Germline truncating mutations in DICER1, an endoribonuclease in the RNase III family that is essential for processing microRNAs, have been observed in families with the pleuropulmonary blastoma–family tumor and dysplasia syndrome. Mutation carriers are at risk for nonepithelial ovarian tumors, notably sex cord–stromal tumors. We sequenced the whole transcriptomes or exomes of 14 nonepithelial ovarian tumors and noted closely clustered mutations in the region of DICER1 encoding the RNase IIIb domain of DICER1 in four samples. We then sequenced this region of DICER1 in additional ovarian tumors and in certain other tumors and queried the effect of the mutations on the enzymatic activity of DICER1 using in vitro RNA cleavage assays. DICER1 mutations in the RNase IIIb domain were found in 30 of 102 nonepithelial ovarian tumors (29%), predominantly in Sertoli–Leydig cell tumors (26 of 43, or 60%), including 4 tumors with additional germline DICER1 mutations. These mutations were restricted to codons encoding metal-binding sites within the RNase IIIb catalytic centers, which are critical for microRNA interaction and cleavage, and were somatic in all 16 samples in which germline DNA was available for testing. We also detected mutations in 1 of 14 nonseminomatous testicular germ-cell tumors, in 2 of 5 embryonal rhabdomyosarcomas, and in 1 of 266 epithelial ovarian and endometrial carcinomas. The mutant DICER1 proteins had reduced RNase IIIb activity but retained RNase IIIa activity. Somatic missense mutations affecting the RNase IIIb domain of DICER1 are common in nonepithelial ovarian tumors. These mutations do not obliterate DICER1 function but alter it in specific cell types, a novel mechanism through which perturbation of microRNA processing may be oncogenic. (Funded by the Terry Fox Research Institute and others.)

Early trials for immune checkpoint inhibitors in sarcomas have delivered mixed results, and efforts to improve outcomes now look to combinatorial strategies with novel immunotherapeutics, including some that target macrophages. To enhance our understanding of the sarcoma immune landscape, we quantified and characterized tumor-associated macrophage infiltration and expression of the targetable macrophage-related immune checkpoint CD47/SIRPα across sarcoma types. We surveyed immunohistochemical expression of CD68, CD163, CD47, and SIRPα in tissue microarrays of 1242 sarcoma specimens (spanning 24 types). Non-translocation sarcomas, particularly undifferentiated pleomorphic sarcoma and dedifferentiated liposarcoma, had significantly higher counts of both CD68+ and CD163+ macrophages than translocation-associated sarcomas. Across nearly all sarcoma types, macrophages outnumbered tumor-infiltrating lymphocytes and CD163+ (M2-like) macrophages outnumbered CD68+ (M1-like) macrophages. These findings were supported by data from The Cancer Genome Atlas, which showed a correlation between increasing macrophage contributions to immune infiltration and several measures of DNA damage. CD47 expression was bimodal, with most cases showing either 0% or >90% tumor cell staining, and the highest CD47 scores were observed in chordoma, angiosarcoma, and pleomorphic liposarcoma. SIRPα scores correlated well with CD47 expression. Given the predominance of macrophage infiltrates over tumor-infiltrating lymphocytes, the bias toward M2-like (immunosuppressive) macrophage polarization, and the generally high scores for CD47 and SIRPα, macrophage-focused immunomodulatory agents, such as CD47 or IDO-1 inhibitors, may be particularly worthwhile to pursue in sarcoma patients, alone or in combination with lymphocyte-focused agents.

A variety of immunohistochemical (IHC) stains have been proposed to mark either benign or malignant mesothelial proliferations. Loss of the p16 tumor suppressor (CDKN2A), through homozygous deletions of 9p21, is a good marker of mesotheliomas but lacks sensitivity. Recent reports indicate that some mesotheliomas are associated with loss of BRCA-associated protein 1 (BAP1) expression. Here we investigate BAP1 and p16 as potential markers of malignancy and compare test characteristics with previously proposed markers using a well-characterized tissue microarray. BAP1 protein expression was interrogated by IHC. The p16 locus was examined by fluorescence in situ hybridization (FISH) directed toward chromosome 9p21. Loss of BAP1 was identified in 7/26 mesotheliomas and 0/49 benign proliferations. Loss of p16 was identified in 14/27 mesotheliomas and 0/40 benign proliferations, yielding 100% specificity and positive predictive value for each marker. Together, BAP1 IHC and p16 FISH were 58% sensitive for detecting malignancy. Various combinations of p53, EMA, IMP3, and GLUT1 showed reasonably high specificity (96% to 98%) but poor to extremely poor sensitivity. Combined BAP1 IHC/p16 FISH testing is a highly specific method of diagnosing malignant mesotheliomas when the question is whether a mesothelial proliferation is benign or malignant and is particularly useful when tissue invasion by mesothelial cells cannot be demonstrated. However, combined BAP1/p16 FISH testing is not highly sensitive, and negative results do not rule out a mesothelioma. The test characteristics of previously proposed markers EMA, p53, GLUT1, IMP3 suggest that, even in combination, these markers are not useful tools in this clinical setting.

Here, we show that the p53 family member, p73, is necessary for survival and long-term maintenance of CNS neurons, including postnatal cortical neurons. In p73−/− animals, cortical neuron number is normal at birth but decreases significantly by postnatal day 14 (P14)–P16 because of enhanced apoptosis. This decrease continues into adulthood, when p73−/− animals have approximately one-half as many cortical cells as their wild-type littermates. Cortical neurons express the ΔNp73α protein, and overexpression of ΔNp73 isoforms rescues cortical neurons from diverse apoptotic stimuli. Thus, ΔNp73 isoforms are survival proteins in cortical neurons, and their deletion causes a gradual loss of cortical neurons in the weeks and months after birth. This decrease in CNS neuron number in p73−/− animals is not limited to the cortex; facial motor neuron number is decreased, and postnatal development of the olfactory bulb is greatly perturbed. These findings, together with our previous work showing that ΔNp73 is essential for survival of peripheral sympathetic neurons (Pozniak et al., 2000), indicate that p73 isoforms are essential survival proteins in CNS as well as PNS neurons, and that they likely play a role not only during developmental cell death but also in the long-term maintenance of at least some adult neurons.