MAX mutant small-cell lung cancers exhibit impaired activities of MGA-dependent noncanonical polycomb repressive complex

Abstract

The MYC axis is disrupted in cancer, predominantly through activation of the MYC family oncogenes but also through inactivation of the MYC partner MAX or of the MAX partner MGA. MGA and MAX are also members of the polycomb repressive complex, ncPRC1.6. Here, we use genetically modified MAX-deficient small-cell lung cancer (SCLC) cells and carry out genome-wide and proteomics analyses to study the tumor suppressor function of MAX. We find that MAX mutant SCLCs have ASCL1 or NEUROD1 or combined ASCL1/NEUROD1 characteristics and lack MYC transcriptional activity. MAX restitution triggers prodifferentiation expression profiles that shift when MAX and oncogenic MYC are coexpressed. Although ncPRC1.6 can be formed, the lack of MAX restricts global MGA occupancy, selectively driving its recruitment toward E2F6-binding motifs. Conversely, MAX restitution enhances MGA occupancy to repress genes involved in different functions, including stem cell and DNA repair/replication. Collectively, these findings reveal that MAX mutant SCLCs have either ASCL1 or NEUROD1 or combined characteristics and are MYC independent and exhibit deficient ncPRC1.6-mediated gene repression.

Polycomb group ring finger protein 6 suppresses Myc-induced lymphomagenesis

“…Formal evidence for this hypothesis was provided in two SCLC mouse models, in which deletion of Max could either abrogate tumorigenesis if combined with a MYCL transgene, or favor it afterloss of the Rb1 and Trp53 tumors suppressors (Augert et al, 2020). Hence, in neuroendocrine tumors loss of Mga/Max/ PRC1.6 repressor function may be sufficient to bypass the requirement for Myc activity, as recently shown in Max-null human SCLC cell lines (Llabata et al, 2021). In other lineages, the essential role of Max as a Myc partner (Amati et al, 1993) may prevent its loss, but may still co-exist with its antagonist activities in complex with either Mga or Mxd/Mnt proteins.…”

“…First, genome sequencing studies revealed loss-of-function mutations in Mga in a wide variety of tumors (Schaub et al, 2018). Loss of Max was also observed, but appears to be restricted to neuroendocrine tumors, including pheochromocytoma (Comino-Mendez et al, 2011;Burnichon et al, 2012) and small-cell lung cancer (SCLC) (Romero et al, 2014;Llabata et al, 2021). In SCLC, mutations affecting the different network members (Max loss, Mga loss, Myc amplification) occur in a mutually exclusive manner, pointing to a common functional consequence (Romero et al, 2014).…”

Killing SCLC: insights into how to target a shapeshifting tumor

“…MAX was found to be a tumor suppressor whose loss leads to derepression of serine and one-carbon pathway genes ( Augert et al 2020 ). Recent studies suggest that MAX-deficient SCLC is predominantly of the ASCL1 subtype and does not depend on any MYC family member ( Llabata et al 2021 ). These studies beg the question of whether a non-MYC family member state of SCLC is phenotypically and/or metabolically distinct from other molecular subsets.…”

How the West(ern) Was Won: Solutions for Immunoblotting Large and Small Proteins

“…MGA is a 350 KDa estimated weight protein composed by 3,065 amino acids [5, 6]. The detection of this protein is arduous, since it cannot be detected following the SWP even after its ectopic overexpression in a cancer cell line.…”

“…Here we describe a set of modifications from the Standard Work Procedure (SWP) of our laboratories to perform Western blots to detect either small or large proteins. These changes were used in different studies involving Small VCP Interacting Protein (SVIP) [4] and Max-Gene Associated Protein (MGA) [5, 6], and they led to overcome the technical issues related with the above-mentioned targets (Fig. 1B).…”