Authors’ contributions
NLM and AJG are co-chief investigators; they conceived the study and participated in its design and coordination. KKL, DL, ROB, RW, AB, CTu, MTHL, AVF, MCW, GR, JB, CTa, DC, CK, JN and DEN participated in the design and coordination. KKL, DC and NLM drafted the manuscript. All authors read and approved the final manuscript.