Functional mechanism and pathogenic potential of MYRF ICA domain mutations implicated in birth defects

Myrf is a membrane-bound transcription factor that plays a key role in various biological processes. The Intramolecular Chaperone Auto-processing (ICA) domain of Myrf forms a homo-trimer, which carries out the auto-cleavage of Myrf. The ICA homo-trimer-mediated auto-cleavage of Myrf is a prerequisite for its transcription factor function in the nucleus. Recent exome sequencing studies have implicated two MYRF ICA domain mutations (V679A and R695H) in a novel syndromic form of birth defects. It remains unknown whether and how the two mutations impact the transcription factor function of Myrf and, more importantly, how they are pathogenic for congenital anomalies. Here, we show that V679A and R695H cripple the ICA domain, blocking the auto-cleavage of Myrf. Consequently, Myrf-V679A and Myrf-R695H do not exhibit any transcriptional activity. Molecular modeling suggests that V679A and R695H abrogate the auto-cleavage function of the ICA homo-trimer by destabilizing its homo-trimeric assembly. We also found that the ICA homo-trimer can tolerate one copy of Myrf-V679A or Myrf-R695H for its auto-cleavage function, indicating that V679A and R695H are not dominant negatives. Thus, if V679A and R695H in a heterozygous state caused birth defects, it would be via haploinsufficiency of MYRF.