Hhex is required for early development of the liver. A null mutation of Hhex results in a failure to form the liver bud and embryonic lethality. Therefore, Hhex null mice are not informative as to whether this gene is required during later stages of hepatobiliary morphogenesis. To address this question, we derived Hhex conditional null mice using the Cre-loxP system and two different Cre transgenics (Foxa3-Cre and Alfp-Cre). Deletion of Hhex in the hepatic diverticulum (Foxa3-Cre;Hhex(d2,3/-)) led to embryonic lethality and resulted in a small and cystic liver with loss of Hnf4alpha and Hnf6 expression in early hepatoblasts. In addition, the gall bladder was absent and the extrahepatic bile duct could not be identified. Loss of Hhex in the embryonic liver (Alfp-Cre;Hhex(d2,3/-)) caused irregular development of intrahepatic bile ducts and an absence of Hnf1beta in many (cystic) biliary epithelial cells, which resulted in a slow, progressive form of polycystic liver disease in adult mice. Thus, we have shown that Hhex is required during multiple stages of hepatobiliary development. The altered expression of Hnf4alpha, Hnf6 and Hnf1beta in Hhex conditional null mice suggests that Hhex is an essential component of the genetic networks regulating hepatoblast differentiation and intrahepatic bile duct morphogenesis.

The homeodomain transcription factor HHEX (hematopoietically expressed homeobox) has been repeatedly linked to type 2 diabetes mellitus (T2DM) using genomewide association studies. We report here that within the adult endocrine pancreas, Hhex is selectively expressed in the somatostatin-secreting d cell. Using two mouse models with Hhex deficiency in the endocrine pancreas, we show that Hhex is required for d-cell differentiation. Decreased somatostatin levels in Hhex-deficient islets cause disrupted paracrine inhibition of insulin release from b cells. These findings identify Hhex as the first transcriptional regulator specifically required for islet d cells and suggest compromised paracrine control as a contributor to T2DM.

An extensive literature documents the existence of pervasive and persistent child health, development, and health care disparities by race, ethnicity, and socioeconomic status (SES). Disparities experienced during childhood can result in a wide variety of health and health care outcomes, including adult morbidity and mortality, indicating that it is crucial to examine the influence of disparities across the life course. Studies often collect data on the race, ethnicity, and SES of research participants to be used as covariates or explanatory factors. In the past, these variables have often been assumed to exert their effects through individual or genetically determined biologic mechanisms. However, it is now widely accepted that these variables have important social dimensions that influence health. SES, a multidimensional construct, interacts with and confounds analyses of race and ethnicity. Because SES, race, and ethnicity are often difficult to measure accurately, leading to the potential for misattribution of causality, thoughtful consideration should be given to appropriate measurement, analysis, and interpretation of such factors. Scientists who study child and adolescent health and development should understand the multiple measures used to assess race, ethnicity, and SES, including their validity and shortcomings and potential confounding of race and ethnicity with SES. The American Academy of Pediatrics (AAP) recommends that research on eliminating health and health care disparities related to race, ethnicity, and SES be a priority. Data on race, ethnicity, and SES should be collected in research on child health to improve their definitions and increase understanding of how these factors and their complex interrelationships affect child health. Furthermore, the AAP believes that researchers should consider both biological and social mechanisms of action of race, ethnicity, and SES as they relate to the aims and hypothesis of the specific area of investigation. It is important to measure these variables, but it is not sufficient to use these variables alone as explanatory for differences in disease, morbidity, and outcomes without attention to the social and biologic influences they have on health throughout the life course. The AAP recommends more research, both in the United States and internationally, on measures of race, ethnicity, and SES and how these complex constructs affect health care and health outcomes throughout the life course.

The homeobox gene Hhex has recently been shown to be essential for normal liver, thyroid and forebrain development. Hhex–/– mice die by mid-gestation (E14.5) and the cause of their early demise remains unclear. Because Hhex is expressed in the developing blood islands at E7.0 in the endothelium of the developing vasculature and heart at E9.0-9.5, and in the ventral foregut endoderm at E8.5-9.0, it has been postulated to play a critical role in heart and vascular development. We show here, for the first time, that a null mutation of Hhex results in striking abnormalities of cardiac and vascular development which include: (1) defective vasculogenesis, (2)hypoplasia of the right ventricle, (3) overabundant endocardial cushions accompanied by ventricular septal defects, outflow tract abnormalities and atrio-ventricular (AV) valve dysplasia and (4) aberrant development of the compact myocardium. The dramatic enlargement of the endocardial cushions in the absence of Hhex is due to decreased apoptosis and dysregulated epithelial-mesenchymal transformation (EMT). Interestingly, vascular endothelial growth factor A (Vegfa) levels in the hearts of Hhex–/– mice were elevated as much as three-fold between E9.5 and E11.5, and treatment of cultured Hhex–/– AV explants with truncated soluble Vegfa receptor 1, sFlt-1, an inhibitor of Vegf signaling, completely abolished the excessive epithelial-mesenchymal transformation seen in the absence of Hhex. Therefore, Hhex expression in the ventral foregut endoderm and/or the endothelium is necessary for normal cardiovascular development in vivo, and one function of Hhex is to repress Vegfa levels during development.

Hypoglycemia is common in critically ill children. It is associated with increased mortality rates in critically ill nondiabetic children. Our data suggest that hypoglycemia is also associated with worsening organ function. Hypoglycemia may merely be a marker of severity of illness. Further investigations are needed to establish the mortality risk with hypoglycemia due to insulin compared to spontaneous hypoglycemia.

Formation of the lymphatic system requires the coordinated expression of several key regulators: vascular endothelial growth factor C (VEGFC), its receptor FLT4, and a key transcriptional effector, PROX1. Yet, how expression of these signaling components is regulated remains poorly understood. Here, using a combination of genetic and molecular approaches, we identify the transcription factor hematopoietically expressed homeobox (HHEX) as an upstream regulator of VEGFC, FLT4, and PROX1 during angiogenic sprouting and lymphatic formation in vertebrates. By analyzing zebrafish mutants, we found that hhex is necessary for sprouting angiogenesis from the posterior cardinal vein, a process required for lymphangiogenesis. Furthermore, studies of mammalian HHEX using tissue-specific genetic deletions in mouse and knockdowns in cultured human endothelial cells reveal its highly conserved function during vascular and lymphatic development. Our findings that HHEX is essential for the regulation of the VEGFC/FLT4/PROX1 axis provide insights into the molecular regulation of lymphangiogenesis.

Unlike clustered HOX genes, the role of nonclustered homeobox gene family members in hematopoiesis and leukemogenesis has not been extensively studied. Here we found that the hematopoietically expressed homeobox gene Hhex is overexpressed in acute myeloid leukemia (AML) and is essential for the initiation and propagation of MLL-ENL-induced AML but dispensable for normal myelopoiesis, indicating a specific requirement for Hhex for leukemic growth. Loss of Hhex leads to expression of the Cdkn2a-encoded tumor suppressors p16INK4a and p19ARF, which are required for growth arrest and myeloid differentiation following Hhex deletion. Mechanistically, we show that Hhex binds to the Cdkn2a locus and directly interacts with the Polycomb-repressive complex 2 (PRC2) to enable H3K27me3-mediated epigenetic repression. Thus, Hhex is a potential therapeutic target that is specifically required for AML stem cells to repress tumor suppressor pathways and enable continued self-renewal.

Hex is a divergent homeobox gene expressed as early as E4.5 in the mouse and in a pattern that suggests a role in anterior‐posterior patterning. Later in embryogenesis, Hex is expressed in the developing thyroid, lung, and liver. We now show Hex expression during thymus, gallbladder, and pancreas development and in the adult thyroid, lung, and liver. At E10.0, Hex is expressed in the 3rd pharyngeal pouch, from which the thymus originates, the endodermal cells of liver that are invading the septum transversum, the thyroid, the dorsal pancreatic bud, and gallbladder primoridum. At E13.5, expression is maintained at high levels in the thyroid, liver, epithelial cells lining the pancreatic and extrahepatic biliary ducts and is present in both the epithelial and mesenchymal cells of the lung. Expression in the thymus at this age is less than in the other organs. In the E16.5 embryo, expression persists in the thyroid, pancreatic, and bile duct epithelium, lung, and liver, with thymic expression dropping to barely detectable levels. By E18.5, expression in the thyroid and bile ducts remains high, whereas lung expression is markedly decreased. At this age, expression in the pancreas and thymus is no longer present. Finally, we show the cell types in the adult thyroid, lung, and liver that express Hex in the mature animal. Our results provide more detail on the potential role of Hex in the development of several organs derived from foregut endoderm and in the maintenance of function of several of these organs in the mature animal. © 2000 Wiley‐Liss, Inc.