Footnotes

• Contributors NP carried out the molecular genetic studies and drafted the manuscript. NS critically revisited the manuscript and contributed to interpretation of the data. RH clinically assessed all patients used in this study and contributed to critical editing of this manuscript. LW contributed in editing the manuscript. MZ is project leader of the study and, therefore, designed and coordinated the study. All authors read and approved the final manuscript.

• Competing interests None.

• Ethics approval Stellenbosch University and the University of Cape Town.

• Patient consent Obtained.

• Provenance and peer review Not commissioned; externally peer reviewed.