Animal models for discovery and assessment of genetic determinants of osteoporosis.

Abstract

Quantitative trait locus (QTL) mapping is the process of identifying, by both chromosomal position and effect, the major genetic contributors to natural-population (or interstrain) variation in the trait of inquiry. The polymorphic alleles, or variants, thus observed are distinct from variants turned up through mutagenesis screens in that each QTL allele exists in nature at a far higher level than could be maintained by the rate of new mutation. This tells us that none of the alleles confers a selective disadvantage, since even a 1% decrease in fitness is sufficient to eliminate that allele within a relatively short evolutionary time span. Thus, each QTL allele is equally normal, or ‘‘wild-type.’’ Several caveats should be noted. If a bone-relevant locus is invariant in a population, or does not happen to differ between the parental strains in an animal cross, it will be invisible to QTL detection. Thus, mapping studies that fail to detect a locus seen in another population or interstrain cross, or that find the same locus at lower significance, should not be interpreted as invalidating the previous positive observations. On the other hand, if a QTL is observed in multiple human populations and/or in the progeny of diverse mouse crosses between distantly related parental strains, that suggests the existence of a sustained polymorphism—in effect, an evolutionarily conserved mechanism for modulating a trait within populations. It may be of benefit to a population to include a range of phenotypes, such as high bone mass to increase bone strength and calcium reserves versus lower bone mass to improve speed. Table 1 summarizes graphically the regions implicated in several such studies of mouse interstrain cross progeny [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]. Although simulation analysis to determine significance has yet to be performed, it is apparent that many loci have been recaptured in 2 or more crosses (see loci on chromosomes 1, 2, 7, 11, 13, 15, and 16). Such data can be used to estimate the number of polymorphic genes of comparable allelic effect [14]. The most significant development over the last decade, in the broad area of QTL mapping, was the determination of near-complete genomic sequences for humans and mice. These sequences, and tools for their alignment, have greatly facilitated positional cloning in each species and the transfer of information from one to the other. With regard specifically to bone QTLs, in just the last 5 years at least four laboratories [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13] have mapped murine QTLs for bone density (spinal or femoral, in each case measured at maturity or 4 months of age) and have used recurrent backcrossing guided by QTL-specific markers to create congenic lines. Such lines feature single blocks of ‘‘donor-strain’’ DNA isolated in the otherwise uniform genetic background of the recipient strain. In each laboratory, congenic lines have recently been tested and were found to display the expected differential allelic effects on the same and related bone traits (unpublished data and references [4, 11, 12, 13]). This is a significant validation of the QTL-mapping procedure and a stage at which some insights can be garnered through exploration of the broader phenotype of each QTL region. Considerable further investment of time and effort will be required to narrow these intervals, via generation and testing of recombinants, to spans comprising less than a few dozen genes. At that point, it will make sense to begin functional testing of individual positional-candidate genes based on both genomic location and putative roles in osteogenesis or its regulation.