Does hepatitis B virus infection cause breast cancer?

Lifestyle and family history are two of the most important risk factors for breast cancer (BC). However, these risk factors cannot explain the differences in the incidence and early BC onset among Chinese females compared to their western counterparts. We propose in this hypothesis the potential mechanism of indirect oncogenesis of hepatitis B virus (HBV) in causing BC through its persistence as occult infection and continuous replication with long term subtle liver damage. Estrogen is mainly deactivated in the liver and long term necro-inflammatory damage to liver may result in persistent high level of estrogen, which is a dominant risk factor for BC. HBV may also directly affect the breast cells through its cis and trans effects of HBx which may act as oncoprotein. Given the recognised aetiologic association between oestrogen and breast cancer risk, there is biological plausibility that dietary soy and vegetable intake which is rich in the Chinese diet may have anti-carcinogenic effect on the breast. The seemingly conflicting phenomenon of early age onset and lower BC incidence in China might be due to wide imbalance in the amount of exposure to carcinogenic factor (e.g., HBV infection) for decades and the carcinoprotective exposure levels (e.g., isoflavonoids and flavonoids intake). For example, the increase in carcinoprotective levels would lead to lower incidence of breast cancer and vice versa. Although the focus of this personal view is on HBV, this by no means negates the roles of other known risk factors in breast-cancer development. Characterisation of the role of HBV in BC could potentially benefit Chinese females by decreasing incidence and increasing overall survival.