Correspondence: Shouichi Ohga, MD PhD, Department of Perinatal and Pediatric Medicine, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan. Email: firstname.lastname@example.org
Genetic predisposition of thromboembolism depends on the racial background. Factor V Leiden (G1691A) and factor II mutation (G20210A) are the leading causes of inherited thrombophilias in Caucasians, but are not found in Asian ancestries. Protein S (PS), protein C (PC) and antithrombin (AT) activity are reportedly low in 65% of adult Japanese patients with deep vein thrombosis. Approximately half of the patients with each deficiency carry the heterozygous mutation of PS (PROS1; 20%), PC (PROC; 10%), and AT genes (SERPINC1: 5%). Recently, several studies have revealed an outline of inherited thrombophilias in Japanese children. Congenital thrombophilias in 48 patients less than age 20 years consisted of 45% PC deficiency, 15% PS deficiency and 10% AT deficiency, along with other causes. All PS- and AT-deficient patients had a heterozygous mutation of the respective gene. On the other hand, PC-deficient patients were considered to carry the homozygous or compound heterozygous mutation in 50%, the heterozygous mutation in 25%, and unknown causes in the remaining 25% of patients. Half of unrelated patients with homozygous or compound heterozygous PROC mutations carried PC-nagoya (1362delG), while their parents with its heterozygous mutation were asymptomatic. Most of the PC-deficient patients developed intracranial lesion and/or purpura fulminans within 2 weeks after birth. Non-inherited PC deficiency also conveyed thromboembolic events in early infancy. The molecular epidemiology of thrombosis in Asian children would provide a clue to establish the early intervention and optimal anticoagulant therapy in pediatric PC deficiency.
Thromboembolic events less commonly occur in children than in adults. The life-threatening conditions are driven by a wide range of triggers, including infection, trauma and surgery, on a background of individual predispositions. Pediatric thrombosis is being recognized with increasing frequency, although the cause remains elusive. The national registry data from North America, Europe and Australia revealed an incidence of 5–8 venous thromboembolic events (VTE)/10 000 hospital admissions, or 0.05–14/10 000 children. Under 20 years of age, VTE occurs at the highest incidence in neonates and infants, and then at the second peak of incidence during puberty and adolescence. Both peak occurrences are considered to be associated with decreased intrinsic fibrinolytic activity of the blood during these periods. The higher prevalence of adult thromboses in the West than that seen in the East might result from the difference in dietary habits and races. The modern lifestyle is being globally Westernized, but genetic backgrounds are hard-wired to change among races. Factor (F) V Leiden (G1691A) and FII mutation (G20210A) are the major thrombophilic predispositions in Caucasians but not in Asian ancestries. In Japan, two major studies revealed that 65% of adult VTE patients had a low activity of protein S (PS), protein C (PC) or antithrombin (AT), and half of them carried the heterozygous mutation of the respective genes.[3, 4] On the other hand, the clinical features and genetic backgrounds of pediatric thrombosis have not been clarified in Japan.
In this review, we first draw an outline of inherited thrombophilias in Japanese children, based on the recent independent studies. Then, we discuss the problems in the diagnosis and treatment of pediatric thromboembolism focusing on PC deficiency.
PC and PC levels in children
PC is a vitamin-K-dependent serine protease, which is synthesized in the liver and circulates at a low concentration in plasma. The anti-coagulant zymogen is activated by the complex formation with thrombin on the endothelial cell receptor thrombomodulin, and more effectively by binding to the endothelial PC receptor. Activated PC cleaves critical sites in the activated procoagulant factor V (FV) and FVIII, and inactivates the two factors. This process is augmented by protein S (PS), FV and lipid cofactors of lipoproteins and phospholipids. PC-deficient individuals have a decreased capacity to control the propagation of thrombin generation by FVa and FVIIIa after the activation of coagulation cascade, according to the circulating amounts of functional PC molecules regardless of the decreased production or increased consumption.
Inherited PC deficiency is an autosomal recessive thrombophilia. Bialellic (homozygous or compound heterozygous) PC gene (PROC) mutants incite purpura fulminans (PF) as a consequence of “severe” PC deficiency in the newborn. Heterozygous PROC mutants are at risk of venous thromboembolism as “moderately severe” or “mild” PC deficiency in young adults. The severity of PC deficiency is defined according to the levels of activity assessed by chromogenic (amidolytic) or coagulometric (clotting) assay (Table 1). The plasma PC activity in healthy adults ranges from 0.65 to 1.35 IU/mL, corresponding to the % interval of references used in Japan. “Mild,” “moderately severe” and “severe” PC deficiencies are defined as the range of >20% (>0.2 IU/mL), 1–20% (0.01–0.2 IU/mL), and <1% (<0.01 IU/mL), respectively. However, the distinction of inherited (heritable) or non-inherited (acquired) PC deficiency is challenging in children who developed acute thrombosis for the following reasons. First, plasma PC activity is physiologically low until adolescence. Second, two major conditions affecting plasma PC levels, vitamin K deficiency and infection, are not rarely found in early infancy. Third, young parents with heterozygous PROC mutation are healthy, and the history of the grandparents may be less informative. Fourth, in critically ill children with thrombotic events, the genetic tests are time-consuming in guiding the management, and the constitutional hypercoagulability is hard to assess by functional assays during the treatment course.
Table 1. Definition of the severity of PC deficiency
Plasma PC activity
Standard values of PC activity were obtained from †the reference; ‡Medical and Biological Laboratories, Tokyo, Japan; and §Special Reference Laboratories, Tokyo, Japan. The levels are determined by clot-based assays. PC, protein C.
Neonatal PC and PS levels are much lower than the adult reference levels. The mean plasma activities of PC and PS in healthy term infants are approximately 35%.[6-8] Preterm infants show much lower levels than term infants because of immaturity of the liver. Both PC and PS levels increase after birth, and reach the lower limit of adult references (∼50 IU/dL) during 6 months to 1 year of age (Fig. 1).[8-10] As PC activity often remains below the adult reference ranges until puberty, PC levels in childhood are hard to screen for inherited thrombophilias.
Several factors affect the physiological levels of PC and PS throughout childhood. Both activities are low in the presence of vitamin K antagonist. Vitamin K deficiency precipitates bleeding (i.e. hemorrhagic diseases of the newborn, vitamin K deficiency bleeding in infancy), and also thrombosis (i.e. warfarin-induced skin necrosis/ paradoxical thrombosis). Infection lowers the plasma concentration of PC and PS. The mechanisms of “infectious PF” are involved in antibody-mediated consumption (i.e. post-varicella PF) or toxic effects (i.e. meningococcemia PF). PC/PS deficiency arises from the loss or consumption in patients with nephrotic syndrome, sepsis and/or disseminated intravascular coagulation, and from the impaired synthesis in those with liver dysfunction. The half-life of plasma PC (6–8 h) is shorter than that of PS and other procoagulant vitamin K dependent factors. The true enzymatic activity of PS depends on free PS concentration. However, the interpretation of PS activity in newborns is not complicated because the binding C4b is at very low levels at birth. Acute inflammation reduces PS activity due to binding with C4b. For the diagnosis of PC deficiency in infants, plasma PC activity should be monitored concurrently with PS activity, protein induced by vitamin K absence or antagonists, d-dimer, anti-phospholipid antibodies, and FVII activity. Unexplained dissociation between PC and PS activity may portend a diagnosis of inherited PC deficiency.
Genetic backgrounds of PC deficiency in Japan
Recent reviews by the experts in the UK and North America described only eight and 12 survivors with long-term therapy, respectively. The nationwide survey of pediatric thrombosis in Japan first collected 301 patients from 2006 to 2010. Forty-eight patients (15%) were diagnosed as having congenital thrombophilia due to PC (n = 22; 46%), PS (n = 7, 15%), AT (n = 5; 10%), or ADAMTS13 (n = 5; 10%) deficiency and other causes (n = 9; 19%). This proportion differed from that of adult Japanese deep vein thrombosis cases (Fig. 2). There were no homozygous or compound heterozygous mutations of PROS1, PROC or SERPINC1 in adult patients. PS deficiency was the most frequent genetic predisposition of adult thrombosis in Japan. It may reflect the higher frequency of heterozygous PROS1 mutants in Japanese (1.12–1.8%) than in Caucasians (0.03–0.13%), accounting for the founder effects of PS-tokushima. On the other hand, approximately half of pediatric patients with congenital thrombophilia had PC deficiency. Half of them carried homozygous or compound heterozygous PROC mutations, in contrast to no PS or AT-deficient patients with biallelic mutations. Homozygous PS or AT-deficient mice result in fetal loss. Only a few patients with homozygous or compound heterozygous mutations of PROS1 or SERPINC1 have ever been reported. The true frequency of Japanese carriers of heterozygous PROC mutation may be higher than the prevalence of PC deficiency of 1/700 screened by amidolytic activity. These studies corroborated that PROC mutants are the leading cause of inherited thrombophilia in Japanese children.
Another study has recently reported the presentation and genotype of pediatric PC deficiency in Japan, based on the combined data with genetic study, post-marketing survey of plasma-derived activated PC concentrate (AnactC, Kaketsuken & Teijin, Kumamoto, Japan), and literature review. Between 1985 and 2010, this study determined 27 Japanese patients with congenital PC deficiency in Japan. These included one pair of twins, and a pair of cousins. Two patients had died. These patients might appreciably cover the 22 patients reported in the aforementioned nationwide survey. Twenty-four (89%) patients presented within 14 days after birth, including three prenatal hydrocephalies. Of the 27 patients, the first presentation was intracranial lesions (thrombosis, hemorrhage, and/or fetal hydrocephaly) in 19, PF in 16, and both in 10 patients (Fig. 3). Intracranial thrombosis/infarction and hemorrhage (ICTH) preceded or concurrently occurred with PF in both affected patients. Low PC activities of 18 mothers and/or 12 fathers indicated 20 heritable PC-deficiencies (two homozygotes, 11 compound heterozygotes, and seven heterozygotes) and seven unidentified causes of PC deficiency. Nine of 11 patients had PROC mutations, and four unrelated patients carried PC-nagoya (1362delG). No PC-deficient parents experienced VTE. Of the 18 patients treated with activated PC concentrate, two died and eight evaluable survivors had neurological sequelae. According to a different survey on neonatal thrombosis in Japan between 1999 and 2009, six newborn infants were diagnosed with inherited thrombophilia (all PC deficiency) of 105 patients with neonatal thrombosis. These studies outlined the features of inherited thrombophilia in Japanese children: (i) PC deficiency is the leading cause of inherited thrombophilia; (ii) the first presentation is preceding ICTH and/or PF within the first 2 weeks of life; (iii) PC-nagoya may be prevalent in pediatric but not adult patients with PC deficiency; and (iv) many survivors with PC deficiency have neurological sequelae, visual impairments and amputated extremities.
Neonatal non-inherited PC deficiency
The important observation was that 25% of patients diagnosed with congenital PC deficiency were born to healthy parents with normal PC activity. Their presentations were indistinguishable from those of patients with PROC mutations. However, the low PC activity gradually increased and attained the normal range, but did not convey any recurrent VTE by the age of 1 year. In this setting, such patients could be diagnosed as having “neonatal non-inherited PC deficiency.”
Manco-Johnson et al.[21, 22] first reported 11 newborn infants with undetectable PC activity and/or antigen, which proved on subsequent follow up to be acquired. Five of them developed renal, aortic and cerebral thrombosis. Severe to moderately severe PC activity (<0.1 U/mL or <10%) could occur and often led to thrombosis in the stressed newborns as non-inherited PC deficiency. Matsunaga et al. recently described a case of neonatal asphyxia and acute renal failure associated with isolated PC deficiency. The term infant had 6% of PC activity, 61% of PS activity, but no mutations in the promoter and coding regions of PROC. The hypercoagulability and dissociated PC and PS levels were unexplained by high d-dimer levels, normal FVII activity and absent vitamin K deficiency. During the early neonatal period, plasma PC activity shows prominent wide range compared with PS or AT activity (Fig. 1). The “transient PC deficiency in infancy” should be noticed as a critical thrombophilia showing a mimicking feature of inherited PC deficiency. Further studies are needed to clarify the mechanisms of delayed PC maturation, focusing on the genetic and epigenetic factors regulating PC concentrations.[24, 25]
Severe or moderately severe PC deficiency occurs in newborn infants, and results in serious conditions. Activated PC products may be limitedly used for the treatment of inherited PC deficiency but not sepsis. However, the clinical dilemma resides in the difficulty in discerning “inherited” from “acquired” PC deficiency at the first presentation. The phenotype and genotype of PC deficiency in Asian children should be clarified more to establish the screening methods, diagnostic guidelines, and optimal managements using PC agents.
We thank the staffs of the Comprehensive Maternity and Perinatal Care Center, and the Department of Clinical Chemistry and Laboratory Medicine, Kyushu University Hospital, Fukuoka, Japan. This work was supported in part by a grant from the Ministry of Health, Labour and Welfare of Japan. The authors have no conflicts of interest to declare.