Rh-mod syndrome: A family study of the translation-initiator mutation in the Rh50 glycoprotein gene

Rh-mod syndrome is a rare genetic disorder thought to result from mutations at a "modifier" but not at the suppressor underlying the regulator type of Rh-null disease. We studied this disorder in a Jewish family with a consan guineous background and analyzed RH and RHAG, the two loci that control Rh- antigen expression and Rh-complex assembly. Despite the presence of a d (D- negative) haplotype, no other gross alteration was found at RH, and cDNA se quencing showed a normal structure for D, Ce, and ce Rh transcripts in fami ly members. However, analysis of RHAG transcript, which encodes Rh50 glycop rotein, identified a single G-->T transversion in the initiation codon, cau sing a missense amino acid change (ATG[Met]-->ATT[Ile]). This point mutatio n also occurred in the genomic region spanning exon 1 of RHAG, and its geno typic status in the mother and two children was confirmed by analysis of si ngle-strand conformation polymorphism. Although blood typing showed a very weak expression of Rh antigens, immunoblotting barely detected the Rh prote ins in the Rh-mod membrane. In vitro transcription-coupled translation assa ys showed that the initiator mutants of Rh-mod-but not those of the wild ty pe-could be translated from ATG codons downstream, Our findings point to in complete penetrance of the Rh-mod mutation, in the form of "leaky" translat ion, leading to some posttranslational defects affecting the structure, int eraction, and processing of Rh50 glycoprotein.