Molecular basis for Glanzmann's thrombasthenia (GT) in a compound heterozygote with glycoprotein IIb gene: a proposal for the classification of GT based on the biosynthetic pathway of glycoprotein IIb-IIIa complex

Abstract

The genetic basis for Glanzmann's thrombasthenia (GT) was elucidated on a compound heterozygote with glycoprotein (GP)IIb gene: an opal mutation at the end of exon 17 (CGA----TGA) results in only a trace amount of GPIIb mRNA, and a splicing mutation at the acceptor site of exon 26 (CAG----GAG) causes an in-frame, exon skipping process from exon 25 to 27. This aberrant transcript encodes a single-chain polypeptide characterized by a 42-amino acid deletion, which includes the proteolytic cleavage site(s) and a unique, proline-rich region at the location corresponding to the carboxyl-terminal of the normal GPIIb alpha-chain. These characteristics are shared by a previously reported defective GPIIb molecule, which is neither assembled with GPIIIa nor transported to the cellular surface. Despite its normal transcription level, expression of the present defective GPIIb molecule was significantly decreased (approximately 6% of the control level). Because the precursor GPIIb molecule is assembled with GPIIIa in the endoplasmic reticulum (ER) and its processing, as well as stability, is dependent on the GPIIIa subunit, the defective GPIIb molecule may be rapidly degraded by the intrinsic quality control system of the ER due to its inability to form a stable heterodimer complex as a consequence of its misfolded structure. Although we did not confirm that the GPIIIa genes of this individual were normal, GPIIIa may be secondarily decreased (approximately 11% of control), because a large part of it could not be complexed, making it vulnerable to proteolysis. To elucidate the molecular basis for GT, we propose here a classification of GT based on the biosynthetic pathway of the GPIIb-IIIa complex.

A nonsense mutation in the GPIIb heavy chain (Ser 870 → stop) impairs platelet GPIIb–IIIa expression

“…One mutation concerned the residue Arg 584 and was found in four patients. Two were from Japan (Kato et al, 1992;Tomiyama et al, 1995), one from China (Gu et al, 1993) and the last from France (Vinciguerra et al, 1995). A 75 bp deletion in mRNA was described in one of these patients resulting in exon 17 skipping.…”

Novel point mutations in the αIIb subunit (Phe289 → Ser, Glu324 → Lys and Gln747 → Pro) causing thrombasthenic phenotypes in four Japanese patients

“…It is generally assumed that nonsense mutations and frameshift mutations which induce a premature stop codon lead to dramatically reduced mRNA levels ( Roos et al , 1996 ). In patients with GT, the transcript levels of mutant alleles with large gene rearrangements of β3 ( Li & Bray, 1993) or a nonsense mutation in αIIb of exon 17 ( Kato et al , 1992 ; Tomiyama et al , 1995 ) were shown to be absent or decreased. Since our screening for mutations was performed by direct sequencing of platelet mRNA, any mutation resulting in reduced levels of transcripts could have been missed, as Bray (1994) pointed out.…”

Novel mutations in Thai patients with glanzmann thrombasthenia

“…The c.1750C>T (p.Arg584Ter) is a mutation previously located in several studies and occurs across continents . The previously reported c.2915dup (p.Leu973Alafs*63) was identified in two families (Family 1 and 3).…”

“…A homozygous mutation was found in one patient. The previously known c.1750C>T (p.Arg584Ter) and c.2915dup (p.L973Afs*63) mutations were identified in patient 1 …”