The telomeric region of BTA18 containing a potential QTL region for health in cattle exhibits high similarity to the HSA19q region in humans☆

Abstract

We have applied a targeted physical mapping approach, based on the isolation of bovine region-specific large-insert clones using homologous human sequences and chromosome microdissection, to enhance the physical gene map of the telomeric region of BTA18 and to prove its evolutionary conservation. The latter is a prerequisite to exploit the dense human gene map for future positional cloning approaches. Partial sequencing and homology search were used to characterize 20 BACs targeted to the BTA18q2.4-q2.6 region. We used fluorescence in situ hybridization (FISH) to create physical maps of 11 BACs containing 15 gene loci; these BACs served as anchor loci. Using these approaches, 12 new gene loci (CKM, STK13, PSCD2, IRF3, VASP, ACTN4, ITPKC, CYP2B6, FOSB, DMPK, MIA, SIX5) were assigned on BTA18 in the bovine cytogenetic map. A resolved physical map of BTA18q2.4-q2.6 was developed, which encompasses 28 marker loci and a comparative cytogenetic map that contains 15 genes. The mapping results demonstrate the high evolutionary conservation between the telomeric region of BTA18q and HSA19q.

Introduction

Genome scans in cattle for quantitative trait loci (QTLs) identified a genomic region affecting udder health and longevity (for example, [1], [2a]). This region is localized on the telomeric end of bovine chromosome 18 (BTA18), in the interval between 70 and 110 cM, which corresponds to BTA18q2.4-q2.6. However, the efficient utilization of the identified QTL in breeding programs or the ultimate positional cloning of the corresponding genes requires a higher mapping resolution.

The positional cloning strategy has been successful in isolating a number of disease genes in humans [3] and pigs [4], and it has often been supplemented with the mapping of candidate genes for traits in farm animals. The latter approach is also called the positional candidate gene approach [3]. In domestic species, where gene maps are poorly developed, this approach is often combined with exploiting the dense maps of the human and mouse by using evolutionarily conserved synteny groups. Large-fragment libraries and radiation hybrid (RH) panels are now available for a variety of domestic animals to facilitate the isolation, identification, physical fine mapping, and ordering of the corresponding species-specific and homologous genes. In addition, flow sorting of individual chromosomes has been used for the construction of chromosome-specific libraries from diverse species [5], [6], [7], [8] that serve a source for chromosome-specific DNA markers. An alternative method for the isolation of chromosome fragment-specific DNA sequences is the microdissection of chromosomes (for example, [9], [10], [11], [12], [13]). This technique has already been successfully applied in a targeted generation of chromosome fragment-specific DNA sequences in cattle and goats [14], [15].

BTA18 represents evolutionarily conserved fragments on human chromosomes (HSA) 16 and 19 that have been identified by cross-species fluorescence in situ hybridization (FISH) [16], [17], [18], somatic hybrid cell (SHC) mapping [19], and RH mapping [20], [21].

In the present study a physical mapping approach using FISH and targeted isolation of bovine chromosome region-specific large-insert clones was applied to improve the physical gene map of the telomeric region of BTA18. Furthermore, a comparative physical map between the telomeric region of BTA18 and the q-arm of HSA19 was established to verify the evolutionary conservation of this region. These results might be useful for the exploitation of the dense gene map in the human for future positional cloning efforts.