Background Germline mutations and deletions of SMARCB1/INI1 in chromosome band 22q11.2 predispose patients to rhabdoid tumor and schwannomatosis. Previous estimates suggested that 15–20% of rhabdoid tumors were caused by an underlying germline abnormality of SMARCB1. However, these studies were limited by case selection and an inability to detect intragenic deletions and duplications. Procedure One hundred matched tumor and blood samples from patients with rhabdoid tumors of the brain, kidney, or soft tissues were analyzed for mutations and deletions of SMARCB1 by FISH, multiplex ligation-dependent probe amplification (MLPA), sequence analysis and high resolution Illumina 610K SNP based oligonucleotide array studies. Results Thirty-five of 100 patients were found to have a germline SMARCB1 abnormality. These abnormalities included point and frameshift mutations, intragenic deletions and duplications, and larger deletions including regions both proximal and distal to SMARCB1. There were 9 cases that demonstrated parent to child transmission of a mutated copy of SMARCB1. In 8 of the 9 cases, one or more family members were also diagnosed with rhabdoid tumor or schwannoma, and 2 of the 8 families presented with multiple affected children in a manner consistent with gonadal mosaicism. Conclusions Approximately one third of newly diagnosed patients with rhabdoid tumor have an underlying genetic predisposition to tumors due to a germline SMARCB1 alteration. Families may demonstrate incomplete penetrance and gonadal mosaicism, which must be considered when counseling families of patients with rhabdoid tumor.

Epithelioid malignant peripheral nerve sheath tumors arising in pre-existing schwannomas are extremely rare. We report an unusual example occurring in a patient with multiple schwannomas (schwannomatosis), all but one of which showed “neuroblastoma-like” histology. By immunohistochemistry, both the epithelioid malignant peripheral nerve sheath tumor and the schwannomas showed a complete loss of the Smarcb1 protein. Subsequent genetic evaluation revealed the presence of a novel germline mutation in the SMARCB1/INI1 gene in the patient and three of her children, two of whom were diagnosed with atypical teratoid/rhabdoid tumors of the brain.

Malignant rhabdoid tumors (MRT) are aggressive tumors associated with mutations in the SMARCB1 gene. In experimental systems, the loss of SMARCB1 is hypothesized to alter p16INK4A pathways resulting in repression of tumor suppressors. To determine whether these pathways are deregulated in human MRT, we used immunohistochemistry on tissue microarrays to evaluate p16INK4A/E2F1/RB and p14ARF/MDM2/p53 pathways in 25 atypical teratoid/ rhabdoid tumors (AT/RT) and 11 non-CNS MRT. p16INK4A was negative or showed focal weak expression. The p16INK4A downstream targets CDK4/cyclin D1/ppRB were variably expressed at moderate to low levels; E2F1 was negative. Unexpectedly, p14ARF expression was seen in many cases, which correlated positively with p53 and inversely with MDM2 immunostaining in AT/RT. TP53 mutational analysis in 19/25 AT/RT and 8/11 non-CNS MRT cases showed point mutations in only 3 AT/RT cases, suggesting that p53 expression was driven mainly by p14ARF. Finally, nucleophosmin, a protein that stabilizes p53, was positive in the majority of cases and colocalized with p53. Together, these data suggest that in MRT there is deregulation of not only p16INK4A, but also the p14ARF pathway. These results provide insights into cell cycle deregulation in the pathogenesis of human MRT and may aid in the design and evaluation of potential therapies for these tumors.