OBJECTIVETo analyze the clinical features and pathogenetic gene mutation in a fetus at his third trimester with familial haemophagocytic lymphohistiocytosis (FHL).

METHODSTarget region sequencing and high-throughput sequencing were used to detect pathogenetic gene mutations for familial haemophagocytic lymphohistiocytosis in a late onset HLH fetus. Pathogenetic gene mutations of the patient and his parents were verified by Sanger dideoxy sequencing.

RESULTSA male neonate, who had right pleural effusion, hepatomegaly and splenomegaly previously revealed by fetus ultrasound, was delivered at full-term by cesarean section. His clinical presentation showed recurrent fever, tachypenea, decreased breath sounds on right side, hepatosplenomegaly etc., which were gradually aggravating Lab.tests results were as follows: WBC 9.88×10/L, Hb 91 g/L, Plt 13×10/L, ALT 18 U/L,AST 69 U/L,TBIL 207.2 µmol/L, DBIL 183.5 µmol/L, TG 3.05 mmol/L, Fib 0.88 g/L, Serum ferritin 3 120 ng/ml and sIL-2R 57 420 U/ml. FCM showed that CD3CD16CD56cells reached to 3.60% in the pripheral blood. Haemophagocytes were occasionally found in the bone marrow. NK/NKT stimulation test showed a severe damage of degranulation of NK cells. Sequence analysis of genomic DNA from his peripheral blood demonstrated the compound heterozygous mutations of UNC13D gene: c.2448-13 G>A in exon26 and c.1055+1 G>A in exon12, both were pathogenetic mutations. In detailed family survey, it was confirmed that the mutation c.2448-13 G>A in exon26 was inherited from his mother and c.1055+1 G>A in exon12 from his father.

CONCLUSIONA rare case of familial haemophagocytic lymphohistiocytosis type 3 (FHL3) with late fetus onset who carried pathogenetic compound heterozygous mutations of UNC13D gene. Those neonates with recurrent fever, serous effusions and multiple organ failure should be screened for FHL. Identifying the pathogenic gene mutations laid the foundation of conceiving disease-free newborns.