One size fits one: pharmacogenetics in gastroenterology.

Individual variability in response and development of adverse effects to drugs is a major challenge in clinical practice. Pharmacogenomics refers to the aspect of personalized medicine where the patient's genetic information instructs the selection and dosage of therapy while also predicting its adverse effects profile. Sequencing of the entire human genome has given us the opportunity to study commonly used drugs as well as newer therapeutic agents in a new light, opening up opportunities for better drug efficacy and decreased adverse effects. This article highlights developments in pharmacogenomics, relates these to practice of gastroenterology, and outlines roadblocks in translation of this knowledge into clinical practice.

Autonomous murine T-cell progenitor production in the extra-embryonic yolk sac before HSC emergence.

The extra-embryonic yolk sac (YS) is the first hematopoietic site in the mouse embryo and is thought to generate only primitive erythroid and myeloerythroid progenitor cells before definitive HSC emergence within the embryo on E10.5. Here, we have shown the existence of T cell-restricted progenitors in the E9.5 YS that directly engraft in recipient immunodeficient mice. T-cell progenitors were also produced in vitro from both YS and para-aortic splanchnopleura hemogenic endothelial cells, and these T-cell progenitors repopulated the thymus and differentiated into mature T-cell subsets in vivo on transplantation. Our data confirm that the YS produces T-lineage-restricted progenitors that are available to colonize the thymus and provide new insight into the YS as a definitive hematopoietic site in the mouse embryo.

Embryonic day 9 yolk sac and intra-embryonic hemogenic endothelium independently generate a B-1 and marginal zone progenitor lacking B-2 potential.

The majority of B lymphocytes in the adult mouse are generated in the bone marrow from hematopoietic stem cells (HSCs) that first appear in the aorta-gonado-mesonephros region of the fetus on embryonic day (E) 10.5-11. Comparatively less is known about B-cell development during embryogenesis. For example, which specific embryonic tissues participate in B lymphopoiesis and whether hematopoietic differentiation is skewed toward specific B-cell subsets in the embryo are unanswered questions, because the systemic circulation is initiated early during embryogenesis, resulting in an admixture of cells potentially originating from multiple sites. We demonstrate, using Ncx1(-/-) mice that lack systemic blood circulation, that the E9 yolk sac (YS) and the intra-embryonic para-aortic splanchnopleura (P-Sp) tissues independently give rise to AA4.1(+)CD19(+)B220(lo-neg) B progenitor cells that preferentially differentiate into innate type B-1 and marginal zone (MZ) B cells but not into B-2 cells upon transplantation. We have further demonstrated that these B-1 progenitor cells arise directly from YS and P-Sp hemogenic endothelium. These results document the initial wave of innate B lymphopoietic progenitor cells available for seeding the fetal liver at E11. The results of these studies expand our knowledge of hemogenic endothelial sites specifying distinct B-1 and MZ cell fates apart from B-2 cells and independent of an HSC origin during development.

An Unusual Case of Gastric Heterotopia Presenting as Rectal Prolapse.

Heterotopic gastric mucosa (HGM) involving the rectum is an uncommon finding. It is especially rare in young children. Rectal prolapse is an uncommon presentation of HGM. We report a case of HGM in the rectum of a 2-year-old previously healthy girl, who presented with rectal prolapse and painless bleeding. Endoscopic mucosal resection was performed to completely resect the lesion after the patient failed to respond to proton pump inhibitors. This case underscores the importance of considering HGM involving the rectum as a cause of rectal prolapse in young pediatric patients.

Maintenance of the BMP4-dependent stress erythropoiesis pathway in the murine spleen requires hedgehog signaling.

The production of mature cells necessitates that lineage-committed progenitor cells be constantly generated from multipotential progenitors. In addition, the ability to respond rapidly to physiologic stresses requires that the signals that regulate the maintenance of progenitor populations be coordinated with the signals that promote differentiation of progenitors. Here we examine the signals that are necessary for the maintenance of the BMP4-dependent stress erythropoiesis pathway. Our previous work demonstrated that BMP4, stem cell factor, and hypoxia act in concert to promote the expansion of a specialized population of stress erythroid progenitors in the spleen during the recovery from acute anemia. Our analysis shows that acute anemia leads to an almost complete mobilization of BMP4-responsive stress erythroid burst-forming units; therefore, new stress progenitors must be recruited to the spleen to replenish this system. We show that bone marrow cells can home to the spleen and, in response to a signal in the spleen microenvironment, Hedgehog, they develop into BMP4-responsive stress progenitors. Hedgehog induces the expression of BMP4, and together these 2 signals are required for the development of BMP4-responsive stress progenitors. These data demonstrate that the interplay between these 2 signals is crucial for maintenance of this stress response pathway.

Identification of TWSG1 as a second novel erythroid regulator of hepcidin expression in murine and human cells.

In thalassemia and other iron loading anemias, ineffective erythropoiesis and erythroid signaling molecules are thought to cause inappropriate suppression of a small peptide produced by hepatocytes named hepcidin. Previously, it was reported that the erythrokine GDF15 is expressed at very high levels in thalassemia and suppresses hepcidin expression. In this study, erythroblast expression of a second molecule named twisted gastrulation (TWSG1) was explored as a potential erythroid regulator of hepcidin. Transcriptome analyses suggest TWSG1 is produced during the earlier stages of erythropoiesis. Hepcidin suppression assays demonstrated inhibition by TWSG1 as measured by quantitative polymerase chain reaction (PCR) in dosed assays (1-1000 ng/mL TWSG1). In human cells, TWSG1 suppressed hepcidin indirectly by inhibiting the signaling effects and associated hepcidin up-regulation by bone morphogenic proteins 2 and 4 (BMP2/BMP4). In murine hepatocytes, hepcidin expression was inhibited by murine Twsg1 in the absence of additional BMP. In vivo studies of Twsg1 expression were performed in healthy and thalassemic mice. Twsg1 expression was significantly increased in the spleen, bone marrow, and liver of the thalassemic animals. These data demonstrate that twisted gastrulation protein interferes with BMP-mediated hepcidin expression and may act with GDF15 to dysregulate iron homeostasis in thalassemia syndromes.

BMP4/Smad5 dependent stress erythropoiesis is required for the expansion of erythroid progenitors during fetal development.

The rapid growth of the embryo places severe demands on the ability of the cardiovascular system to deliver oxygen to cells. To meet this need, erythroid progenitors rapidly expand in the fetal liver microenvironment such that by E14.5, erythropoiesis predominates in the fetal liver. In this report we show that the BMP4/Smad5 dependent stress erythropoiesis pathway plays a key role in the expansion of erythroid progenitors in the fetal liver. These data show that the fetal liver contains two populations of erythroid progenitors. One population resembles the steady state erythroid progenitors found in the adult bone marrow. While the second population exhibits the properties of stress erythroid progenitors found in adult spleen. Here we demonstrate that defects in BMP4/Smad5 signaling preferentially affect the expansion of the stress erythroid progenitors in the fetal liver leading to fetal anemia. These data suggest that steady state erythropoiesis is unable to generate sufficient erythrocytes to maintain the rapid growth of the embryo leading to the induction of the BMP4 dependent stress erythropoiesis pathway. These observations underscore the similarities between fetal erythropoiesis and stress erythropoiesis.

Friend virus utilizes the BMP4-dependent stress erythropoiesis pathway to induce erythroleukemia.

More than 50 years of genetic analysis has identified a number of host genes that are required for the expansion of infected cells during the progression of Friend-virus-induced erythroleukemia. In this report, we show that Friend virus induces the bone morphogenetic protein 4 (BMP4)-dependent stress erythropoiesis pathway in the spleen, which rapidly amplifies target cells, propagating their infection and resulting in acute splenomegaly. This mechanism mimics the response to acute anemia, in which BMP4 expressed in the spleen drives the expansion of a specialized population of stress erythroid progenitors. Previously we demonstrated that these progenitors, termed stress BFU-E, are targets for Friend virus in the spleen (A. Subramanian, H. E. Teal, P. H. Correll, and R. F. Paulson, J. Virol. 79:14586-14594, 2005). Here, we extend those findings by showing that Friend virus infects two distinct populations of bone marrow cells. One population, when infected, differentiates into mature erythrocytes in an Epo-independent manner, while a second population migrates to the spleen after infection, where it induces BMP4 expression and acts as a reservoir of virus. The activation of the stress erythropoiesis pathway in the spleen by Friend virus results in the rapid expansion of stress BFU-E, providing abundant target cells for viral infection. These observations suggest a novel mechanism by which a virus induces a stress response pathway that amplifies target cells for the virus, leading to acute expansion of infected cells.

Overcoming obstacles in the search for the site of hematopoietic stem cell emergence.

The murine embryo has become a valuable tool to examine the ontogeny of hematopoiesis. However, the onset of the systemic circulation has long been a confounding developmental variable that may mask the site of blood cell emergence. This Minireview examines some approaches that have been applied to overcome this obstacle.

An intronic sequence mutated in flexed-tail mice regulates splicing of Smad5.

Recent work has identified a growing body of evidence that subtle changes in noncoding sequences can result in significant pathology. These mutations, which would have been called silent polymorphisms in the past, affect gene transcription and mRNA splicing and lead to drastic changes in gene expression. Previous work from our lab has characterized the murine flexed-tail (f) mutation, which encodes Smad5, a transcription factor that functions downstream of the receptors for bone morphogenetic proteins (BMPs). f/f mice are unable to rapidly respond to acute anemia. Our analysis of these mice led to the development of a new model for stress erythropoiesis, where BMP4 expression in the spleen leads to the Smad5-dependent expansion of a specialized population of stress erythroid progenitors during the recovery from acute anemia. f/f mutant mice exhibit a defect in Smad5 mRNA splicing in the spleen such that the majority of Smad5 transcripts are two misspliced mRNAs. One of these mRNAs encodes a truncated form of Smad5 that inhibits BMP4 signaling when overexpressed. Here we show that a mutation in a poly(T) element in intron 4 causes the splicing defect in f/f mutant mice. This subtle mutation (loss of 1 or 2 Ts in a 16-T element) results in defects in splicing throughout the Smad5 gene. Furthermore, we show that this mutation results in tissue-specific splicing defects, which may explain why f/f mice are viable when Smad5-/- mice are embryonic lethal.