Hereditary antithrombin deficiency caused by heterozygous Cambridge II mutation in combination with a large deletion

The antithrombin (AT) p.Ala384Ser mutation, also known as the Cambridge II variant, is associated with a mild reduction of AT activity. The thrombotic risk associated with this variant is under discussion.
In this report, we describe a family with hereditary AT deficiency caused by a large gene deletion in combination with the p.Ala384Ser mutation on the same allele.
AT activity was measured using a chromogenic anti-Xa assay and AT antigen was determined by rocket electrophoresis. Molecular analysis of the AT gene included complete sequencing of all exons and
intron-exon junctions and multiplex ligation-dependent probe amplification (MLPA).
We received samples from a mother and daughter for genotyping of AT deficiency. Both patients had a strong history of venous thrombosis and received lifelong oral anticoagulants. AT antigen and activity
levels suggested type I deficiency. Sequencing of all exons showed heterozygous Cambridge II mutation in both patients. As this mutation is known to cause a type II reactive site deficiency, this finding
was not in agreement with the AT plasma values. Thus, exon dosage was performed using the MLPA technique which revealed a large deletion including exons 3A and 3B, in both patients. This deletion
was confirmed by long range PCR.
The occurrence of both mutations in mother and daughter implies that they are located on the same allele. This is, to our knowledge, the first report of a complex AT allele. The lack of two exons in the protein is most likely to cause the thrombotic phenotype in these patients. Our findings, in combination with previous reports, might support that the p.Ala384Ser mutation is a polymorphism with low thrombotic risk.