Case 266: Pontine Tegmental Cap Dysplasia.

History A 1-year-old boy was referred for cochlear implant assessment after he received a diagnosis of bilateral profound sensorineural hearing loss at neonatal hearing screening shortly after birth. The child was born at term via uneventful delivery, and there was no history of familial hearing loss or maternal illness. Tympanic membranes were normal, and hearing loss was confirmed with auditory brainstem testing, which showed no response from either ear. Hearing aids were provided from 3 months of age, but no behavioral responses were noted when these were worn. He was also noted to have some mild developmental delay throughout his 1st year of life and was slow to crawl, roll over, and stand up. Physical examination showed no syndromic features or physical abnormalities. Ophthalmology confirmed normal vision and visual movements but bilateral anesthetic corneas. He had corneal abrasions due to minor repeated corneal trauma, and left-sided tarsorraphy was performed at 6 months. Facial nerve function, swallow, and voice quality were normal. To assess suitability for a cochlear implant, the patient underwent MRI of the temporal lobe and brain and thin-section CT of the temporal bones. The patient subsequently underwent left cochlear implantation.

Case 266.

History A 1-year-old boy was referred for cochlear implant assessment after he received a diagnosis of bilateral profound sensorineural hearing loss at neonatal hearing screening shortly after birth. The child was born at term via uneventful delivery, and there was no history of familial hearing loss or maternal illness. Tympanic membranes were normal, and hearing loss was confirmed with auditory brainstem testing, which showed no response from either ear. Hearing aids were provided from 3 months of age, but no behavioral responses were noted when these were worn. He was also noted to have some mild developmental delay throughout his 1st year of life and was slow to crawl, roll over, and stand up. Physical examination showed no syndromic features or physical abnormalities. Ophthalmology confirmed normal vision and visual movements but bilateral anesthetic corneas. He had corneal abrasions due to minor repeated corneal trauma, and left-sided tarsorraphy was performed at 6 months. Facial nerve function, swallow, and voice quality were normal. To assess suitability for a cochlear implant, the patient underwent MRI of the temporal lobe and brain ( Figs 1 - 4 ) and thin-section CT of the temporal bones ( Figs 5 , 6 ). The patient subsequently underwent left cochlear implantation. Figure 1: Sagittal midline T1-weighted 1.5-T MR image (repetition time msec/echo time msec, 541/15). Figure 2: Axial T2-weighted MR image (6703/116, 4-mm section thickness) at the level of the pontomesencephalic junction. Figure 3: Axial T2-weighted MR image one level inferior to that shown in Figure 2 . Figure 4a: (a) Axial 1.5-T single-slab three-dimensional (3D) turbo spin-echo MR image (1200/271) at the level of the right internal auditory canal. (b) Corresponding axial 1.5-T single-slab 3D turbo spin-echo MR image at the level of the left internal auditory canal. Figure 4b: (a) Axial 1.5-T single-slab three-dimensional (3D) turbo spin-echo MR image (1200/271) at the level of the right internal auditory canal. (b) Corresponding axial 1.5-T single-slab 3D turbo spin-echo MR image at the level of the left internal auditory canal. Figure 5: Unenhanced axial thin-section (0.7-mm section thickness) CT image of the temporal bone at the level of the internal auditory canals and middle cochlear turns. Figure 6: Unenhanced axial thin-section (0.7-mm section thickness) CT image of the temporal bone at a level slightly inferior to that shown in Figure 5 .

Rep provides a second motor at the replisome to promote duplication of protein-bound DNA.

Nucleoprotein complexes present challenges to genome stability by acting as potent blocks to replication. One attractive model of how such conflicts are resolved is direct targeting of blocked forks by helicases with the ability to displace the blocking protein-DNA complex. We show that Rep and UvrD each promote movement of E. coli replisomes blocked by nucleoprotein complexes in vitro, that such an activity is required to clear protein blocks (primarily transcription complexes) in vivo, and that a polarity of translocation opposite that of the replicative helicase is critical for this activity. However, these two helicases are not equivalent. Rep but not UvrD interacts physically and functionally with the replicative helicase. In contrast, UvrD likely provides a general means of protein-DNA complex turnover during replication, repair, and recombination. Rep and UvrD therefore provide two contrasting solutions as to how organisms may promote replication of protein-bound DNA.

Unwinding of forked DNA structures by UvrD.

Many studies have demonstrated the need for processing of blocked replication forks to underpin genome duplication. UvrD helicase in Escherichia coli has been implicated in the processing of damaged replication forks, or the recombination intermediates formed from damaged forks. Here we show that UvrD can unwind forked DNA structures, in part due to the ability of UvrD to initiate unwinding from discontinuities within the phosphodiester backbone of DNA. UvrD does therefore have the capacity to target DNA intermediates of replication and recombination. Such an activity resulted in unwinding of what would be the parental duplex DNA ahead of either a stalled replication fork or a D-loop formed by recombination. However, UvrD had a substrate preference for fork structures having a nascent lagging strand at the branch point but no leading strand. Furthermore, at such structures the polarity of UvrD altered so that unwinding of the lagging strand predominated. This reaction is reminiscent of the PriC-Rep pathway of replication restart, suggesting that UvrD and Rep may have at least partially redundant functions.

PriA helicase and SSB interact physically and functionally.

PriA helicase is the major DNA replication restart initiator in Escherichia coli and acts to reload the replicative helicase DnaB back onto the chromosome at repaired replication forks and D-loops formed by recombination. We have discovered that PriA-catalysed unwinding of branched DNA substrates is stimulated specifically by contact with the single-strand DNA binding protein of E.coli, SSB. This stimulation requires binding of SSB to the initial DNA substrate and is effected via a physical interaction between PriA and the C-terminus of SSB. Stimulation of PriA by the SSB C-terminus may act to ensure that efficient PriA-catalysed reloading of DnaB occurs only onto the lagging strand template of repaired forks and D-loops. Correlation between the DNA repair and recombination defects of strains harbouring an SSB C-terminal mutation with inhibition of this SSB-PriA interaction in vitro suggests that SSB plays a critical role in facilitating PriA-directed replication restart. Taken together with previous data, these findings indicate that protein-protein interactions involving SSB may coordinate replication fork reloading from start to finish.

Stimulation of UvrD helicase by UvrAB.

Helicases play critical roles in all aspects of nucleic acid metabolism by catalyzing the remodeling of DNA and RNA structures. UvrD is an abundant helicase in Escherichia coli with well characterized functions in mismatch and nucleotide excision repair and a possible role in displacement of proteins such as RecA from single-stranded DNA. The mismatch repair protein MutL is known to stimulate UvrD. Here we show that the nucleotide excision repair proteins UvrA and UvrB can together stimulate UvrD-catalyzed unwinding of a range of DNA substrates containing strand discontinuities, including forked DNA substrates. The stimulation is specific for UvrD, as UvrAB failed to stimulate Rep helicase, a UvrD homologue. Moreover, although UvrAB can promote limited strand displacement, stimulation of UvrD did not require the strand displacement function of UvrAB. We conclude that UvrAB, like MutL, modulate UvrD helicase activity. This stimulation likely plays a role in DNA strand and protein displacement by UvrD in nucleotide excision repair. Promotion of UvrD-catalyzed unwinding of nicked duplexes by UvrAB may also explain the need for UvrAB and UvrD in Okazaki fragment processing in cells lacking DNA polymerase I. More generally, these data support the idea that helicase activity is regulated in vivo, with helicases acting as part of multisubunit complexes rather than in isolation.

Implications of recent statin trials for primary care practice.

3-Hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors (statins) are the first-line treatment for dyslipidemia and the results of large statin trials have a significant impact on guidelines for cardiovascular disease (CVD) management, such as those set by the National Cholesterol Education Program Adult Treatment Panel. The benefit of statin therapy in CVD prevention has traditionally been demonstrated in clinical trials by the superior efficacy of statins vs placebo in lowering low-density lipoprotein cholesterol (LDL-C) and preventing hard coronary heart disease (CHD) outcomes including myocardial infarction and CHD death. However, due to earlier and improved treatment of CVD, the clinical manifestations of atherosclerosis are changing and other forms of CVD are now thought to predominate (such as revascularization and stroke). These changes in how CVD manifests in the patient population may have consequences for selection of endpoints when designing future clinical trials. Recent statin trials have also demonstrated the early and improved clinical benefit of lowering LDL-C beyond traditional goals with intensive statin therapy vs more moderate lipid-lowering therapy. This review assesses the impact of early statin trials on current CVD management guidelines, summarizes results of recent landmark statin trials, and evaluates the potential implications of these studies for future clinical trials and CVD management guidelines.

PriB stimulates PriA helicase via an interaction with single-stranded DNA.

The frequency with which replication forks break down in all organisms requires that specific mechanisms ensure completion of genome duplication. In Escherichia coli a major pathway for reloading of the replicative apparatus at sites of fork breakdown is dependent on PriA helicase. PriA acts in conjunction with PriB and DnaT to effect loading of the replicative helicase DnaB back onto the lagging strand template, either at stalled fork structures or at recombination intermediates. Here we showed that PriB stimulates PriA helicase, acting to increase the apparent processivity of PriA. This stimulation correlates with the ability of PriB to form a ternary complex with PriA and DNA structures containing single-stranded DNA, suggesting that the known single-stranded DNA binding function of PriB facilitates unwinding by PriA helicase. This enhanced apparent processivity of PriA might play an important role in generating single-stranded DNA at stalled replication forks upon which to load DnaB. However, stimulation of PriA by PriB is not DNA structure-specific, demonstrating that targeting of stalled forks and recombination intermediates during replication restart likely resides with PriA alone.