Transfusion-transmitted malaria masquerading as sickle cell crisis with multisystem organ failure.

BACKGROUND: Fever accompanying vaso-occlusive crisis is a common presentation in patients with sickle cell disease (SCD) and carries a broad differential diagnosis. Here, we report a case of transfusion-transmitted malaria in a patient with SCD presenting with acute vaso-occlusive crisis and rapidly decompensating to multisystem organ failure (MSOF). CASE REPORT: An 18-year-old African American male with SCD was admitted after multiple days of fever and severe generalized body pain. He received monthly blood transfusions as stroke prophylaxis. A source of infection was not readily identified, but treatment was initiated with continuous intravenous fluids and empiric antibiotics. The patient developed acute renal failure, acute hypoxic respiratory failure, and shock. He underwent red blood cell (RBC) exchange transfusion followed by therapeutic plasma exchange and continuous veno-venous hemodialysis. A manual peripheral blood smear revealed intraerythrocytic inclusions suggestive of Plasmodium, and molecular studies confirmed Plasmodium falciparum infection. Intravenous artesunate was given daily for 1 week. A look-back investigation involving two hospitals, multiple blood suppliers, and state and federal public health departments identified the source of malaria as a unit of RBCs transfused 2 weeks prior to admission. CONCLUSIONS: Clinical suspicion for transfusion-related adverse events, including hemolytic transfusion reactions and transfusion-transmitted infections, should be high in typically and atypically immunocompromised patient populations (like SCD), especially those on chronic transfusion protocols. Manual blood smear review aids in the evaluation of patients with SCD presenting with severe vaso-occlusive crisis and MSOF and can alert clinicians to the need for initiating aggressive therapy like RBC exchange and artesunate therapy.

Structure of the DNA deaminase domain of the HIV-1 restriction factor APOBEC3G.

The human APOBEC3G (apolipoprotein B messenger-RNA-editing enzyme, catalytic polypeptide-like 3G) protein is a single-strand DNA deaminase that inhibits the replication of human immunodeficiency virus-1 (HIV-1), other retroviruses and retrotransposons. APOBEC3G anti-viral activity is circumvented by most retroelements, such as through degradation by HIV-1 Vif. APOBEC3G is a member of a family of polynucleotide cytosine deaminases, several of which also target distinct physiological substrates. For instance, APOBEC1 edits APOB mRNA and AID deaminates antibody gene DNA. Although structures of other family members exist, none of these proteins has elicited polynucleotide cytosine deaminase or anti-viral activity. Here we report a solution structure of the human APOBEC3G catalytic domain. Five alpha-helices, including two that form the zinc-coordinating active site, are arranged over a hydrophobic platform consisting of five beta-strands. NMR DNA titration experiments, computational modelling, phylogenetic conservation and Escherichia coli-based activity assays combine to suggest a DNA-binding model in which a brim of positively charged residues positions the target cytosine for catalysis. The structure of the APOBEC3G catalytic domain will help us to understand functions of other family members and interactions that occur with pathogenic proteins such as HIV-1 Vif.

A Comparison of Two Single-Stranded DNA Binding Models by Mutational Analysis of APOBEC3G.

APOBEC3G is the best known of several DNA cytosine deaminases that function to inhibit the replication of parasitic genetic elements including the lentivirus HIV. Several high-resolution structures of the APOBEC3G catalytic domain have been generated, but none reveal how this enzyme binds to substrate single-stranded DNA. Here, we constructed a panel of APOBEC3G amino acid substitution mutants and performed a series of biochemical, genetic, and structural assays to distinguish between "Brim" and "Kink" models for single-strand DNA binding. Each model predicts distinct sets of interactions between surface arginines and negatively charged phosphates in the DNA backbone. Concordant with both models, changing the conserved arginine at position 313 to glutamate abolished both catalytic and restriction activities. In support of the Brim model, arginine to glutamate substitutions at positions 213, 215, and 320 also compromised these APOBEC3G activities. Arginine to glutamate substitutions at Kink model residues 374 and 376 had smaller effects. These observations were supported by A3G catalytic domain-ssDNA chemical shift perturbation experiments. The overall data set is most consistent with the Brim model for single-stranded DNA binding by APOBEC3G.

Crystal structure of the APOBEC3G catalytic domain reveals potential oligomerization interfaces.

APOBEC3G is a DNA cytidine deaminase that has antiviral activity against HIV-1 and other pathogenic viruses. In this study the crystal structure of the catalytically active C-terminal domain was determined to 2.25 A. This structure corroborates features previously observed in nuclear magnetic resonance (NMR) studies, a bulge in the second beta strand and a lengthening of the second alpha helix. Oligomerization is postulated to be critical for the function of APOBEC3G. In this structure, four extensive intermolecular interfaces are observed, suggesting potential models for APOBEC3G oligomerization. The structural and functional significance of these interfaces was probed by solution NMR and disruptive variants were designed and tested for DNA deaminase and anti-HIV activities. The variant designed to disrupt the most extensive interface lost both activities. NMR solution data provides evidence that another interface, which coordinates a novel zinc site, also exists. Thus, the observed crystallographic interfaces of APOBEC3G may be important for both oligomerization and function.

An extended structure of the APOBEC3G catalytic domain suggests a unique holoenzyme model.

Human APOBEC3G (A3G) belongs to a family of polynucleotide cytidine deaminases. This family includes APOBEC1 and AID, which edit APOB mRNA and antibody gene DNA, respectively. A3G deaminates cytidines to uridines in single-strand DNA and inhibits the replication of human immunodeficiency virus-1, other retroviruses, and retrotransposons. Although the mechanism of A3G-catalyzed DNA deamination has been investigated genetically and biochemically, atomic details are just starting to emerge. Here, we compare the DNA cytidine deaminase activities and NMR structures of two A3G catalytic domain constructs. The longer A3G191-384 protein is considerably more active than the shorter A3G198-384 variant. The longer structure has an alpha1-helix (residues 201-206) that was not apparent in the shorter protein, and it contributes to catalytic activity through interactions with hydrophobic core structures (beta1, beta3, alpha5, and alpha6). Both A3G catalytic domain solution structures have a discontinuous beta2 region that is clearly different from the continuous beta2 strand of another family member, APOBEC2. In addition, the longer A3G191-384 structure revealed part of the N-terminal pseudo-catalytic domain, including the interdomain linker and some of the last alpha-helix. These structured residues (residues 191-196) enabled a novel full-length A3G model by providing physical overlap between the N-terminal pseudo-catalytic domain and the new C-terminal catalytic domain structure. Contrary to predictions, this structurally constrained model suggested that the two domains are tethered by structured residues and that the N- and C-terminal beta2 regions are too distant from each other to participate in this interaction.