Detection, cloning, and distribution of minisatellites in some mammalian genomes

Centromere Repeats: Hidden Gems of the Genome

Satellite DNAs are now regarded as powerful and active contributors to genomic and chromosomal evolution. Paired with mobile transposable elements, these repetitive sequences provide a dynamic mechanism through which novel karyotypic modifications and chromosomal rearrangements may occur. In this review, we discuss the regulatory activity of satellite DNA and their neighboring transposable elements in a chromosomal context with a particular emphasis on the integral role of both in centromere function. In addition, we discuss the varied mechanisms by which centromeric repeats have endured evolutionary processes, producing a novel, species-specific centromeric landscape despite sharing a ubiquitously conserved function. Finally, we highlight the role these repetitive elements play in the establishment and functionality of de novo centromeres and chromosomal breakpoints that underpin karyotypic variation. By emphasizing these unique activities of satellite DNAs and transposable elements, we hope to disparage the conventional exemplification of repetitive DNA in the historically-associated context of ‘junk’.

Molecular and cytogenetic analysis of familial Xp deletions

Five families in which an Xp deletion is segregating and two families in which an X chromosome rearrangement including a deletion of the short arm is segregating were ascertained for study. Normal fertility was seen in all families. Members from 5 of the 7 families manifested short stature (height <5th centile), while normal height was present in two families. Studies of both the FMR-1 and the androgen receptor loci using PCR based X-inactivation analysis demonstrated that in all families analyzed, there is preferential inactivation of one X chromosome. Molecular cytogenetic analysis showed that members of 3 of the 7 families share a common breakpoint in an approximate 2-3 Mb region at Xp22.12, suggesting a possible hotspot for chromatin breakage. Previous genotype-phenotype correlations and deletion mapping have indicated that a gene for stature resides within the pseudoautosomal region in Xp22.33. Our findings indicate that the loss of this region is not always associated with short stature, suggesting that other factors may be involved.

Finding New Human Minisatellite Sequences in the Vicinity of Long CA-Rich Sequences

Microsatellites and minisatellites are two classes of tandem repeat sequences differing in their size, mutation processes, and chromosomal distribution. The boundary between the two classes is not defined. We have developed a convenient, hybridization-based human library screening procedure able to detect long CA-rich sequences. Analysis of cosmid clones derived from a chromosome 1 library show that cross-hybridizing sequences tested are imperfect CA-rich sequences, some of them showing a minisatellite organization. All but one of the 13 positive chromosome 1 clones studied are localized in chromosomal bands to which minisatellites have previously been assigned, such as the 1pter cluster. To test the applicability of the procedure to minisatellite detection on a larger scale, we then used a large-insert whole-genome PAC library. Altogether, 22 new minisatellites have been identified in positive PAC and cosmid clones and 20 of them are telomeric. Among the 42 positive PAC clones localized within the human genome by FISH and/or linkage analysis, 25 (60%) are assigned to a terminal band of the karyotype, 4 (9%) are juxtacentromeric, and 13 (31%) are interstitial. The localization of at least two of the interstitial PAC clones corresponds to previously characterized minisatellite-containing regions and/or ancestrally telomeric bands, in agreement with this minisatellite-like distribution. The data obtained are in close agreement with the parallel investigation of human genome sequence data and suggest that long human (CA)s are imperfect CA repeats belonging to the minisatellite class of sequences. This approach provides a new tool to efficiently target genomic clones originating from subtelomeric domains, from which minisatellite sequences can readily be obtained.

[The sequence data described in this paper have been submitted to the EMBL data library under accession nos.AJ000377–AJ000383.]

Comparative Sequence Analysis of Human Minisatellites Showing Meiotic Repeat Instability

The highly variable human minisatellites MS32 (D1S8), MS31A (D7S21), and CEB1 (D2S90) all show recombination-based repeat instability restricted to the germline. Mutation usually results in polar interallelic conversion or occasionally in crossovers, which, at MS32 at least, extend into DNA flanking the repeat array, defining a localized recombination hotspot and suggesting that cis-acting elements in flanking DNA can influence repeat instability. Therefore, comparative sequence analysis was performed to search for common flanking elements associated with these unstable loci. All three minisatellites are located in GC-rich DNA abundant in dispersed and tandem repetitive elements. There were no significant sequence similarities between different loci upstream of the unstable end of the repeat array. Only one of the three loci showed clear evidence for putative coding sequences near the minisatellite. No consistent patterns of thermal stability or DNA secondary structure were shared by DNA flanking these loci. This work extends previous data on the genomic environment of minisatellites. In addition, this work suggests that recombinational activity is not controlled by primary or secondary characteristics of the DNA sequence flanking the repeat array and is not obviously associated with gene promoters as seen in yeast.

[The sequence data described in this paper have been submitted to the GenBank data library under accession nos. AF048727(CEB1), AF048728 (MS31A), and AF048729 (MS32).]

Mapping of quantitative trait loci for blood pressure and cardiac mass in the rat by genome scanning of recombinant inbred strains

In the HXB and BXH recombinant inbred strains derived from the spontaneously hypertensive rat and the normotensive Brown Norway rat, we determined the strain distribution patterns of 500 genetic markers to scan the rodent genome for quantitative trait loci regulating cardiac mass and blood pressure. The markers spanned approximately 1,139 cM of the genome and were tested for correlations with left ventricular mass adjusted for body weight, and with systolic, diastolic, and mean arterial pressures. The marker for the dopamine 1A receptor (Drd1a) on chromosome 17 showed the strongest correlation with left ventricular heart weight (P = .00038, r = -0.59) and the relationship to heart weight was independent of blood pressure. The markers showing the strongest correlations with systolic, diastolic, and mean arterial pressure were D19Mit7 on chromosome 19 (P = .0012, r = .55), D2N35 on chromosome 2 (P = .0008, r = .56), and Il6 on chromosome 4 (P = .0018, r = .53), respectively. These studies demonstrate that the HXB and BXH strains can be effectively used for genome scanning studies of complex traits and have revealed several chromosome regions that may be involved in the genetic control of blood pressure and cardiac mass in the rat.