CSL controls telomere maintenance and genome stability in human dermal fibroblasts

Abstract

Genomic instability is a hallmark of cancer. Whether it also occurs in Cancer Associated Fibroblasts (CAFs) remains to be carefully investigated. Loss of CSL/RBP-Jκ, the effector of canonical NOTCH signaling with intrinsic transcription repressive function, causes conversion of dermal fibroblasts into CAFs. Here, we find that CSL down-modulation triggers DNA damage, telomere loss and chromosome end fusions that also occur in skin Squamous Cell Carcinoma (SCC)-associated CAFs, in which CSL is decreased. Separately from its role in transcription, we show that CSL is part of a multiprotein telomere protective complex, binding directly and with high affinity to telomeric DNA as well as to UPF1 and Ku70/Ku80 proteins and being required for their telomere association. Taken together, the findings point to a central role of CSL in telomere homeostasis with important implications for genomic instability of cancer stromal cells and beyond.

NOTCH1 gene amplification promotes expansion of Cancer Associated Fibroblast populations in human skin

“…In agreement with previous findings 3,19 , silencing of the CSL but not NOTCH1 gene in f-HDFs resulted in up-regulation of a number of CAF effector genes with a key tumor-promoting function ( Fig. 3a and Supplementary Fig.…”

“…As expected from previous findings 19 , knockdown of CSL in foreskin-derived HDFs resulted in DNA damage and growth suppression, which were not elicited by NOTCH1 silencing (Fig. 3d-f).…”

“…In contrast to f-HDFs and m-HDFs, NOTCH1 silencing suppressed the proliferation of CAFs (Fig. 3g, left columns), without affecting the elevated levels of DNA damage resulting from loss of CSL in these cells 19 (Supplementary Fig. 2f, g), but inducing downstream events of the DNA Damage Response (DDR) shown further below.…”

“…CSL functions as a constitutive negative repressor of a large battery of CAF effector genes, which are all induced by decreased CSL expression as it occurs at early steps of CAF activation 3,17,23,24 . Separately from its role in transcription, CSL is essential for maintenance of genomic stability as part of a telomere protective complex that is lost in CAFs 19 . CAF effector genes under negative CSL control can be induced by increased levels of activated NOTCH1, which, by binding to CSL, converts it from a repressor into an activator of transcription 3 .…”

“…We have recently unveiled a role of CSL in human dermal fibroblasts (HDFs) and CAFs, separate from gene transcription, as an essential component of a telomere binding/ protective complex. CSL loss or down-modulation in HDFs and CAFs results in DNA damage and genomic instability 19 . Here we show that in CAFs the NOTCH1 gene is frequently amplified and overexpressed, preventing the DNA damage response (DDR) through ATM association and suppression of downstream signaling.…”