BRG1-deficient NSCLCs have been more intriguing recently for its highly aggressive clinical behavior and no effective therapies. We summarized the clinical, pathomorphological and immunohistochemical characteristics of BRG1-deficient NSCLCs in detail. Patients with BRG1-deficient NSCLCs tend to be male with tobacco exposure, as a previous study reported (Decroix et al. 2020). About 27.8% of patients suffered recurrence in the first year after lung surgery. For twenty-nine patients who were in the advanced stage, 82.8%(24/29) patients died before the end of this study, with the median OS of only 7.0 months.
SMARCA4 mutation usually coexists with mutation of KRAS, LKB1, KEAP1, P16 and P53 genes, which suggests that BRG1-deficient NSCLC may be insensitive to traditional treatments such as chemotherapy, radiotherapy and targeted inhibitors (Dagogo-Jack et al.2020). The ICIs-based immunotherapy has achieved remarkable success in advanced NSCLCs. Existing evidences revealed that when compared to wild-type subgroups, the inactiviation of SMARCA4/BRG1 may predict poorer clinical response to ICIs (Zhou et al.2020). However, little is known about the ICIs efficacy of the BRG1-deficient NSCLC when compared to the traditional therapeutic settings. In our cohort, for advanced BRG1-deficient NSCLCs, ICIs ever treated patients had better OS than those who received non-ICIs treatment settings including chemotherapy, anti-angiogenesis therapy, TKI, radiotherapy, Gamma Knife and palliative therapy (median OS, 27.0m versus 6.0m, p=0.02). Moreover, patients received ICIs have an improved median PFS of 17.0 months, while median PFS on platinum doublet chemotherapy were only 6.0 months (p=0.04). Notably, three patients benefited from ICIs therapy as second or third-line treatment even after failing from traditional settings. These data suggest that patients with BRG1-deficient NSCLC may be more sensitive to immunotherapy than traditional settings such as chemotherapy. However, as a retrospective study, the number, age, stage and the treatment lines among the groups have unbalanced differences, without setting the control or matching the patients, which may affect the conclusion. Therefore, randomized controlled trials are necessity to examine efficacy difference between ICIs-based immunotherapy and traditional therapies for BRG1-deficient NSCLCs.
The PD-L1 expression level and tumor mutation burden(TMB) were important factors for predicting ICIs response (Carbone et al. 2017; Rizvi et al. 2015). Recently, a case report indicated that BRG1-deficient NSCLC had higher level of PD-L1 and TMB, and therefore could generate sustained response to nivolumab, the inhibitor of PD-L1(Naito et al. 2019). However, two cases in our cohort showed sustainable response to ICIs even with low PD-L1 expression level. Furthermore, another study indicated that NSCLC with SMARCA4 mutation harbored high TMB but infrequently(15%) had high PD-L1(Dagogo-Jack et al.2020)(13). Therefore, despite the expression level of PD-L1, other effective predictors such as TMB may also account for the response of BRG1-deficient NSCLCs to immunotherapy.
In our study, BRG1-deficient NSCLCs showed varied histological types, including adenocarcinoma, squamous-cell carcinoma, adenosquamous carcinoma and pleomorphic carcinoma, as early studies had reported (Agaimy et al.2017; Naito et al.2019; Nambirajan et al. 2021; Herpel et al. 2017). Consistent with previous results, no lepidic pattern was found in our study (Matsubara et al.2013). Notably, a smaller area of micro-papillary pattern was observed in one solid predominant adenocarcinoma, which has not been reported previously. Significant morphological diversity makes it difficult to predict BRG1 deficiency only by histomorphometric analysis. Accordingly, we consider it necessary to detect BRG1 staining in all NSCLCs. Tumor cells of BRG1-deficient NSCLCs showed overt cytological atypia even in papillary or acinus predominant adenocarcinoma. More or less cytoplasmic hyaline droplets (their nature remains unknown) can be seen in most cases. Multifocal and irregular tumor necrosis accompanied by inflammatory infiltration were the prominent features. Frequent necrosis, overt cytological atypia, high mitotic rate and high proliferation index all predicted the aggressive behavior of BRG1-deficient NSCLCs. The inflammatory cells were seen in both interstitium and tumor nests, suggesting inflammation may play an important role in the establishment of tumor microenvironment.
Partial loss and significant reduction of BRG1 in NSCLCs have only been occasionally described in literatures (Dagogo-Jack et al.2020; Nambirajan et al. 2021; Herpel et al. 2017). Partial loss of BRG1 were frequently seen in our study possibly due to the whole-section immunohistochemistry staining rather than tissue microarray. There was a relatively clear boundary between the BRG1-deficient and BRG1-intact areas in the 6 cases with partial loss. Among them, 5 showed no significant morphological difference between the two areas while one showed loss of BRG1 in solid growth pattern but intact BRG1 in acinus pattern. One study presumed that loss of BRG1 occurs at the early phase of carcinogenesis of lung adenocarcinoma (Matsubara et al.2013). However, findings in our cases with partial loss of BRG1 suggested that this presumption needs to be reconsidered. BRG1-deficient lung adenocarcinomas were typically negative for TTF-1 and positive for CK7 and HepPar-1(Hirsch et al.2017; Nambirajan et al.2020). In our study, TTF-1 and NapsinA was negative in 64.3% and 75.6% BRG1-deficient adenocarcinomas respectively. Interestingly, in BRG1-partially lost cases, 4 were positive for TTF-1 and 3 were positive for NapsinA only in the area with BRG1 expression intact. CK7 was positive in 92.9% of adenocarcinomas and 93.6% of all NSCLCs, which was consistent with previous studies. HepPar-1 was reported to be positive in 85% BRG1-deficient lung adenocarcinomas by Agaimy et al(2017).However, in our study, HepPar-1 was only positive in 48.7% of adenocarcinomas and 46.5% of all NSCLCs. HepPar-1 staining showed strong granular cytoplasmic(mitochondrial) pattern as seen in normal liver and hepatoid and hepatocellular carcinomas. Therefore, when HepPar-1 is positive in biopsy sample, it is necessary to search for BRG1-deficient lung cancer, hepatocellular carcinoma and hepatoid carcinoma. As a further test, we examined the expression of other three subunits of the SWI/SNF complex in BRG1-deficient NSCLCs. Four cases (11.8%) had loss or reduction of BRM and no case showed ARID1A or ARID1B loss. Interestingly, in one case with partial loss of BRG1 (No.41), the BRM expression decreased exactly in BRG1 loss area, which further suggested the underlying relationship between BRG1 and BRM. In a study, ARID1A, ARID1B, BRG1 and BRM were lost in 1.3%, 0.3% 2.4% and 2.4% of NSCLCs respectively, suggesting that loss of ARID1A or ARID1B was not common, especially for ARID1B (Naito et al.2019).
In our cohort, there was minimal overlap between BRG1 deficiency and actionable oncogenic driver alterations. KRAS mutations rather than EGFR mutations occurred frequently in BRG1-deficient NSCLC. As we all known that, KRAS gene mutations are the most prevalent molecular alteration detected in human lung adenocarcinoma, accounting for up to 25% of all genetic mutations. The prior efforts to develop specific targeted inhibitors for the KRAS-mutant patients has been unsuccessful (Ferrer et al.2018). A recent study had revealed that SMARCA4 mutation was a risk factor for KRAS-mutant patients with lung adenocarcinoma (Liu et al.2021). On the other hand, IHC for BRG1 can capture SMARCA4-deficient tumor which is associated with SMARCA4 mutations (Dagogo-Jack et al.2020). Therefore, evaluation of BRG1 expression by IHC may further of KRAS-mutant patients with lung adenocarcinoma.