Conventional chondrosarcoma is the most common primary bone tumor in adults. Prognosis corresponds with tumor grade but remains variable, especially for individuals with grade (G) II disease. There are currently no biomarkers available for monitoring or prognostication of chondrosarcoma. Circulating tumor DNA (ctDNA) has recently emerged as a promising biomarker for a broad range of tumor types. To date, little has been done to study the presence of ctDNA and its potential utility in the management of sarcomas, including chondrosarcoma. In this study, we have assessed ctDNA levels in a cohort of 71 patients, 32 with sarcoma, including 29 individuals with central chondrosarcoma (CS) and 39 with locally aggressive and benign bone and soft tissue tumors, using digital PCR. In patients with CS, ctDNA was detected in pretreatment samples in 14/29 patients, which showed clear correlation with tumor grade as demonstrated by the detection of ctDNA in all patients with GIII and dedifferentiated disease (n = 6) and in 8/17 patients with GII disease, but never associated with GI CS. Notably detection of ctDNA preoperatively in GII disease was associated with a poor outcome. A total of 14 patients with CS had ctDNA levels assessed at multiple time points and in most patients there was a clear reduction following surgical removal. This research lays the foundation for larger studies to assess the utility of ctDNA for chondrosarcoma diagnosis, prognostication, early detection of residual disease and monitoring disease progression.

PTEN is one of the most commonly inactivated tumour suppressor genes in sporadic cancer. Germline heterozygous PTEN gene alterations also underlie PTEN hamartoma tumour syndrome (PHTS), a rare human cancer-predisposition condition. A key feature of systemic PTEN deregulation is the inability to adequately dampen PI3-kinase (PI3K)/mTORC1 signalling. PI3K/mTORC1 pathway inhibitors such as rapamycin are therefore expected to neutralise the impact of PTEN loss, rendering this a more druggable context compared with those of other tumour suppressor pathways such as loss of TP53. However, this has not been explored in cancer prevention in a model of germline cancer predisposition, such as PHTS. Clinical trials of short-term treatment with rapamycin have recently been initiated for PHTS, focusing on cognition and colon polyposis. Here, we administered a low dose of rapamycin from the age of 6 weeks onwards to mice with heterozygous germline Pten loss, a mouse model that recapitulates most characteristics of human PHTS. Rapamycin was well tolerated and led to a highly significant improvement of survival in both male and female mice. This was accompanied by a delay in, but not full blockade of, the development of a range of proliferative lesions, including gastro-intestinal and thyroid tumours and endometrial hyperplasia, with no impact on mammary and prostate tumours, and no effect on brain overgrowth. Our data indicate that rapamycin may have cancer prevention potential in human PHTS. This might also be the case for sporadic cancers in which genetic PI3K pathway activation is an early event in tumour development, such as endometrial cancer and some breast cancers. To the best of our knowledge, this is the first report of a long-term treatment of a germline cancer predisposition model with a PI3K/mTOR pathway inhibitor.

PTEN negatively regulates the Class I PI 3-kinases by metabolizing phosphatidylinositol(3,4,5)trisphosphate (PIP3) and acts as a tumor suppressor. Heterozygous germline PTEN mutations in humans manifest into a complex multiorgan disorder known as PTEN hamartoma tumor syndrome (PHTS). Patients present with hamartomatous skin lesions, benign GI polyps, have an increased risk of developing certain types of cancer (breast, colon, endometrium, thyroid, GI tract and kidney) and have macrocephaly and autism spectrum disorders (ASD). There is huge variability in disease symptoms in PHTS patients, the cause of which is unclear. It has been proposed that this correlates with the nature of the aberration in PTEN and its impact on PTEN expression and function, allowing to discriminate the following cancer-risk groups in PHTS patients: 1. High cancer risk: Patients expressing a catalytically inactive but stable PTEN protein (mis-sense PTEN mutations such as the C124S, G129R, R130Q etc.) have a severe phenotype, with benign tumors and aggressive malignant cancers at a young age of one of more tissues. 2. Medium risk: Patients with complete loss of PTEN expression due to insertions, deletions and truncations in the PTEN gene present with an intermediate severity with benign and malignant tumors. 3. Low risk: Patients with PTEN protein characteristics that do not fall into Group 1 or 2 but with some detectable suppression of AKT signaling. This covers a range of PTEN mutants, with often a mix of characteristics such as being unstable but active or even overactive, stable but partially inactive or no apparent activity defect. These patients have a mild phenotype, mainly related to a broad range of ASD symptoms such as macrocephaly, developmental delay and mental retardation. Some patients are severely autistic. Patients may have benign tumors and skin hamartomas, but malignant cancer is rare. Here we expand our analysis by characterizing a range of additional PHTS PTEN mutations using biochemical methods. Of the several mutations characterized, the R173C mutation leads to a PTEN enzyme that retains its activity but becomes partially unstable, putting it in Group 3. This mutation was of particular interest because the R173 site in PTEN is also the third most commonly somatically mutated PTEN site in cancer (COSMIC database; accessed September 2018). Mouse models such as the heterozygous PTEN heterozygous mice (PTEN +/- mice) and PTEN +/C124S represent Groups 1 and 2, and their characteristics coincide with the predicted phenotypes. However, no clinically relevant mouse models for the low-risk PHTS group have been reported. We have generated and characterized a PTEN +/R173C PHTS mouse model. We show that the PTEN +/R173C mice have macrocephaly, fewer tumors and a significantly longer tumor-free survival compared to the PTEN +/-, thus providing further evidence that the above-proposed PTEN genotype-phenotype correlation holds true in PHTS. Acknowledgment: This work has been supported by PTEN Research. Citation Format: Wayne Pearce, Nicoletta Kessaris, Nicholas R. Leslie, Bart Vanhaesebroeck, Priyanka Tibarewal, Gala Classen, Virginia A Garcia, Victoria Rathbone, Nisha Kriplani, Georgia Constantinou. Investigation of PTEN genotype-phenotype correlations in the PTEN hamartoma tumor syndrome (PHTS) using in vitro and in vivo approaches [abstract]. In: Proceedings of the AACR Special Conference on Targeting PI3K/mTOR Signaling; 2018 Nov 30-Dec 8; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(10_Suppl):Abstract nr B22.

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