The number of reports on
genetic predisposition to pediatric
thrombosis is increasing. The risk of
thrombosis in childhood varies according to patient age, and the contribution of
genetic predisposition also differs. The term early-onset
thrombophilia, which occurs until the age of 20 years in patients with genetic diagnosis, was defined. Then, the registry in Japan was established. Further, publications were reviewed comprehensively, and results revealed the genetic and clinical characteristics of patients. Less than 60% of patients presented with
protein C (PC) deficiency, and over half of them had PC-gene monoallelic variants. The number of patients with
protein S or
antithrombin deficiency increased with age. None of them were aged between 6 and 8 years. PC-Tottori and
protein S-Tokushima, which are high-frequency and low-risk variants in Japanese, contributed to the development of
thrombosis. However, PC-Tottori did not affect the development of severe PC deficiency. One exceptional de novo PC-deficient variant was identified in 32 EOT families, and
thrombosis developed concurrently in three pairs of mothers-newborns. Appropriate EOT screening tests targeting PC deficiency are required to prevent maternal and neonatal
thromboses.