The Role of Promoter Hypermethylation and Expression of Spleen Tyrosine Kinase in Mammary Carcinogenesis
Pornchai O-charoenrat MD*, Bhusarakorn Thakrairach MD**
Affiliation : * Division of Head-Neck & Breast Surgery, Department of Surgery, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand ** Department of Immunology, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand
Background : The spleen tyrosine kinase (SYK) has been considered as an important inhibitor of cancer cell growth and a
potential tumor suppressor in human breast carcinomas. Loss of SYK expression occurs frequently in breast cancer cells
together with the promoter hypermethylation suggests that methylation may play an important role in inactivating SYK. In
contrast to full-length form of SYK (SYKL), a spliced variant or SYKS is frequently expressed in breast cancer cells.
Objective : To determine whether the expression of SYKL and its variant SYKS and the presence of DNA methylation in
promoter of SYK might be associated with the risk and progression of mammary carcinoma.
Material and Method: Five breast cancer cell lines were examined for expression of SYKL and SYKS mRNA and SYK DNA
methylation. One hundred and eight breast cancer tissues, 13 benign tumor tissues, and 35 adjacent non-cancerous tissues
(ANCT) were extracted for examination of SYKL and SYKS mRNA. Genomic DNA of 83 breast cancer tissues and 13 benign
tissues were isolated for examination of SYK DNA methylation. SYKL and SYKS were determined by quantitative reverse
transcription-PCR. SYK DNA methylation was assayed by methylation-specific PCR.
Results : Two breast cancer cell lines with metastatic phenotype showed complete loss of SYKL mRNA together with SYK DNA
methylation. The significant reduction of the SYKL mRNA expression in breast cancers and benign tumors in comparison to
ANCT was observed (mean mRNA levels = 1.0297, 0.6294 and 0.3446 in ANCT, benign tumors and breast cancers,
respectively). Complete loss of SYKL was found in 50% of breast cancer tissues but not in benign tumors and ANCT. SYK was
methylated in 45% of breast cancer tissues compared with only 15% in benign tumor tissues. Furthermore, a significant
correlation between SYK methylation and loss of its expression was observed. There was no significant association between
SYKL expression or methylation and clinicopathological parameters. Two breast carcinoma cell lines showed SYKS expression.
The expression of SYKS mRNA was found most frequently in breast cancer tissues (49%) and less in benign tumors (46%)
and ANCT (25%). The SYKS expression showed a significant correlation with larger tumor size.
Conclusion : These findings suggest that SYK signaling pathway may play a crucial role in breast cancer development. The
mechanisms responsible for SYKL inactivation may occur through DNA methylation or the presence of SYKS.
Keywords : Breast cancer, Methylation, Spleen tyrosine kinase, Tumor suppressor
