Abnormalities of Fetal Situs: An Overview and Literature Review.

The incidence of congenital heart disease is most commonly reported in 8 per 1000 live births. Situs anomalies are among the least common forms of congenital heart disease. This study defines situs and describes the variations of fetal situs anomalies. Situs refers to the arrangement of the viscera, atria, and vessels within the body. Situs solitus describes the normal arrangement; situs inversus describes inverted arrangement of the viscera and atria; and situs ambiguous (also referred to as heterotaxy) describes disturbances in arrangements that can neither be identified as solitus nor inversus. This review will concentrate on heterotaxy, as it is the most complicated situs abnormality to define, classify, and study. Prognosis of heterotaxy is variable but most correlated with cardiac anatomy. Management is concentrated on numerous cardiac operations and requires a multidisciplinary approach to address coexisting congenital anomalies.

Distinguishing the roles of Topoisomerases I and II in relief of transcription-induced torsional stress in yeast rRNA genes.

To better understand the role of topoisomerase activity in relieving transcription-induced supercoiling, yeast genes encoding rRNA were visualized in cells deficient for either or both of the two major topoisomerases. In the absence of both topoisomerase I (Top1) and topoisomerase II (Top2) activity, processivity was severely impaired and polymerases were unable to transcribe through the 6.7-kb gene. Loss of Top1 resulted in increased negative superhelical density (two to six times the normal value) in a significant subset of rRNA genes, as manifested by regions of DNA template melting. The observed DNA bubbles were not R-loops and did not block polymerase movement, since genes with DNA template melting showed no evidence of slowed elongation. Inactivation of Top2, however, resulted in characteristic signs of slowed elongation in rRNA genes, suggesting that Top2 alleviates transcription-induced positive supercoiling. Together, the data indicate that torsion in front of and behind transcribing polymerase I has different consequences and different resolution. Positive torsion in front of the polymerase induces supercoiling (writhe) and is largely resolved by Top2. Negative torsion behind the polymerase induces DNA strand separation and is largely resolved by Top1.