PT-VWD
What is Platelet type von Willebrand disease?
Clinical Features
Laboratory Findings
2B or not 2B?
Treatment of Platelet type VWD
Molecular Characterization of Platelet type VWD
Platelet GPIBA gene
Platelet GPIB/IX/V complex
Platelet GP1BA gene Sequence
Nomenclature
What is Platelet type von Willebrand disease?
Platelet type von Willebrand disease (VWD) is a very rare autosomal dominant bleeding disorder first described by Weiss et al., in 1982. The genetic defect is in platelets rather than von Willerband factor (VWF). The disease is characterized by abnormally high binding affinity of the platelets to the VWF, leading to a characteristic platelet hyper responsiveness as evidenced by the ristocetin- induced platelet agglutination (RIPA) test (Miller et al., 1991) (Weiss et al., 1982).
The common features of Platelet type VWD are frequent and severe nosebleeds, excessive bleeding following tooth extraction, tonsillectomy and other surgical operations. Bleeding becomes more pronounced after aspirin ingestion or drugs that have antiplatelet activity (Miller 1996). Loss of HMW multimers due to their deposition on the platelet surface together with thrombocytopenia caused by an increased removal of the bound platelets are the two mechanisms behind the bleeding diathesis in platelet-type VWD.
The laboratory features of Platelet type VWD include loss from plasma of HMW multimers and characteristically enhanced RIPA (to 0.5 mg/ml or less of ristocetin) (Miller et al., 1991)(Miller et al., 1983). The VWD screen shows low or near normal factor VIII level, low or near normal VWF antigen level and low ristocetin cofactor activity. Platelet count can be normal. However, mild or moderate intermittent thrombocytopenia can be also seen and usually becomes aggravated by conditions that increase the endogenous release of VWF such as pregnancy, stress and infection (Miller 1996).
In the diagnosis of Platelet type VWD, the key feature that raises the question about the possibility of the disease is an enhanced RIPA reflecting the increased platelet-VWF binding. The more commonly encountered the type 2B VWD shares most of the clinical and laboratory features of Platelet type VWD, including the unique enhanced RIPA. The discrimination between type 2B and Platelet type VWD can be very difficult. Methods of discrimination include
- RIPA mixing studies
- Cryoprecipitate challenge
- DNA analysis:
- A1 domain (exon 28) VWF gene
- Platelet GPIBA gene
Treatment of Platelet type VWD:
The hyperresponsiveness of the platelet receptor results in increased interaction with VWF in response to minimal or no stimulation in vivo. This leads to a fall in plasma VWF and typically to a decreased or low normal platelet count. Replacement therapy in the form of VIII/VWF preparations or drugs aiming at increasing the release of endogenous VWF will exacerbate the condition and lead to further reduction of the platelet count. However, an ideal treatment would be to infuse platelet concentrates or, if possible, a VIII/VWF preparation in a low enough dosage to increase the haemostatic activity to a limit that does not induce a fall in the platelet count (VWF:RCo of 40-47u/dl) (Miller 1996).
Molecular Characterization of Platelet typeVWD:
The abnormality in Platelet type VWD resides in the platelet receptor for VWF, the glycoprotein Ib/IX/V complex being hyper-responsive rather than a defect in plasma VWF itself. In all studied cases, the abnormality has been located precisely in the GP Ib alpha component of the receptor complex.
To date, four mutations have been identified within the GPIBA gene. Functional characterization of each of these mutations was found responsible for the disease phenotype:
- Gly 233 Val (Miller et al, 1991;Russel and Roth 1993)
- Met 239 Val (Takahashi et al., 1995)
- Gly 233 Ser (Matsubara et al. 2003;Nurden et al., 2007)
- 27bp deletion (Othman et al., 2005)
The GPIBA gene cloned in 1987 by Lopez and coworkers is a simple gene spanning ~ 2.4 kb of genomic DNA, with two exons. The entire protein coding sequence is contained within exon 2 and the mRNA comprises ~1882 bp. The gene codes for a leader sequence of 16 aa, a mature 610 aa protein and a stop codon. There are also 42 nucleotides of 5’ non-coding sequence and 497 nucleotides of 3’ non-coding sequence, including the poly ‘A’ tail. (Lopez et al., 1987).
The sequence numbering is according to the GP1BA sequence of Lopez et al “Cloning of the alpha chain of human platelet glycoprotein Ib: a transmembrane protein with homology to leucine-rich alpha 2-glycoprotein.” Proc Natl Acad Sci U S A. 1987 84(16):5615-5619.
The GP Ib/IX/V comprises four transmembrane polypeptides, each of which belongs to the leucine-rich repeat family of proteins. They are: GPIbα (GPIBA) disulfide linked to GPIb, GPIX and GPV each of which is non-covalently associated with the complex with a stoichiometry revised recently of 2:4:2:1 (Luo et al., 2007). Platelet GPIBA is the largest component of the GPIb/IX/V receptor complex (610 amino acids) and carries the VWF binding site. The His 1- Glu 282 stretch within the 45 kDa amino terminal domain is known to contribute to direct VWF binding. (Vicente et al., 1990; Peterson et al., 1992; Murata et al., 1991).
Figure 1 Stereo view of a ribbon representation of the GpIb-A1 complex.
Source: Eric G. Huizinga, Shizuko Tsuji, Roland A. P. Romijn, Marion E. Schiphorst, Philip G. de Groot, Jan J. Sixma, Piet Gros, Structures of Glycoprotein Ib and Its Complex with von Willebrand Factor A1 Domain, Science 16 August 2002, Vol. 297. no. 5584, pp. 1176 – 1179
Four types of polymorphism have been described in the GP Ib alpha gene:
- The T or C at position –5 T/C from the initiator ATG codon (Kozak sequence (Kozak, 1987).
- The 70 Leu/Phe polymorphism (Matsubara et al., 2002)
- The Thr/Met polymorphism at residue 145 (Kuijpers et al., 1992).
- The Variable number of tandem repeats (VNTR) of 39 bp (13 aminoacids repeats) in the macroglycopeptide within a region flanked by Glu 396 and Thr 411. Four alleles have been described: D allele (one repeat) C allele (2 repeats) B allele (3 repeats), A allele (4 repeats) Lopez et al.,1992).
The abnormality in PT-VWD lies within the platelet, rather than VWF, leading to a VWD- like phenotype, yet not part of the ISTH VWD classification (Sadler et al., 2006). PT-VWD was first described by Weiss and coworkers in 1982 and was referred to as pseudo- VWD (Weiss etal., 1982). It was once classified under ‘VWD-mimic’ disorders (Favaloro et al., 1999) and later studies have since used the term ‘platelet-type’ VWD to describe these patients. The term ‘platelet type pseudo’ VWD was also recommended in several occasions. Until a final agreement on nomenclature is reached, we will continue to use the term ‘platelet type’ VWD. We believe that this nomenclature is more descriptive of the pathophysiology and has already achieved universal utility.