Thrombophilia refers to a group of diseases characterized by a high propensity for thromboembolism due to genetic or acquired defects in anticoagulant proteins, coagulation factors, fibrin, or acquired risk factors[5]. PC belongs to the serine protease family and is present in plasma as a single-chain zymogen. It serves as a physiologically important anticoagulant in the body. PC activation occurs through cleavage of its heavy chain at the arginine-isoleucine site (Arg12-Leu13) by thrombin or the thrombin-thrombomodulin complex. This cleavage generates activated protein C (APC), which, in the presence of Ca2+ and phospholipids, combines with its cofactor protein S (PS) to inactivate activated coagulation factor V and activated coagulation factor VIII, reducing thrombin production. APC also inhibits the activation of prothrombin by activated coagulation factor X, stimulates the release of fibrinogen activator, and enhances fibrinolytic activity, thus playing a dual role in anticoagulation and pro-fibrinolysis[6–7]. Consequently, a decrease in PC levels directly affects the normal anticoagulation and fibrinolytic functions of the body, predisposing individuals to thrombosis.
The PROC gene, located on autosome 2q13-q14, spans approximately 11.2 kb and consists of nine exons and eight introns[8]. Hereditary PC deficiency is an autosomal dominant disorder caused by mutations in the PROC gene and manifests with clinical heterogeneity[9]. In recent years, advancements in testing techniques and tools have led to an increasing incidence of PC deficiency. Heterozygous carriers have a prevalence of 1/500 to 1/200 in the population, while purely heterozygous types occur in approximately 1/4,000 to 1/25,000 individuals[10]. Venous Thrombus Embolism (VTE) is a complex condition that arises from a combination of acquired and genetic risk factors. These factors include Deep Venous Thrombosis (DVT), Pulmonary Embolism (PE), or a combination of both. [11]. According to the American College of Medical Genetics and Genomics (ACMG) classification criteria and guidelines for genetic variants, the specific evidence supporting the mutations identified in this study as causes of venous embolism are as follows: (1) The variant is a nonfunctional variant causing hereditary PC deficiency (PVS1); (2) The variant's impaired gene function has been validated through in vivo and in vitro functional experiments [12] (PS3); (3) The variant is located in a hot spot variant region[13–14] (PM1); (4) The mutation is not found in the normal population database (PM2); (5)The variant is a deletion variant leading to the formation of a truncated protein (PM4); (6) It shares the same locus change as the identified pathogenic variant[15] (PM5); (7) The variant has been detected in members of the affected lineage, except for the prior witness (PP1); (8) Multiple statistical methods have predicted the deleterious effects of the variant on the gene or gene product, including conservativeness prediction and pathogenicity analysis (PP3); (9) Both PC:A and PC:Ag were reduced in the proband’s plasma, and the family history strongly correlated with coagulation factor PC deficiency (PP4). Combining the aforementioned variant evidence and classification, the p.Phe181Val variant had one strong evidence of pathogenicity (PS3) and three supporting evidence (PP1 + PP3 + PP4). The p.Lys192deletion variant had two moderate evidence of pathogenicity (PM1 + PM4) and three supporting evidence (PP1 + PP3 + PP4). The p.Arg220Gln variant had two moderate evidence of pathogenicity (PM1 + PM5) and two supporting evidence (PP1 + PP4). The p.Glu383Argfs*38 variant had one very strong evidence of pathogenicity (PVS1), two moderate evidence of pathogenicity (PM2 + PM4), and three supporting evidence (PP1 + PP3 + PP4). The p.Glu383Argfs*38 variant was rated as pathogenic, while the p.Phe181Val, p.Lys192deletion, and p.Arg220Gln variants were all rated as potentially pathogenic. We speculate that these four heterozygous mutations can potentially decrease protein synthesis, reduce stability or impair secretion. As a result, the levels of plasma PC may be reduced. Furthermore, in the venous system where blood flow is slow, the body's anticoagulant effect plays a crucial role in balancing the procoagulant proteins and platelet phospholipids exposed to the vessel wall over an extended period of time. [16]. The deficiency of PC disrupts the dynamic balance between procoagulation and anticoagulation, making individuals susceptible to recurrent thromboembolism [16], and is considered one of the significant causes of VTE in young patients.
While PC deficiency leading to arterial thrombosis is relatively rare compared to venous thrombosis, studies have reported a relationship between PC deficiency and ischemic cerebrovascular disease. In 1989, Sasaki et al.[17] reported the first case of PC deficiency leading to recurrent multiple cerebral infarctions in a young patient with only 28% PC:A in plasma. Faryal et al. [18] found that PC and PS defects may be risk factors for arterial thrombosis in young patients based on a study of a young case with sequential myocardial infarction and cerebral infarction. Li P et al. [19] discovered an association between PC gene mutations and atherothrombosis through an analysis of a 35-year-old patient with cerebellar infarction. A systematic review and meta-analysis conducted by Chiasakul et al. [20] demonstrated that PC deficiency contributes to a genetic thrombotic tendency with a significant risk of adult arterial thrombosis, especially in younger patients. All these reports concluded that PC deficiency likely plays a role in the development of cerebral infarction. In this study, we identified a heterozygous p.Trp83Serfs*54 variant in proband 2. The catalytic structural domain of PC spans amino acids 212 to 461, and PC secretion is dependent on the carboxy-terminal region located after the 28 amino acid loop (positions 398 to 426) [1]. By constructing a protein model, we observed that the c.247-238insCT insertion mutation prematurely generates a stop codon at position 136, resulting in the loss of amino acids 136 to 461 and the production of a truncated protein. The majority of the lost amino acids are located in the catalytic domain of PC, suggesting that the mutation likely disrupts enzyme activity. Furthermore, the c.247-238insCT insertion mutation introduces a premature termination codon, leading to truncation of the carboxyl terminus and affecting the secretion of PC. The pathogenicity of the mutated site was assessed using ACMG. The p.Trp83Serfs*54 variant exhibited one very strong evidence of pathogenicity (PVS1), one moderate evidence of pathogenicity (PM4), and three supporting evidence (PP1 + PP3 + PP4), leading to its classification as a pathogenic mutation. This classification aligns with our initial prediction, as thrombophilia is a multifactorial disease influenced by a combination of genetic and environmental factors, which determine the presence or absence of thrombotic events in patients carrying PROC gene mutations [4]. Since no other known common risk factors are present in Proband 2, the occurrence of cerebral infarction in this individual was primarily attributed to hereditary PC deficiency.
The treatment approach for patients with PC deficiency leading to venous thrombosis is essentially the same as that for patients with other causes of venous thrombosis. Due to the high risk of VTE recurrence in PC-deficient patients, which can be as high as 60%, long-term anticoagulation is typically recommended. The exact duration of anticoagulation should be determined based on the patient's risk of recurrence and bleeding risk stratification[21], particularly after a spontaneous thromboembolic event. In patients with vitamin K-dependent PC defects identified through early screening, the use of newer anticoagulants such as rivaroxaban tablets or dabigatranate is recommended for regular monitoring. This is because oral anticoagulants like vitamin K antagonists (eg warfarin) can further decrease their anticoagulant protein levels[5]. For patients with hereditary PC deficiency without embolic events, prophylactic anticoagulation is usually unnecessary, but predisposing factors should be avoided[22]. Therefore, we strongly recommend screening for PC deficiency in young patients presenting with unexplained venous thromboembolism or cerebral infarction. This screening is particularly important to determine the underlying cause and establish appropriate prophylactic treatment.