The DNA sequence and comparative analysis of human chromosome 20

The finished sequence of human chromosome 20 comprises 59,187,298 base pairs (bp) and represents 99.4% of the euchromatic DNA. A single contig of 26 megabases (Mb) spans the entire short arm, and five contigs separated by gaps totalling 320 kb span the long arm of this metacentric chromosome. An additional 234,339 bp of sequence has been determined within the pericentromeric region of the long arm. We annotated 727 genes and 168 pseudogenes in the sequence. About 64% of these genes have a 5' and a 3' untranslated region and a complete open reading frame. Comparative analysis of the sequence of chromosome 20 to whole-genome shotgun-sequence data of two other vertebrates, the mouse Mus musculus and the puffer fish Tetraodon nigroviridis, provides an independent measure of the efficiency of gene annotation, and indicates that this analysis may account for more than 95% of all coding exons and almost all genes.

Cochlear protection from carbon monoxide exposure by free radical blockers in the guinea pig

Acute carbon monoxide exposure produces a significant impairment in high-frequency auditory sensitivity that can be prevented using the N-methyl-D-aspartate receptor blocker MK-801. This finding suggests an excitotoxic component to carbon monoxide ototoxicity and establishes the potential for free radical formation. Free radical scavengers and inhibitors are protective in many organs, including the brain and cochlea, during hypoxic events such as ischemia/reperfusion and, in the cochlea, during noise exposure. This study evaluated the protection afforded by two such agents, phenyl-n-tert-butyl-nitrone (PBN), which acts as a general free radical scavenger, and allopurinol, which acts as a free radical inhibitor specific to the xanthine oxidase metabolic pathway. Guinea pigs were pretreated with PBN (100 mg/kg i.p.), allopurinol (100 mg/kg i.p.), or saline 1 hr prior to exposure to carbon monoxide (35 ml/kg i.p.) or to an equal volume of air. They were monitored at 15, 30, and 60 min after carbon monoxide exposure for alterations in compound action potential threshold and cochlear microphonic amplitude. The groups receiving carbon monoxide alone displayed characteristic compound action potential threshold elevations particularly at the higher test frequencies (16-40 kHz), consistent with earlier studies; no loss of cochlear microphonic amplitude was exhibited. Both free radical inhibitors, PBN and allopurinol, blocked loss of auditory threshold sensitivity produced by carbon monoxide. These data suggest that free radical generation may play a significant role in the impairment of high-frequency auditory sensitivity resulting from carbon monoxide.