Homozygous SERPINC1 Mutation in Congenital Antithrombin Deficiency: A Unique First Case of Neonatal Cardiac Thrombosis

Antithrombin (AT) serves as the primary inhibitor of blood coagulation proteases. Hereditary AT deficiency, caused by mutations in the SERPINC1 gene represents an autosomal-dominant thrombophilic disorder, contributing to an increased risk of thrombotic disease.1, 2 Various SERPINC1 defects have been identified in gene analyses of patients with AT deficiency, encompassing missense, nonsense, deletion, and insertion mutations.1, 2, 3 Most mutations manifest in a heterozygous manner, inheriting autosomal dominance. However, our report unveils a unique case of hereditary AT deficiency characterized by a homozygous SERPINC1 mutation, presenting with cardiac thrombosis during the neonatal period.

We approved the weiver of consent for this case report from the Samsung Medical Center Institutional Review Board (IRB No. 2023-06-105). A male 37 weeks and 3 days of gestational age weighing 2,470 g (10-25 percentile), length 48.5 cm (25-50 percentile), head circumference 32.5 cm (10-25 percentile) was born to a 37-year-old mother by vaginal delivery. The Apgar score was 5 at 1 minute and 7 at 5 minutes, and positive pressure ventilation was administered due to weak spontaneous breathing. Subsequently, spontaneous breathing was maintained.

The patient was the first baby and family history of thrombosis was denied. In the prenatal ultrasonography, there were no abnormal findings. After birth, there were symptoms of respiratory distress, the neonate was admitted to the neonatal intensive care unit (NICU) and necessitated noninvasive respiratory support with continuous positive airway pressure device. At date of birth, initial echocardiography for evaluation of respiratory difficulty, revealed a mass-like lesion measuring 4×5 mm on the tricuspid valve (TV). Consequently, the patient was referred to Samsung Medical Center NICU on the date of birth for further assessment of an intracardiac mass suggestive of thrombus. Subsequent echocardio-graphy performd on the date of birth identified a mobile mass of approximately 5×6 mm on the TV (Fig. 1). Initial abdominal ultrasonography and brain ultrasonography performed on the date of birth, there are no thrombus lesion. One week follow up abdominal ultrasonography detected a thrombus lesion measuring about 7×15 mm in the suprarenal inferior vena cava (IVC). Initial laboratory studies showed AT III activity at 5%, white blood cell count at 11,290/µL, hemoglobin at 19.5 g/dL, and platelet count at 7,000/µL.

Fig. 1
Intracardiac mass on the tricuspid valve on echocardiography. The arrow indicates intracardiac mass. Written informed consent was obtained for publication of this case report and accompanying images.

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Due to persistent thrombocytopenia, we suspected disseminated intravascular coagulation, and administered immunoglobulin G (400 mg/kg/day) for 5 days to the patient. Methylprednisolone (2 mg/kg/day) was administered to address potential neonatal alloimmune thrombocytopenia. During hospitalization, follow-up echocardiography performed on 7 days of age exhibited consistent 4×6 mm TV mass dimensions but increased echogenicity. We performed surgical removal of the TV mass, measuring 0.5×0.5 cm, at 20 days of age (Fig. 2). Although the surgery addressed the TV mass, multiple thrombi persisted in the IVC and left portal vein, necessitating ongoing monitoring and anticoagulation therapy. Enoxaparin (1.72 mg/kg/day) was administered for 17 days, and the plan is to switch to warfarin for maintenance before discharge.

Fig. 2
The intraoperative finding of calcified thrombus on tricuspid valve. The arrow indicates calcified thrombus. Written informed consent was obtained for publication of this case report and accompanying images.

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After surgery, respiratory distress improved without respiratory support. The initial AT III activity level was 5% to 13%, which was very low compared to 50% of reference range for adults (normal adult AT III level reference range, 83%-123%). Considering the thrombosis at the early age of life and the low AT III levels, we were concerned about the possibility of congenital AT III deficiency.

Genetic panel testing at 19 days of age identified a mutation in the SERPINC1 gene with a homozygous variants of NM_000488.3 (SERPINC1):c.235C>T, p. (Arg79Cys) (Fig. 3). While the homozygous detection of this mutation aligned with coagulation test results, the potential for compound heterozygosity involving heterozygous Arg79Cys mutation and heterozygous large deletion mutation could not be excluded and subsequent multiplex ligation-dependent probe amplification was performed, showing no evidence of SERPINC1 gene deletion or duplication. Targeted gene variant analysis of the parents revealed heterozygosity for the mutation c.235C>T in both the father and mother. This, coupled with the father’s AT activity of 53% and Ag 111%, and the mother's AT activity of 44% and Ag 99%, suggested findings consistent with congenital type II AT deficiency caused by homozygous mutation in SERPINC1 gene.

Fig. 3
Genetic testing results of the patient and his parients. (A) Next-generation sequencing showed a homozygous pathogenic variant of SERPINC1, NC_00001.10:g.173883864G>A, visualized by Integrative Genomic Viewer. (B) The targeted Sanger sequencing of the SERPINC1 gene revealed the proband inherited the Arg79Cys from each of his parents. Written informed consent was obtained for publication of this case report and accompanying images.

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Brain ultrasonography conducted 2-weeks after birth detected a left focal periventricular white matter lesion, indicative of focal hemorrhage or infarction. Brain magnetic resonance imaging at 1-month of age confirmed multiple punctate white matter lesions in periventricular areas and a small hemorrhagic infarction in the right thalamus. At one month of age, an assessment of infantile motor and performance was conducted, resulting in a total score of 53, which falls within the 5 to 16th percentile range. The patient was discharged at 1-month of age on warfarin (0.1 mg/kg/day) therapy for prophylaxis thromboembolic complications. At the time of discharge from NICU, the patient without the need for respiratory support in a stable condition with good oral intake. Regular follow-up with rehabilitation therapy was maintained, and at one year of age, a Denver Developmental Screening Test suggested motor developmental delay. The patient is currently 20-month-old and is undergoing rehabilitation therapy for mild motor developmental delay.

AT deficiency is widely recognized as one of the most severe congenital prothrombotic disorders. This deficiency can either be inherited or acquired. Inherited forms are primarily attributed to mutations in the AT gene. A significant proportion of AT deficiency cases can be elucidated by genetic mutations within the SERPINC1 gene, which encodes AT, a member of the serine protease inhibitor (SERPIN) superfamily. This gene is situated on chromosome 1q25.1 and comprises a total of 7 exons and 6 introns. The resulting phenotypes are divided into type 1 and type 2 deficiencies, corresponding to reduced levels of AT antigen and activity, or to functionally defective AT present at normal antigen levels.4, 5 Most of patients with AT had genetic defects in SERPINC1, mainly missense mutations.6 In this case, a SERPINC1 gene mutation with a homozygous variants of NM_000488.3 (SERPINC1):c.235C>T, p. (Arg79Cys) was also identified in the patient. This missense varient is already reported, but uniquely, the mutation of this case confimred to be homozygous through parental genetic testing.

There were 3 cases diagnosed with AT during the neonatal period, and they were confimred to be heterozygous. Most of the patients were found to have developed the thromboembolic condition during child hood or adulthood.7 In the future, if reports on homozygous AT deficiency cases emerge, it is expected that a comparison of clinical symptoms between heterozygous and homozygous indivisuals can be made to identify potential differences. Although there are cases where heterozygous AT deficiency has occurred in the neonatal period, it is presumed that in this case, the symptoms manifested in the early neonatal period due to homozygocity.

It is known that the common causes of cardiac thrombosis in newborns are AT deficiency and protein C deficiency.8, 9 Therefore, it is essential to conduct tests for these 2 anticoagulant protein deficiencies. In this case, AT deficiency has been conformed, but assessing AT deficiency in newborns is not straightforward. Diagnosing inherited AT deficiency in neonates presents challenges due to nonspecific manifestations, difficulties in blood sampling, and a lack of reliable reference ranges for AT III levels in neonates. In this case, rigorous coagulation test evaluation and targeted variant analysis of the patient and parents identified a homozygous SPERINC1 gene mutation. To our best knowledge, this is the first AT III deficiency case of clinically presenting intracardiac thrombus with a homozygous SPERINC1 gene mutation. More interesting point of this case, most intracardiac thrombus in neonate or preterm infant risk factor is related to central lines or after the surgery for congenital heart disease.10 Furthemore, the activity of both procoagulant and anticoagulant factors was reduced in neonate group.10, 11 So intracardiac thrombus is the key condition of AT deficiency in neonate without risk factors. In neonate who had no risk factors like this case patient, a rigorous evaluation of AT deficiency is essential. Furthermore, the diagnosis in the neonatal period may have been homozygous, so it is necessary to confirm zigosity through parental testing.12

In conclusion, we present the interesting case of a genetically confirmed homozygous SPERINC1 gene mutation resulting in congenital AT III deficiency with multiple thrombosis presenting in neonatal period. In neonates displaying unprovoked thrombosis, early evaluation for congenital thrombophilia, including AT deficiency, alongside vigilant anticoagulation therapy, is imperative. It also emphasize the importance of evaluatiing the underlying cause through parental genetic testing for early diagnosis.