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To the Editor: Preservation of RNA integrity is important in microarray techniques for identifying differentially expressed genes so that results reflect true biological differences and not differences in RNA degradation (1 ). RNA degradation is usually low in RNA isolated from cultured cells (2 ). When samples isolated from RNaserich tissues are used, however, RNA degradation may introduce bias (3, 4). RNA extraction using Trizol (Invitrogen Life Technologies) is a common procedure in microarray experiments. Tissue homogenization at room temperature (15–25 °C), as specified in the manufacturer’s protocol, generates heat that may increase RNase activity. When snapfrozen tissue pieces are processed, only the outer surfaces are initially in contact with Trizol, and RNase activity in the deeper regions could adversely affect RNA integrity. We modified the Trizol protocol by (a) performing homogenization of frozen placental specimens in cold Trizol on wet ice (0–4 °C), (b) limiting homogenization time to 30-s intervals for up to 1.5 min to minimize heat generation at the expense of complete tissue disruption, and (c) pelleting cellular debris at 12 000g for 10 min at 4 °C rather than at room temperature (see the Supplemental Methods section in the Data Supplement that accompanies the online version of this Letter at http://www. clinchem.org/content/vol52/issue1/). Using capillary electrophoresis [RNA 6000 Nano LabChip (Agilent)] on an Agilent Bioanalyzer 2100 system (5 ), we compared RNA obtained by the standard method and by our modified methods. RNA degradation, if present, would appear as a decrease in the 28S/18S ratio (Fig. 1A), with an increase in the small RNAs peak area (peak I) and the baseline signal (bracket). We analyzed capillary electrophoresis data with Degradometer Ver. 1.41 software (available at http://www. dnaarrays.org) (2 ), which automatically rescaled the time axis so that the synchronization peak occurred at 23.0 s and the 28S peak occurred at 48.0 s. Rescaling allowed analysis of fixed time intervals: 30–41 s (degradation signal range), 41–42.5 s (18S peak range), and 48 s (28S peak range). The degradation factor (DF) was calculated as the ratio of the mean degradation signal value to the 18S peak value, expressed as a percentage. The 28S/18S rRNA ratio was also calculated. RNA degradation was categorized as follows: strong (DF 24%), severe (DF 16%), and detectable (DF 8%) (2 ). We evaluated 7 specimens from nonpathologic and pathologic placentas of various gestational ages (see Table 1 in the online Data Supplement). The 260/280 nm ratio for all samples was 1.97–2.13 and did not differ between the standard and modified methods. The mean (SE) total RNA yield (RNA weight/pla-