Evolution of DNA replication origin specification and gene silencing mechanisms.

DNA replication in eukaryotic cells initiates from replication origins that bind the Origin Recognition Complex (ORC). Origin establishment requires well-defined DNA sequence motifs in Saccharomyces cerevisiae and some other budding yeasts, but most eukaryotes lack sequence-specific origins. A 3.9 Å structure of S. cerevisiae ORC-Cdc6-Cdt1-Mcm2-7 (OCCM) bound to origin DNA revealed that a loop within Orc2 inserts into a DNA minor groove and an α-helix within Orc4 inserts into a DNA major groove. Using a massively parallel origin selection assay coupled with a custom mutual-information-based modeling approach, and a separate analysis of whole-genome replication profiling, here we show that the Orc4 α-helix contributes to the DNA sequence-specificity of origins in S. cerevisiae and Orc4 α-helix mutations change genome-wide origin firing patterns. The DNA sequence specificity of replication origins, mediated by the Orc4 α-helix, has co-evolved with the gain of ORC-Sir4-mediated gene silencing and the loss of RNA interference.

Comparison of the theoretical and real-world evolutionary potential of a genetic circuit.

With the development of next-generation sequencing technologies, many large scale experimental efforts aim to map genotypic variability among individuals. This natural variability in populations fuels many fundamental biological processes, ranging from evolutionary adaptation and speciation to the spread of genetic diseases and drug resistance. An interesting and important component of this variability is present within the regulatory regions of genes. As these regions evolve, accumulated mutations lead to modulation of gene expression, which may have consequences for the phenotype. A simple model system where the link between genetic variability, gene regulation and function can be studied in detail is missing. In this article we develop a model to explore how the sequence of the wild-type lac promoter dictates the fold-change in gene expression. The model combines single-base pair resolution maps of transcription factor and RNA polymerase binding energies with a comprehensive thermodynamic model of gene regulation. The model was validated by predicting and then measuring the variability of lac operon regulation in a collection of natural isolates. We then implement the model to analyze the sensitivity of the promoter sequence to the regulatory output, and predict the potential for regulation to evolve due to point mutations in the promoter region.

Preoperative gadolinium-enhanced magnetic resonance pulmonary venography in an adolescent with atrial septal defect.

The conventional preoperative evaluation of congenital heart disease in the adult using only echocardiography and cardiac catheterization is frequently limited. We report a case in which newer magnetic resonance imaging with gadolinium angiographic techniques proved helpful by conclusively documenting normal pulmonary venous return in a patient with secundum [corrected] atrial septal defect. This evaluation obviated the need for referral to a more specialized tertiary-care center for congenital heart disease and dramatized the value of an interdisciplinary approach to the adult patient with newly diagnosed congenital heart disease.

Contributions of newer MR imaging strategies for congenital heart disease.

Magnetic resonance (MR) imaging is a valuable noninvasive adjunct in the evaluation of congenital heart disease (CHD). With its multiplanar image acquisition, good spatial resolution, and large-field-of-view image display, MR imaging can allow appreciation of vascular connections not readily apparent at echocardiography or angiography. Evaluation of CHD with MR imaging has two components: definition of cardiovascular anatomy and characterization of blood flow. However, the variety of MR imaging options is large and often confusing. Besides spin-echo and gradient-echo imaging, MR imaging techniques for evaluation of CHD include MR angiography, cine MR imaging, and flow quantification. An understanding of the potential MR imaging options enables formulation and more efficient application of MR imaging strategies. When performed well, MR imaging greatly enhances surgical planning and can even obviate cardiac catheterization. MR imaging is also an excellent modality for serial evaluation of surgical results and complications in cases of CHD.

Ruptured sinus of Valsalva aneurysm: cine phase-contrast MR characterization.

Congenital sinus of Valsalva aneurysms are rare anomalies that usually go undetected unless they rupture. We report a case of a ruptured congenital sinus of Valsalva aneurysm, detected with cine phase-contrast MRI.

Quantitative perfusion imaging assessing acquired discrete peripheral pulmonary artery stenosis.

We present the case an 8-yr-old boy evaluated for anastomotic stenosis of the right pulmonary artery after surgical repair of hemitruncus at 6 wk of age. Pulmonary angiography revealed only mild narrowing and a 10-mm pressure gradient across the anastomosis, but quantitative perfusion imaging demonstrated that the right lung only received 16% of pulmonary blood flow. Subsequently, balloon angioplasty of the anastomotic site was performed, resulting in complete resolution of the stenosis and gradient. Early postangioplasty perfusion imaging demonstrated increased perfusion of the right lung to 35% of total pulmonary blood flow. It is theorized that initially a chronically hyperperfused lung may develop more capacious vessels and recruit new capillaries during the years of hyperperfusion such that a "perfect" angioplasty may result in less than symmetric perfusion. The inexpensive, noninvasive quantitative perfusion study is more sensitive and accurate in evaluating acquired (postsurgical) pulmonary artery stenoses.

Atypical Kawasaki disease: coronary aneurysms and thrombocytopenia.

The patient we have described had coronary artery aneurysms pathognomonic for Kawasaki disease after a nonspecific febrile illness not meeting the classic clinical criteria. The child had significant thrombocytopenia, which in the past has been thought to militate against the diagnosis of Kawasaki disease. Echocardiography should be considered in febrile infants with thrombocytopenia of uncertain etiology.