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Somatic and germline mutations in young patients with breast cancer

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Author(s):
Giselly Encinas Zanetti
Total Authors: 1
Document type: Doctoral Thesis
Press: São Paulo.
Institution: Universidade de São Paulo (USP). Faculdade de Medicina (FM/SBD)
Defense date:
Examining board members:
Maria Aparecida Azevedo Koike Folgueira; Helena Paula Brentani; Dirce Maria Carraro; Roger Chammas; René Aloisio da Costa Vieira
Advisor: Maria Aparecida Azevedo Koike Folgueira
Abstract

Approximately 4% of the breast cancer patients have their diagnosis under the age of 36 years and germline mutations in BRCA1 or BRCA2 genes is one of the predisposing factors. Identification of patients who are mutation carriers may contribute for the adoption of targeted therapies and for genetic counseling of their family members for early detection or preventive measures. However, most patients present with sporadic cancer where predisposing factors are less understood. In the latter group of patients, the identification of somatic mutations may contribute to a better understanding of the biology of the disease and to the development of molecular targeted therapies. Therefore, our goals were to characterize early onset breast cancer patients for family history, lifestyle, germline mutations in BRCA1 and BRCA2 genes and somatic mutations. Eighty-three breast cancer patients aged 18-35 years were interviewed using a questionnaire. DNA was extracted from peripheral blood and all coding regions of BRCA1/2 genes were screened by Sanger sequencing and large deletions and insertions were examined by Multiplex Ligation-Dependent Probe Amplification (MLPA). Results were analyzed through Mutation Surveyor v.3.20 and Chromas version 2.13 and searched in BIC Database, LOVD, LOVD-IARC, UMD and ClinVar. Whole exome sequencing was performed in blood and fresh-frozen tumor samples from eight patients (hormone receptor positive, HER2 negative and BRCA1/2 wild type) using Nextera Rapid Capture Enrichment in an Illumina HiSeq 1000. To detect somatic SNVs from whole exome sequencing data, SomaticSniper (v1.0.2) was used. Somatic mutation in PIK3CA gene was then searched in Formaldehyde Fixed-Paraffin Embedded samples from another group of young and older breast cancer patients. In addition, a metaanalysis was performed to evaluate whether somatic mutations in the five genes most frequently mutated in breast cancer patients were associated with an early age onset. Median age of 83 patients at diagnosis was 32 years (22-35y), median age of menarche was 13 years (8-19y); 71.1% patients had at least one born child, median age of first pregnancy was 22 years (14-34y); 80.7% were never smokers; 74.7% reported no regular alcoholic drinking; median body mass index was 25.26 (10.78-41.57). Most patients presented with invasive ductal carcinoma (89.3%), histological grade III (49.4%), luminal subtype (65.9%) and Ki67 expression > 14% (89.2). Approximately 52% of the patients reported a positive family history for breast, ovarian or prostate cancer. Deleterious mutations in BRCA1/2 genes ware detected in 13 patients (BRCA1, n= 4 and BRCA2, n= 9). One novel mutation was detected in BRCA1 gene: a stop codon in exon 6 (c.483T > A; p.Cys161Ter). Exome sequencing was evaluated in eight luminal tumors fromBRCA1/2 wild type patients and a median of 39 somatic alterations/tumor was detected, varying from 17 to 74. Potential driver alterations were detected in 53 genes, such as PIK3CA, PRKD1, PRKAR1A and PIK3AP1, that encode tyrosine kinase proteins or proteins involved in tyrosine kinases regulation; and POLD1 gene, that encodes the catalytic subunit of DNA polymerase delta which plays a critical role in DNA replication and repair. A Meta-analysis showed that somatic mutation in PIK3CA gene was frequently detected in both young and older patients. In conclusion, more than 15% of the young breast cancer patients are BRCA1 or BRCA2 mutation carriers. In patients with sporadic cancer somatic mutations were found in genes that encode tyrosine kinase proteins or are involved in the DNA repair. In agreement with what was observed in tumors from older patients, somatic mutation in PIK3CA gene is relatively frequent in tumors from young patients (AU)