeostasis of somatic tissue, as targets to curtail malignant growth with minimal impact on overall health.