LINE1 insertion into the NACC2 locus disrupts the HDM2 axis and activates lung oncogenic signaling

Although long interspersed element-1 (LINE-1) retrotransposons are poor prognostic indicators of non-small cell lung cancer, the genetic consequences of aberrant LINE-1 expression remain poorly understood. In this study, human bronchial epithelial cells were exposed chronically to 25 µM NiCl₂ to induce malignant transformation and compared to PBS-treated control cells or lung cancer cell lines. A 4 kb intronic LINE-1 insertion into the NACC2 (nucleus accumbens-associated protein 2) locus in nickel-transformed cells was associated with significant reductions in NACC2 mRNA and protein, along with constitutive increases in HDM2 and TP53α mRNAs, and aberrant expression of TP53β and TP53γ mRNAs. Steady-state levels of p53 and RB decreased, while EGFR and LINE-1 ORF1p increased. Silencing of NACC2 in TP53 wildtype NCI-H460 or TP53-null NCI-1299 cell lines replicated many of these changes, with profiles varying as a function of p53 status. Stable overexpression of HDM2 increased LINE-1 ORF1p in cancer cell lines. Human lung adenocarcinomas with wild-type TP53 showed sexually dimorphic profiles, with higher HDM2 levels in females than males, and shifts in LINE-1 ORF1p, p53, and RB that mirrored cancer cell lines. This study identifies the NACC2 locus as a target of LINE-1 retrotransposition and highlights critical interactions with the HDM2/TP53/RB regulatory axis.