Phosphatidylserine is localized exclusively to the inner leaflet of the membrane lipid bilayer of most cells, including erythrocytes. This asymmetric distribution is critical for the survival of erythrocytes in circulation since externalized phosphatidylserine is a phagocytic signal for splenic macrophages. Flippases are P-IV ATPase family proteins that actively transport phosphatidylserine from the outer to inner leaflet. It has not yet been determined which of the 14 members of this family of proteins is the flippase in human erythrocytes. Herein, we report that ATP11C encodes a major flippase in human erythrocytes, and a genetic mutation identified in a male patient caused congenital hemolytic anemia inherited as an X-linked recessive trait. Phosphatidylserine internalization in erythrocytes with the mutant ATP11C was decreased 10-fold compared to that of the control, functionally establishing that ATP11C is a major flippase in human erythrocytes. Contrary to our expectations phosphatidylserine was retained in the inner leaflet of the majority of mature erythrocytes from both controls and the patient, suggesting that phosphatidylserine cannot be externalized as long as scramblase is inactive. Phosphatidylserine-exposing cells were found only in the densest senescent cells (0.1% of total) in which scramblase was activated by increased Ca²⁺ concentration: the percentage of these phosphatidylserine-exposing cells was increased in the patient’s senescent cells accounting for his mild anemia. Furthermore, the finding of similar extents of phosphatidylserine exposure by exogenous Ca²⁺-activated scrambling in both control erythrocytes and the patient’s erythrocytes implies that suppressed scramblase activity rather than flippase activity contributes to the maintenance of phosphatidylserine in the inner leaflet of human erythrocytes.

Precise genetic diagnosis of inherited bone marrow failure syndromes (IBMFS), a heterogeneous group of genetic disorders, is challenging but essential for precise clinical decision making. We analyzed 121 IBMFS patients using a targeted sequencing covering 184 associated genes and 250 IBMFS patients using whole-exome sequencing (WES). We achieved successful genetic diagnoses for 53 of 121 patients (44%) using targeted sequencing and for 68 of 250 patients (27%) using WES. In the majority of cases (targeted sequencing: 45/53, 85%; WES: 63/68, 93%), the detected variants were concordant with, and therefore supported, the clinical diagnoses. However, in the remaining 13 cases (8 patients by target sequencing and 5 patients by WES), the clinical diagnoses were incompatible with the detected variants. Our approach utilizing targeted sequencing and WES achieved satisfactory diagnostic rates and supported the efficacy of massive parallel sequencing as a diagnostic tool for IBMFS. Genet Med advance online publication 19 January 2017

The type-I isoform of pyrimidine 5′-nucleotidase (P5N-I) has an important role in the catabolism of pyrimidine mononucleotides during erythroid maturation. Two alternatively spliced forms of P5N-I mRNA have been identified, and we found another alternatively spliced form in reticulocytes, which included an additional 87-bp sequence. The sequence is located 6·2-kb downstream of the exon 2 and 2·7-kb upstream of the exon 3 sequence; consequently, the P5N-I gene encodes 11 exons, which span approximately 48 kb. We identified five novel mutations in nine families with P5N-I deficiency: two missense mutations (425C, 721C), one splice mutation (339C), one 1-bp insertion (251-insA-252) and one 9-bp deletion (del 192-200). All patients were homozygous for each mutation. The mutant P5N-I with 721C (G241R) had lower affinity for cytidine monophosphate, suggesting that Gly241 is important for substrate binding. Haplotype analysis showed that 721C, which had been identified in five unrelated families, was a founder mutation. The mutant P5N was then expressed in Cos-7. The degradation of P5N with 425C (L142P) was significantly faster than a wild-type control, and proteasome inhibitors restored the stability of L142P. These data suggest that L142P increases susceptibility to the degradation by the ubiquitin-proteasome pathway.