Epigenomic analysis of the HOX gene loci reveals mechanisms that may control canonical expression patterns in AML and normal hematopoietic cells

HOX genes are highly expressed in many acute myeloid leukemia (AML) samples, but the patterns of expression and associated regulatory mechanisms are not clearly understood. We analyzed RNA sequencing data from 179 primary AML samples and normal hematopoietic cells to understand the range of expression patterns in normal versus leukemic cells. HOX expression in AML was restricted to specific genes in the HOXA or HOXB loci, and was highly correlated with recurrent cytogenetic abnormalities. However, the majority of samples expressed a canonical set of HOXA and HOXB genes that was nearly identical to the expression signature of normal hematopoietic stem/progenitor cells. Transcriptional profiles at the HOX loci were similar between normal cells and AML samples, and involved bidirectional transcription at the center of each gene cluster. Epigenetic analysis of a subset of AML samples also identified common regions of chromatin accessibility in AML samples and normal CD34(+) cells that displayed differences in methylation depending on HOX expression patterns. These data provide an integrated epigenetic view of the HOX gene loci in primary AML samples, and suggest that HOX expression in most AML samples represents a normal stem cell program that is controlled by epigenetic mechanisms at specific regulatory elements.

State of the art in myeloid sarcoma

INTRODUCTION

Myeloid sarcomas are extramedullary lesions composed of myeloid lineage blasts that typically form tumorous masses and may precede, follow, or occur in the absence of systemic acute myeloid leukemia. They most commonly involve the skin and soft tissues, lymph nodes, and gastrointestinal tract and are particularly challenging to diagnose in patients without an antecedent history of acute myeloid leukemia.

METHODS

We conducted a search of the English language medical literature for recent studies of interest to individuals involved in the diagnosis of myeloid sarcoma.

RESULTS

The differential diagnosis includes non-Hodgkin lymphoma, blastic plasmacytoid dendritic cell neoplasm, histiocytic sarcoma, melanoma, carcinoma, and (in children) small round blue cell tumors. The sensitivity and specificity of immunohistochemical markers must be considered when evaluating a suspected case of myeloid sarcoma. A high percentage of tested cases have cytogenetic abnormalities.

CONCLUSION

A minimal panel of immunohistochemical markers should include anti-CD43 or anti-lysozyme as a lack of immunoreactivity for either of these sensitive markers would be inconsistent with a diagnosis of myeloid sarcoma. Use of more specific markers of myeloid disease, such as CD33, myeloperoxidase, CD34 and CD117 is necessary to establish the diagnosis. Other antibodies may be added depending on the differential diagnosis. Identification of acute myeloid leukemia-associated genetic lesions may be helpful in arriving at the correct diagnosis.

von Hippel-Lindau protein mutants linked to type 2C VHL disease preserve the ability to downregulate HIF

von Hippel-Lindau (VHL) disease is a hereditary cancer syndrome caused by germ line mutation of the von Hippel-Lindau tumor suppressor gene (VHL). Tumors observed in this disorder include retinal and central nervous system hemangioblastomas, clear cell renal carcinomas and pheochromocytomas. The VHL gene product, pVHL, is a component of a ubiquitin ligase which targets the transcription factor known as hypoxia-inducible factor (HIF) for degradation in the presence of oxygen. pVHL also plays roles in the control of extracellular matrix formation and cell-cycle exit. Different VHL mutations confer different site-specific risks of cancer. Type 2C VHL mutations confer an increased risk of pheochromocytoma without the other stigmata of VHL disease. Here we report that the products of such type 2C VHL alleles retain the ability to down regulate HIF but are defective for promotion of fibronectin matrix assembly. Furthermore, pVHL L188V, a well studied type 2C mutant, retained the ability to suppress renal carcinoma growth in vivo. These studies strengthen the notion that HIF deregulation plays a causal role in hemangioblastoma and renal carcinoma, and raises the possibility that abnormal fibronectin matrix assembly contributes to pheochromocytoma pathogenesis in the setting of VHL disease.

Suppression of tumor growth through disruption of hypoxia-inducible transcription

Chronic hypoxia, a hallmark of many tumors, is associated with angiogenesis and tumor progression. Strategies to treat tumors have been developed in which tumor cells are targeted with drugs or gene-therapy vectors specifically activated under hypoxic conditions. Here we report a different approach, in which the normal transcriptional response to hypoxia is selectively disrupted. Our data indicate that specific blockade of the interaction of hypoxia-inducible factor with the CH1 domain of its p300 and CREB binding protein transcriptional coactivators leads to attenuation of hypoxia-inducible gene expression and diminution of tumor growth. Thus, disrupting the normal co-activational response to hypoxia may be a new and useful therapeutic strategy.