1Texas Tech University Health Science Center, Lubbock, Texas

2University of Iowa, Iowa City, Iowa

Correspondence: Julie Stanik-Hutt; Tel: 410-802-4717; E-mail: [email protected]

Competing interests: The authors report no conflicts of interest.

Authors' contributions: G. Sun: Substantial contributions to the conception, design, drafting, and final approval of the manuscript including agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. J. Stanik-Hutt: Substantial contributions to the conception, design, drafting, and final approval of the manuscript including agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.