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Ryr1 I4895T/wt (IT/+) mice express a knockin mutation corresponding to the human I4898T EC-uncoupling mutation in the type 1 ryanodine receptor/Ca 2+ release channel (RyR1), which causes a severe form of central core disease (CCD). IT/+ mice exhibit a slowly progressive congenital myopathy, with neonatal respiratory stress, skeletal muscle weakness, impaired mobility, dorsal kyphosis, and hind limb paralysis. Lesions observed in myofibers from diseased mice undergo age-dependent transformation from minicores to cores and nemaline rods. Early ultrastructural abnormalities include sarcomeric misalignment, Z-line streaming, focal loss of cross-striations, and myofibrillar splitting and intermingling that may arise from defective myofibrillogenesis. However, manifestation of the disease phenotype is highly variable on a Sv129 genomic background. Quantitative RT-PCR shows an equimolar ratio of WT and mutant Ryr1 transcripts within IT/+ myofibers and total RyR1 protein expression levels are normal. We propose a unifying theory in which the cause of core formation lies in functional heterogeneity among RyR1 tetramers. Random combinations of normal and either leaky or EC-uncoupled RyR subunits would lead to spatial differences in Ca 2+ transients; the resulting heterogeneity of contraction among myofibrils would lead to focal, irreversible tearing and shearing, which would, over time, enlarge to form minicores, cores, and nemaline rods. The IT/+ mouse line is proposed to be a valid model of RyR1-related congenital myopathy, offering high potential for elucidation of the pathogenesis of skeletal muscle disorders arising from impaired EC coupling.

The nuclear RNA from a large variety of kappa-producing plasmacytomas was size fractionated and analyzed with a series of cloned probes representing sequences encoding variable (V), joining (J), and constant (C) regions and selected intervening sequences. All of the plasmacytomas produce a nuclear RNA component that contains V kappa and C kappa sequences as well as the intervening sequence between J kappa and C kappa, and that has a distinctive size depending on which of the four J kappa segments is expressed (i.e., is present in the secreted kappa chain). These RNAs are the precursors of kappa mRNAs, which are transcribed from productively rearranged C kappa genes. Half of the plasmacytomas examined produce, in addition to a kappa mRNA precursor, a discrete component of about 8.4 kilobases that contains C kappa and upstream flanking sequences but lacks the expressed V region sequence. The ability to produce this component is always associated with the persistence in the tumor genome of an unrearranged (germline) J kappa-C kappa region. In tumors rearranged at both kappa loci the nonproductive allele is either transcriptionally silent or, in a minority of cases, transcribed and processed into a "fragment" mRNA lacking V region sequences. These results reveal that allelic exclusion can be effected at several levels of gene expression. They also provide some insight into the relative contributions of the V and C gene elements to this expression.

We have identified at least six EcoRI fragments of mouse DNA that encode variable-region gene sequences closely related to the mouse kappa light chain, MOPC-149. Two of these fragments have been cloned, and the entire nucleotide sequence of the variable-region genes encoded on each has been determined. Both genes encode closely related variable-region sequences extending from codon position 1 through position 97. Neither fragment encodes a constant-region sequence. Although both genes are closely related, they differ from one another and from the sequence expressed in the MOPC-149 cell from which they were cloned. These few differences cluster within the complementarity-determining regions although several occur in framework sequences as well. We therefore conclude that an antibody-producing cell contains genetic information corresponding to its expressed sequence and several other closely related but silent sequences. These initial results raise the possibility that similar sets of genes might exist corresponding to each of the many subgroups already identified among mouse kappa light chains. If true, this would further suggest that the mouse genome might be rich enough in variable-region genes so as to encode a major portion of the variable-region repertoire.