Summary: biology of aggressive ACC

Conventional ACC is composed of luminal ductal cells and abluminal myoepithelial cells. MYB activation, through MYB-NFIB or MYBL1-NFIB fusion or other mechanisms, is the common oncogenic event, and MYB/MYBL1 fusion rates do not differ between ACC-I (65%) and ACC-II (64%). The aggressive phenotype is associated with loss of myoepithelial differentiation, solid growth, MYC upregulation (through NOTCH activation or MYC amplification), and suppression of p63 signaling. NOTCH1 alterations are enriched in recurrent or metastatic tumors (26.3% versus 8.5% in primary tumors). High-grade transformation shows chromosomal gains confined to the transformed areas, including C-MYC gains. ACC-I tumors are immune-excluded and overexpress the immune checkpoint B7-H4, whose expression level was an independent predictor of poor survival.

Tumor architecture: biphasic ductal and myoepithelial cells with perineural invasion

“Conventional ACC is a basaloid biphasic tumor with abluminal myoepithelial cells and luminal ductal cells arranged in tubular, cribriform and solid growth patterns, consistently demonstrating isomorphic, angulated, and hyperchromatic nuclei.” (opens the source at this quote in a new tab)
“Despite its monomorphic cytonuclear appearance, ACC stands at odds with other basaloid biphasic tumors in that it is highly infiltrative and neurotropic, requiring a wider resection and radiotherapy for disease control” (opens the source at this quote in a new tab)

“the loss of myoepithelial cells is associated with solid histology, aggressive clinical behavior, and poor prognosis” (opens the source at this quote in a new tab)

MYB activation: loss of microRNA regulation and downstream targets

“The minimal common part of MYB deleted as the result of fusion was exon 15 including the 3′-UTR, which contains several highly conserved target sites for miR-15a/16 and miR-150 microRNAs.” (opens the source at this quote in a new tab)
“We suggest that deletion of these target sites may disrupt repression of MYB leading to overexpression of MYB-NFIB transcripts and protein and to activation of critical MYB targets, including genes associated with apoptosis, cell cycle control, cell growth/angiogenesis, and cell adhesion.” (opens the source at this quote in a new tab)

NOTCH-MYC axis in ACC-I

“An association between MYC gene activation and the loss of myoepithelial cells can be empirically deduced from the high expression of the MYC gene in the solid form and the lack of expression in normal and neoplastic myoepithelial cells as observed in the ACC-I subgroup.” (opens the source at this quote in a new tab)
“At the genetic level, target DNA-sequencing revealed that ACC-I exhibited a higher tumor mutation burden (TMB) as compared to ACC-II (median mutations per megabase = 2.3 vs 0.7, p=0.004)” (opens the source at this quote in a new tab)

“NOTCH1 mutations occurred predominantly (14 of 15 patients) in the negative regulatory region and Pro-Glu-Ser-Thr–rich domains, the same two hotspots seen in T-cell acute lymphoblastic leukemias, and led to pathway activation in vitro.” (opens the source at this quote in a new tab)

“MYC expression was 1.61-fold higher (39.8 vs. 24.7; P < 0.0001) among NOTCH1-mutant ACC-I tumors, whereas MYB/L1 fusion rates were similar among ACC-I/II.” (opens the source at this quote in a new tab)

Progression to recurrent or metastatic disease: enrichment of NOTCH and chromatin-remodeling alterations

“Compared with primary tumors, R/M tumors were enriched for alterations in key Notch (NOTCH1, 26.3% vs. 8.5%; NOTCH2, 4.6% vs. 2.3%; NOTCH3, 5.7% vs. 2.3%; NOTCH4, 3.6% vs. 0.6%) and chromatin-remodeling (KDM6A, 15.2% vs. 3.4%; KMT2C/MLL3, 14.3% vs. 4.0%; ARID1B, 14.1% vs. 4.0%) genes.” (opens the source at this quote in a new tab)

High-grade transformation: progressive chromosomal alterations

“Regions that showed the significant losses included 9q34, 4p16, 1p36.1, and 11q22.” (opens the source at this quote in a new tab)
“High-grade transformation in adenoid cystic carcinoma is a complex process that is reflected by several chromosomal alterations.” (opens the source at this quote in a new tab)

“Moreover, HGT regions typically showed reduced or absent p63 expression, along with high-grade pathomorphology.” (opens the source at this quote in a new tab)

Tumor microenvironment: immune exclusion and B7-H4 overexpression in ACC-I

“ACC-I tumors with a poor prognosis harbored a higher density of immune cells; however, spatial analysis by IMC revealed that ACC-I immune cells were significantly restricted to the stroma, characterizing an immune-excluded TME. ACC-I tumors overexpressed the immune checkpoint B7-H4, and the degree of immune exclusion was directly correlated with B7-H4 expression levels, an independent predictor of poor survival.” (opens the source at this quote in a new tab)

Genomic landscape in 438 profiled tumors: fusion, mutation, and copy-number frequencies

“MYB/L1 fusions were the most common genomic alteration observed (56.9%) across the cohort, followed by NOTCH1 (17.0%)” (opens the source at this quote in a new tab)
“High TMB (≥10 mutations/Mb) was infrequent, observed in only 6 (1.5%) of all ACC tumors sampled, and none were mismatch repair deficient/microsatellite instability-high (dMMR/MSI-H).” (opens the source at this quote in a new tab)
“Overall, copy-number amplification (CN amp) events were infrequent but were most commonly observed for genes located on the 4q12 amplicon, including KIT (3.2%), PDGFRA (3.2%), CHIC2 (3.0%), and KDR (2.5%), which were often co-amplified.” (opens the source at this quote in a new tab)

Immune microenvironment by subtype: B7-H4 expression and immune cell fractions

“The median cell fraction (%) of B cells in the TME was higher among ACC-II (7.1% vs. 5.8%; P < 0.01), along with greater proportions of M2 macrophages (4.8% vs. 3%, P < 0.05) and neutrophils (2.8% vs. 1.3%, P < 0.05; Fig. 2B).” (opens the source at this quote in a new tab)
“Expression of B7-H4, an important negative regulator of T-cell immunity, was significantly higher in ACC-I (P < 0.001).” (opens the source at this quote in a new tab)
“PD-L1 positivity was observed in 7 (1.9%) of all samples” (opens the source at this quote in a new tab)

Molecular changes underlying high-grade transformation: p53 and copy-number alterations

“The pathogenesis of HGT in AdCC is not well understood, although p53 alterations may play a significant role in the evolution of this phenomenon.” (opens the source at this quote in a new tab)
“Using high-resolution microarray comparative genomic hybridization, Costa et al” (opens the source at this quote in a new tab)
“studied 6 cases of AdCC-HGT as well as 5 cases of solid AdCC. Interestingly, the authors found that AdCC-HGT carried fewer copy number changes compared with solid AdCC.” (opens the source at this quote in a new tab)

Markers associated with solid histology: NGF, EphA2, ILK, and EN1

“Increased expression of NGF is observed in 79% of solid type AdCCs (or 68% of all studied AdCC) and may relate to its neurotropism” (opens the source at this quote in a new tab)
“The overexpression is significantly greater in solid AdCC than in tubular and cribriform types and correlates with microvessel density, TNM staging and perineural and vascular invasion” (opens the source at this quote in a new tab)
“Expression of ILK correlates strongly with solid type AdCC, advanced TNM stage and increased risk of recurrence.” (opens the source at this quote in a new tab)
“EN1 protein expression seems typical for solid AdCC and implies a significantly lower survival rate.” (opens the source at this quote in a new tab)

Emiltatug ledadotin (Emi-Le)

Pathophysiology