Author Contributions
L Tang and H Bo drafted the manuscript and revised and confirmed the final manuscript.
Financial & competing interests disclosure
This work was supported by the Changsha Municipal Natural Science Foundation (conducted by H Bo, No. kq2014033) and Scientific Research Fund of Hunan Provincial Health and Family Planning Commission (conducted by L Tang, No. 2015-145). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.
No writing assistance was utilized in the production of this manuscript.