322P Clinicopathological characteristics and genomic profiling of pure mucinous breast cancer

Pure mucinous breast cancer (PMC) is a rarely seen histological type with favorable prognosis. Nevertheless, cases with recurrence have been reported and diagnosed in clinical practice. The recurrent patterns and mechanisms of PMC are unknown. Thus, in this study, we aim to discover the prognostic factors of PMC and reveal the genomic profiling of PMC with positive lymph nodes. A total of 166 cases diagnosed as PMC were included. We compared the clinicopathologic characteristics across different groups using Mann-Whitney U test or chi square test according to variables. 21-gene assay was done in 10 patients with recurrence and 20 TNM staging-matched patients. Whole-exon sequencing was done in 12 PMC primary tumor and 4 positive lymph nodes. We compared the genomic alterations between two groups in single nucleotide polymorphism, copy number variation and enriched pathways. The median age at diagnosis was 45 years old in total 166 patients. After median follow-up of 84 months, 21/166 patients developed recurrence with/without distant metastasis. The significant different factors between patients with recurrence and without recurrence were tumor size, lymph node status and TNM staging. 21-gene recurrence scores were not statistically significant between two groups. Whole-exon sequencing showed that lymph node-positive(LNP) PMC primary tumors were more recurrently with mutations of ADCY10 (3/6) and SHANK3 (3/6), while lymph node-negative(LNN) PMC did not harbor these genes. Moreover, LNP group prevalently harbored gains of 15q23, 17q23.2, 20p11.21 and loss of 21p11.2, while LNN group did not harbored any of these copy number variations. In terms of pathway enrichment, LNP group was involved in several pathways but LNN group was not enriched in any pathways. Overall prognosis is good in PMC. TNM staging is an important prognostic factor of PMC. PMC with positive lymph node more recurrently harbored gains of 15q23,17q23.2, 20p11.21 and loss of 21p11.2. More researches, especially integration of multi-omics studies, are needed in revealing the underlying mechanism of lymph node metastasis in PMC.