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Blood, Vol. 92 No. 8 (October 15), 1998:
pp. 2750-2758
A Gln747 Pro Substitution in the IIb Subunit Is
Responsible for a Moderate IIb 3
Deficiency in Glanzmann Thrombasthenia
Seiji Tadokoro,
Yoshiaki Tomiyama,
Shigenori Honda,
Morio Arai,
Naomasa Yamamoto,
Masamichi Shiraga,
Satoru Kosugi,
Yuzuru Kanakura,
Yoshiyuki Kurata, and
Yuji Matsuzawa
From The Second Department of Internal Medicine, Osaka University
Medical School, Osaka, Japan; the Department of Blood Transfusion,
Osaka University Hospital, Osaka, Japan; the Department of Clinical
Pathology, Tokyo Medical College, Tokyo, Japan; and the Department of
Cardiovascular Research, The Tokyo Metropolitan Institute of Medical
Science, Tokyo, Japan.
To clarify a molecular defect responsible for moderate
IIb 3 deficiency, we examined two
unrelated patients, MT and MS, suffering from type II and type I
Glanzmann thrombasthenia (GT), respectively. Sequence analysis of
polymerase chain reaction (PCR) fragments derived from platelet mRNA
showed a single A C substitution at nucleotide (nt) 2334 leading to a
Gln747 Pro in IIb in both patients. Allele-specific
restriction enzyme analysis (ASRA) of genomic DNA demonstrated that
patient MT was homozygous for the Gln747 Pro substitution and patient
MS was compound heterozygous for this substitution and for an RNA
splice mutation at the consensus sequence of the splice acceptor site
of exon 18 (AG AA). Furthermore, ASRA showed that, among 17 unrelated
Japanese GT patients, this Gln747 Pro substitution was detected in 4 patients, including MT and MS (homozygous, 2 patients; heterozygous, 2 patients). Cotransfection of Pro747 IIb and
3 constructs into 293 cells resulted in moderate
reduction in the amount of IIb 3 within the transfected cells as well as on the cell surface. However, Pro747 IIb 3 bound the ligand mimetic
monoclonal antibody (MoAb) PAC-1 after activation of
IIb 3 by the MoAb PT25-2, suggesting that
the mutant IIb 3 possesses the
ligand-binding function. The association between the mutant
pro IIb and 3 was not disturbed. Surface
labeling and pulse chase study showed that the Gln747 Pro substitution moderately impaired both intracellular transport of the
IIb 3 heterodimers to the Golgi apparatus
and endoproteolytic cleavage of pro IIb into heavy and
light chains. By contrast, replacement of Gln747 with Ala by
mutagenesis did not impair IIb 3 expression on the cell surface. These results suggest that the presence
of Pro, rather than the absence of Gln, at amino acid residue 747 on
IIb is responsible for moderate
IIb 3 deficiency.
© 1998 by The American Society of Hematology.

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