SMARCA4: Implications of an Altered Chromatin-Remodeling Gene for Cancer Development and Therapy.

The SWI/SNF chromatin remodeling complex, via nucleosome topology modulation, regulates transcription. The SMARCA4 (BRG1) subunit codes for the ATPase energy engine of the SWI/SNF complex. SMARCA4 is a tumor suppressor that is aberrant in ∼5% to 7% of human malignancies. Class I SMARCA4 alterations (truncating mutations, fusions, and homozygous deletion) lead to loss of function whereas class II alterations (missense mutations) have a dominant negative/gain-of-function effect and/or loss-of function. SMARCA4 alterations typify the ultra-rare small cell carcinomas of the ovary hypercalcemic type (SCCOHT) and SMARCA4-deficient thoracic and uterine sarcomas; they are also found in a subset of more common tumors, for example, lung, colon, bladder, and breast carcinomas. Germline variants in the SMARCA4 gene lead to various hereditary conditions: rhabdoid tumor predisposition syndrome-2 (RTPS2), characterized by loss-of-function alterations and aggressive rhabdoid tumors presenting in infants and young children; and Coffin-Siris syndrome, characterized by dominant negative/gain-of function alterations and developmental delays, microcephaly, unique facies, and hypoplastic nails of the fifth fingers or toes. A minority of rhabdoid tumors have a germline SMARCA4 variant as do >40% of women with SCCOHT. Importantly, immune checkpoint blockade has shown remarkable, albeit anecdotal, responses in SCCOHT. In addition, there is ongoing research into BET, EZH2, HDAC, CDK4/6, and FGFR inhibitors, as well as agents that might induce synthetic lethality via DNA damage repair impairment (ATR inhibitors and platinum chemotherapy), or via the exploitation of mitochondrial oxidative phosphorylation inhibitors or AURKA inhibitors, in SMARCA4-aberrant cancers.

Temporal and spatial effects and survival outcomes associated with concordance between tissue and blood KRAS alterations in the pan-cancer setting.

We investigated the impact of time interval, primary vs. metastatic biopsy site, variant allele fraction (VAF) and histology on concordance of KRAS alterations in tissue vs. circulating tumor DNA (ctDNA), and association of concordance with survival. Blood and tissue were evaluated by next-generation sequencing in 433 patients with diverse cancers. Altogether, 101 patients (23.3%) had KRAS alterations: 56, ctDNA (12.9%); 81, tissue (18.7%); and 36, both (8.3%). The overall blood and tissue concordance rate for KRAS alterations was 85%, but was mainly driven by the large negative/negative subset. Therefore, specificity of one test for the other was high (88.1-94.3%), while sensitivity was not high (44.4-64.3%) and was lower still in patients with >6 vs. ≤2 months between blood and tissue sampling (31.0-40.9% vs. 51.2-84.0%; p = 0.14 time interval-dependent sensitivity of blood for tissue; p = 0.003, tissue for blood). Positive concordance rate for KRAS alterations was 57.1% vs. 27.4% (colorectal vs. noncolorectal cancer; p = 0.01), but site of biopsy (primary vs. metastatic) and VAF (%ctDNA) was not impactful. The presence of KRAS alterations in both tests was independently associated with shorter survival from diagnosis (hazard ratio, 1.72; 95% confidence interval, 1.04-2.86) and from recurrent/metastatic disease (1.70; 1.03-2.81). Positive concordance of KRAS alterations between ctDNA and tissue was negatively affected by a longer time period between blood and tissue sampling and was higher in colorectal cancer than in other malignancies. The presence of KRAS alterations in both tests was an independent prognostic factor for poor survival.