SMARCAD1, a novel human helicase family-defining member associated with genetic instability: cloning, expression, and mapping to 4q22-q23, a band rich in breakpoints and deletion mutants involved in several human diseases.

Members of the DEAD/H box-containing helicase superfamily include proteins essential to genome replication, repair, and expression. We report here the cloning and initial characterization of a novel human member of this protein family, designated hHel1 (human helicase 1), now designated SMARCAD1 by HUGO. This DEAD/H box-containing molecule has seven highly conserved sequence regions that allow us to place it in the SNF2 family of the helicase superfamily. Uniquely, though, hHel1 contains two DEAD/H box motifs, a property not reported to be shared by any other SNF2 family members. This defines a new subfamily consisting of hHel1 and its homologues. In addition to these DEAD/H box/ATP-binding motifs, hHel1 has a putative nuclear localization signal and several regions that may mediate protein-protein interactions. Expression analysis indicates that hHel1 transcripts are ubiquitous, with particularly high levels in endocrine tissue. We have mapped the gene for hHel1 to human chromosome 4q22-q23; this region is rich in breakpoints and deletion mutants of genes involved in several human diseases, notably soft tissue leiomyosarcoma, hepatocellular carcinoma, and hematologic malignancies. Our observation that human Hel1 gene overexpression is present in an E1A-expressing cell line with increased capacity for gene reactivation events by genomic rearrangement suggests that human Hel1 may play a role in genetic instability development.

Passage to nonselective media transiently alters growth of mycophenolic acid-resistant mammalian cells expressing the escherichia coli xanthine-guanine phosphoribosyltransferase gene: implications for sequential selection strategies.

The Escherichia coli xanthine-guanine phosphoribosyltransferase gene (Ecogpt) rescues mammalian cells from inhibition of purine nucleotide biosynthesis by mycophenolic acid (MPA). We used Ecogpt and other selectable markers to obtain subclones of NIH 3T3 derivatives (EN/NIH) stably expressing transfected genes of interest. In their respective selective mediums, growth of MPA-resistant (MPA(R)) isolates was indistinguishable from that of aminoglycoside-resistant counterparts expressing selectable marker genes conferring resistance to protein synthesis inhibitors hygromycin B, puromycin, and G418. Growth of aminoglycoside-resistant isolates remained unaltered on passage to nonselective media. In contrast, MPA(R) cells transferred from MPA complete media to nonselective media displayed morphologic changes with static growth. These findings resolved completely by third passage in nonselective media and were independent of the gene of interest cis-linked to the selectable marker. Sequential selection strategies involving cell culture conditions resulting in these altered growth characteristics significantly impaired detection (by selection in G418) of genomic events associated with reactivation of enhancerless, transcriptionally silent neointegrants present in MPA(R) EN/NIH isolates. We explored the cause of these cell culture findings and defined transfection and sequential selection strategies for MPA(R) derivatives that successfully circumvented these effects.