Multicenter evaluation of a commercial cytomegalovirus quantitative standard: effects of commutability on interlaboratory concordance.

Commutability of quantitative reference materials has proven important for reliable and accurate results in clinical chemistry. As international reference standards and commercially produced calibration material have become available to address the variability of viral load assays, the degree to which such materials are commutable and the effect of commutability on assay concordance have been questioned. To investigate this, 60 archived clinical plasma samples, which previously tested positive for cytomegalovirus (CMV), were retested by five different laboratories, each using a different quantitative CMV PCR assay. Results from each laboratory were calibrated both with lab-specific quantitative CMV standards ("lab standards") and with common, commercially available standards ("CMV panel"). Pairwise analyses among laboratories were performed using mean results from each clinical sample, calibrated first with lab standards and then with the CMV panel. Commutability of the CMV panel was determined based on difference plots for each laboratory pair showing plotted values of standards that were within the 95% prediction intervals for the clinical specimens. Commutability was demonstrated for 6 of 10 laboratory pairs using the CMV panel. In half of these pairs, use of the CMV panel improved quantitative agreement compared to use of lab standards. Two of four laboratory pairs for which the CMV panel was noncommutable showed reduced quantitative agreement when that panel was used as a common calibrator. Commutability of calibration material varies across different quantitative PCR methods. Use of a common, commutable quantitative standard can improve agreement across different assays; use of a noncommutable calibrator can reduce agreement among laboratories.

Reduction of Purkinje cell pathology in SCA1 transgenic mice by p53 deletion.

The expansion of a polyglutamine tract in the ataxin-1 protein beyond a critical threshold causes spinocerebellar ataxia type 1 (SCA1). To investigate the mechanism of neuronal degeneration in SCA1, we analyzed the phenotype of an SCA1 transgenic mouse model in the absence of p53, an important regulator of cell death. p53 deficiency did not affect the early features of SCA1 mice such as impaired motor coordination and ataxin-1 nuclear inclusion formation but caused a notable reduction in later pathological features, including Purkinje cell heterotopia, dendritic thinning, and molecular layer shrinkage. To determine if this protective effect was mediated by an anti-apoptotic property of p53 deficiency, we looked for apoptosis in SCA1 mice but failed to detect any evidence of it even in the presence of p53. We propose that p53 acts after the initial pathogenic events in SCA1 to promote the progression of neuronal degeneration in SCA1 mice, but this activity may be unrelated to apoptosis.

Molecular genetics of Rett syndrome and clinical spectrum of MECP2 mutations.

Rett syndrome, a neurodevelopmental disorder that is a leading cause of mental retardation in females, is caused by mutations in the X-linked gene encoding methyl-CpG-binding protein 2 (MeCP2). MECP2 mutations have subsequently been identified in patients with a variety of clinical syndromes ranging from mild learning disability in females to severe mental retardation, seizures, ataxia, and sometimes neonatal encephalopathy in males. In classic Rett syndrome, genotype-phenotype correlation studies suggest that X chromosome inactivation patterns have a more prominent effect on clinical severity than the type of mutation. When the full range of phenotypes associated with MECP2 mutations is considered, however, the mutation type strongly affects disease severity. MeCP2 is a transcriptional repressor that binds to methylated CpG dinucleotides throughout the genome, and mutations in Rett syndrome patients are thought to result in at least a partial loss of function. Abnormal gene expression may thus underlie the phenotype. Discovering which genes are misregulated in the absence of functional MeCP2 is crucial for understanding the pathogenesis of this disorder and related syndromes.