A novel missense mutation in ABCA1 results in altered protein trafficking and reduced phosphatidylserine translocation in a patient with Scott syndrome

Christiane Albrecht,John H Mcvey,James I Elliott,Alessandro Sardini,Ildiko Kasza,Andrew D Mumford,Rossi P Naoumova,Edward G.D Tuddenham,Katalin Szabo,Christopher F Higgins

doi:10.1182/blood-2004-05-2056

Abstract

AbstractScott syndrome (SS) is a bleeding disorder characterized by a failure to expose phosphatidylserine (PS) to the outer leaflet of the platelet plasma membrane. Because the adenosine triphosphate (ATP)–binding cassette transporter A1 (ABCA1) is implicated in the exofacial translocation of PS, we assessed its role in the pathophysiology of a patient with SS. Substantially reduced levels of ABCA1 mRNA were found in the patient's leukocytes, compared with controls. The SS patient was heterozygous for a novel missense mutation c.6064G>A (ABCA1 R1925Q), absent from unaffected family members and controls. Both mutant and wild-type alleles were reduced in mRNA expression, and no causative mutation for this phenomenon was identified in the ABCA1 gene or its proximal promoter, suggesting a putative second mutation in a trans-acting regulatory gene may also be involved in the disorder in this patient. In vitro expression studies showed impaired trafficking of ABCA1 R1925Q to the plasma membrane. Overexpression of wild-type ABCA1 in SS lymphocytes complemented the Ca2+-dependent PS exposure at the cell surface. These data identify a mutation in ABCA1 that contributes to the defective PS translocation phenotype in our patient with SS.

Highlights

Scott syndrome (SS) is a rare, moderately severe bleeding disorder (Online Mendelian Inheritance in Man [OMIM] database: 262890)
ABCA1 mRNA levels were quantified by real-time reverse transcription (RT)-polymerase chain reaction (PCR) relative to expression of glyceraldehyde-3-phosphate dehydrogenase (G3PDH) using a TaqMan probe located in exon 3 of the ABCA1 gene
We identified a novel missense mutation in ABCA1 (R1925Q) in the SS patient, which results in severely impaired trafficking and reduced expression of functional ABCA1 protein at the cell surface

Summary

Introduction

Scott syndrome (SS) is a rare, moderately severe bleeding disorder (Online Mendelian Inheritance in Man [OMIM] database: 262890). Full-length human ABCA1 cDNA was generated by RT-PCR of mRNA obtained from leukocytes of a healthy individual and the patient as described previously.[13] Wild-type cDNA was cloned into pGEM-11Zf vector (Promega, Madison, WI) and replaced with the corresponding cDNA fragment (Asp718-HpaI) containing the patient’s mutation (c.6064GϾA).

Results

Conclusion