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Published online 7 August 2000. doi:10.1083/jcb.150.3.417
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© The Rockefeller University Press, 0021-9525/2000/8/417/ $5.00
The Journal of Cell Biology, Volume 150, Number 3, August 7, 2000 417-432

Original Article

Tracking COL1A1 RNA in Osteogenesis Imperfecta: Splice-defective Transcripts Initiate Transport from the Gene but Are Retained within the SC35 Domain

Carol Johnsona, Dragan Primoracb, Monique McKinstryb, John McNeila, David Roweb, and Jeanne Bentley Lawrencea
a Department of Cell Biology, University of Massachusetts Medical School, Worcester, Massachusetts 01655
b Department of Pediatrics, University of Connecticut Health Center, Farmington, Connecticut 06030

Correspondence to: Jeanne Bentley Lawrence, Department of Cell Biology, University of Massachusetts Medical School, 55 Lake Avenue North, Worcester, MA 01655. Tel:(508) 856-6015 Fax:(508) 856-5178 E-mail:jeanne.lawrence{at}umassmed.edu.

This study illuminates the intra-nuclear fate of COL1A1 RNA in osteogenesis imperfecta (OI) Type I. Patient fibroblasts were shown to carry a heterozygous defect in splicing of intron 26, blocking mRNA export. Both the normal and mutant allele associated with a nuclear RNA track, a localized accumulation of posttranscriptional RNA emanating to one side of the gene. Both tracks had slightly elongated or globular morphology, but mutant tracks were cytologically distinct in that they lacked the normal polar distribution of intron 26. Normal COL1A1 RNA tracks distribute throughout an SC-35 domain, from the gene at the periphery. Normally, almost all 50 COL1A1 introns are spliced at or adjacent to the gene, before mRNA transits thru the domain. Normal COL1A1 transcripts may undergo maturation needed for export within the domain such as removal of a slow-splicing intron (shown for intron 24), after which they may disperse. Splice-defective transcripts still distribute thru the SC-35 domain, moving ~1–3 µm from the gene. However, microfluorimetric analyses demonstrate mutant transcripts accumulate to abnormal levels within the track and domain. Hence, mutant transcripts initiate transport from the gene, but are impeded in exit from the SC-35 domain. This identifies a previously undefined step in mRNA export, involving movement through an SC-35 domain. A model is presented in which maturation and release for export of COL1A1 mRNA is linked to rapid cycling of metabolic complexes within the splicing factor domain, adjacent to the gene. This paradigm may apply to SC-35 domains more generally, which we suggest may be nucleated at sites of high demand and comprise factors being actively used to facilitate expression of associated loci.

Key Words: osteogenesis imperfecta, RNA transport, RNA splicing, nuclear structure, collagen


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