Sandbox:Roukoz

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Roukoz A. Karam, M.D.[2]

Overview

Protein S deficiency is an autosomal dominant thrombophilia, which leads to an increased risk of thromboembolic events. Protein S is a vitamin K-dependent glycoprotein and plays a role in anticoagulation. It is mainly a cofactor to the activated protein C (APC), which inactivates coagulation factors Va and VIIa and thereby controlling the coagulation cascade.

Historical Perspective

Protein S was first discovered and purified in Seattle, Washington in 1979, and it was arbitrarily named protein S after the city it was discovered in. The function of this protein was still unknown; however, it was hypothesized that protein S plays a role in activating protein C. Protein S deficiency was first discovered in 1984 when two related individuals with recurrent thromboembolic events and normal coagulation tests were studied. At the time, protein C deficiency was usually associated with recurrent familial thrombosis. These individuals were found to have diminished anticoagulation activity with normal coagulation tests (including a normal protein C level), and when purified human protein S was added to their plasma, effective anticoagulation was restored. (1)

Classification

Protein S deficiency can be subdivided into three types depending on whether the abnormality affects total protein S level, free protein S level, and/or protein S function:

• Type I: Reduced total protein S, free protein S, and protein S function

It is the classic form of hereditary protein S deficiency. Total protein S levels drop to approximately 50% of normal values while free protein S levels collapse to almost 15% of the normal. On a genetic level, type I deficiency usually results from missense or nonsense mutations. On few occasions, microinsertions, microdeletions, and splice site mutations have occurred with this type.

• Type II: Normal total and free protein S, reduced protein S function

This form results from a qualitative defect and is very rare. Reports describe missense mutations that affect the protein's ability to bind to activated protein C. (2,3,4) These mutations may alter the conformation of protein S or interfere with carboxylation of the gamma-carboxyglutamic acid domain of the protein. In a series of 118 French patients with thromboembolism associated with protein S deficiency, 26 had a serine to proline substitution at amino acid 460 (the Heerlen polymorphism), which affects protein S metabolism. The low free plasma protein S may result from increased binding of the abnormal protein S to C4b-binding protein [38,39]. The thrombophilic risk with this polymorphism has been questioned.

• Type III: Normal total protein S, reduced free protein S and protein S function

This is a quantitative defect.

Type	Total Protein S	Free Protein S	Protein S Function
I	↓	↓	↓
II	↔	↔	↓
III	↔	↓	↓