Inhibition of nuclear export restores nuclear localization and residual tumor suppressor function of truncated SMARCB1/INI1 protein in a molecular subset of atypical teratoid/rhabdoid tumors
Creators: name: Pathak, Rajiv
name: Zin, Francesca
name: Thomas, Christian
name: Bens, Susanne
name: Gayden, Tenzin
name: Karamchandani, Jason
name: Dudley, Roy W.
name: Nemes, Karolina
name: Johann, Pascal D.
name: Oyen, Florian
name: Kordes, Uwe
name: Jabado, Nada
name: Siebert, Reiner
name: Paulus, Werner
name: Kool, Marcel
name: Frühwald, Michael C.
name: Albrecht, Steffen
name: Kalpana, Ganjam V.
name: Hasselblatt, Martin
Publisher: name: Universität Ulm
Year: 2021
Subjects: subject: Atypical teratoid
subject: rhabdoid tumor
subject: Malignant rhabdoid tumor
subject: INI1
subject: SMARCB1
subject: BAF47
subject: Cytoplasmic
subject: Nuclear export signal
subject: Selinexor
subject: SELINEXOR KPT-330
subject: ANTITUMOR EFFICACY
subject: IMMUNOHISTOCHEMICAL ANALYSIS
subject: MUTATIONS
subject: TRANSLATION
subject: EXPRESSION
subject: HSNF5/INI1
subject: SUBGROUP
subject: FEATURES
subject: DDC 610 / Medicine &amp; health

description: Abstract Loss of nuclear SMARCB1 (INI1/hSNF5/BAF47) protein expression due to biallelic mutations of the SMARCB1 tumor suppressor gene is a hallmark of atypical teratoid/rhabdoid tumors (ATRT), but the presence of cytoplasmic SMARCB1 protein in these tumors has not yet been described. In a series of 102 primary ATRT, distinct cytoplasmic SMARCB1 staining on immunohistochemistry was encountered in 19 cases (19%) and was highly over-represented in cases showing pathogenic sequence variants leading to truncation or mutation of the C-terminal part of SMARCB1 (15/19 vs. 4/83; Chi-square: 56.04, p = 1.0E−10) and, related to this, in tumors of the molecular subgroup ATRT-TYR (16/36 vs. 3/66; Chi-square: 24.47, p = 7.6E−7). Previous reports have indicated that while SMARCB1 lacks a bona fide nuclear localization signal, it harbors a masked nuclear export signal (NES) and that truncation of the C-terminal region results in unmasking of this NES leading to cytoplasmic localization. To determine if cytoplasmic localization found in ATRT is due to unmasking of NES, we generated GFP fusions of one of the SMARCB1 truncating mutations (p.Q318X) found in the tumors along with a p.L266A mutation, which was shown to disrupt the interaction of SMARCB1-NES with exportin-1. We found that while the GFP-SMARCB1(Q318X) mutant localized to the cytoplasm, the double mutant GFP-SMARCB1(Q318X;L266A) localized to the nucleus, confirming NES requirement for cytoplasmic localization. Furthermore, cytoplasmic SMARCB1(Q318X) was unable to cause senescence as determined by morphological observations and by senescence-associated β-galactosidase assay, while nuclear SMARCB1(Q318X;L266A) mutant regained this function. Selinexor, a selective exportin-1 inhibitor, was effective in inhibiting the nuclear export of SMARCB1(Q318X) and caused rapid cell death in rhabdoid tumor cells. In conclusion, inhibition of nuclear export restores nuclear localization and residual tumor suppressor function of truncated SMARCB1. Therapies aimed at preventing nuclear export of mutant SMARCB1 protein may represent a promising targeted therapy in ATRT harboring truncating C-terminal SMARCB1 mutations.