The PROC gene in humans is situated at 2q13-q14 and covers a length of 11.2 kb. It comprises of 9 exons and is responsible for the production, aggregation, secretion and functioning of protein C (PC). Mutations in the PROC gene commonly cause hereditary PC deficiency. The Human Gene Mutation Database (HGMD) lists 391 (513) PROC gene mutations associated with hereditary protein C deficiency. Majority of these mutations are missense/nonsense mutations with only a few being insertion, deletion and splicing mutations.

Most of the genetic defects causing hereditary PC deficiency are heterozygous. Patients with the heterozygous type usually develop the disease in adulthood and have an increased risk of deep vein thrombosis. However, the symptoms of the homozygous or compound heterozygous type are more severe and often occur in the newborn period. These patients may experience purpura fulminans, pulmonary embolism, and disseminated intravascular coagulation [3]. In this case, genetic analysis of the family revealed that the proband and his brother inherited a heterozygous mutation c.1204 A > G in exon 9 of the PROC gene from their mother.

The genetic variation of the PROC gene c.1204 A > G mutation in a brother and sister was analyzed according to the genetic variation interpretation rules of ACMG/AMP [4] and ClinGen framework. The software predicted a REVEL score of 0.957 points, which indicates that the function was predicted to be harmful. This is considered as strong supporting evidence (PP3). Additionally, this mutation is rare in the general population, which adds supporting evidence (PM2) according to the latest guidelines. Furthermore, the protein activity measurement and clinical phenotype are consistent with PROC gene-related diseases, which adds evidence for PP4. Based on the fact that these three items are met at the same time, the variant site is considered a possible pathogenic variant. It is worth noting that this mutation site is reported for the first time.

The risk of developing VTE due to protein C deficiency varies from person to person, depending on the degree of deficiency and other acquired or inherited factors that increase the risk of blood clots [5]. Studies conducted abroad have shown that the incidence of heterozygous PC deficiency in families with inherited thrombophilia is 6%, while healthy people have a rate of only 0.2% [6]. In China, a study found that around 8% of 202 patients with venous thrombosis had protein C deficiency [7]. Moreover, individuals with protein C deficiency are 10–15 times more likely to develop VTE than those without the deficiency [2].

Compared to healthy individuals, heterozygous individuals have PC levels that are approximately 50% of the standard value. Generally, no clinical symptoms or delayed venous thromboembolism occur [8]. Our patients had PC activities of 53%, 40%, and 45%. The proband and his brother developed superficial vein thrombosis, while the mother was asymptomatic, which is consistent with the literature. Asymptomatic patients with PC deficiency may also be at risk for future VTE, and prospective studies have shown that these patients with heterozygous protein C deficiency have a 2.5% increased risk of thrombosis per year compared to wild-type individuals [9]. The mother needs to be followed up for later thrombosis.

Our study revealed that three family members have PC deficiency. The proband and his brother suffered from venous thrombosis, while the mother did not. The reason for this discrepancy could be attributed to α-thalassemia and splenectomy that the proband and his brother underwent. Patients with thalassemia are vulnerable to VTEs such as deep vein thrombosis, pulmonary embolism, and portal vein thrombosis due to a hypercoagulable state [10]. Several mechanisms lead to hypercoagulability, including chronic platelet activation, changes in RBC membranes, abnormal expression of adhesion molecules on vascular endothelial cells, and dysregulation of hemostasis [10]. Low levels of PC and protein S have been observed in thalassemia patients from different ethnic backgrounds [11]. Regular blood transfusion can reduce the risk of thrombosis, but splenectomy increases it [12]. Acquired risk factors such as pregnancy, exogenous estrogen therapy, immobilization, or surgery have also been shown to increase the incidence of VTE in individuals with PC deficiency [13]. The proband and his brother underwent splenectomies in 2018 and 2019, respectively. This surgery may be a predisposing factor for VTE formation because of procoagulants and abnormal platelets present on the surface of red blood cells [12]. These platelets do not disappear after splenectomy and get removed from circulation, leading to increased PC consumption to control the hypercoagulable state [14,15,16].

In this study, we discussed a novel mutation in the PROC gene that leads to protein C deficiency and recurrent venous thrombosis after surgery. The mutation was found in a family, and it is the first time this particular mutation site has been reported. Our study adds to the knowledge of the protein C gene’s mutation sites and clinical phenotypes. If a child experiences unexplained venous thromboembolism, protein C deficiency should be considered as a possible cause. It is important to conduct timely genetic testing to confirm the diagnosis.