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Haemophagocytosis by myeloid precursors in lysinuric protein intolerance

An 8-year-old boy was referred with a normochromic anaemia, thrombocytopenia, hepatosplenomegaly and persistent pyrexia despite broad spectrum antibiotics. Ferritin, triglycerides and lactate dehydrogenase (LDH) were elevated. Liver enzymes and fibrinogen were normal. Polymerase chain reaction for Epstein–Barr virus (EBV) was strongly positive. Bone marrow examination showed striking haemophagocytosis by myeloid precursors (figures). The patient had a complex history. He had been noted to have hepatosplenomegaly at birth and subsequently suffered from poor weight gain and motor delay. At 2 years of age he became unwell following a diarrhoeal illness. On commencement of enteral feeding he deteriorated and developed encephalopathy with abnormal movements and bilateral slow wave changes on electroencephalography. Hyperammonaemia and generalised aminoaciduria were detected. A diagnosis of lysinuric protein intolerance (LPI) was subsequently made after sequencing identified a homozygous deletion of exon 4 of the LPI gene (SLC7A7) on chromosome 14. Lysinuric protein intolerance is a rare multisystem disorder characterised by failure to thrive, protein intolerance, pulmonary alveolar proteinosis, osteoporosis and an increased susceptibility to haemophagocytic lymphohistocytosis (HLH). Inheritance is autosomal recessive. LPI is a transport disorder for lysine, cystine ornithine and arginine. Treatment is with citrulline supplements. The clinical and biochemical expression is chronic and intermittent; the only permanent findings are hyperferritinemia and elevated serum LDH. There is no profound cellular immunodeficiency. High protein feeding causes clinical deterioration and encephalopathy secondary to hyperammonaemia. Haemophagocytic lymphohistocytosis as described here differs from the HLH seen in other acquired or familial diseases because, in addition to macrophages, myeloid precursors are very actively involved. This is said to be a specific sign of LPI. This condition should be seriously considered in any patient presenting with a haemophagocytic syndrome and failure to thrive.