Molecular genetics of the brown (b)-locus region of mouse chromosome 4. II. Complementation analyses of lethal brown deletions.

Abstract

Numerous new mutations at the brown (b) locus in mouse chromosome 4 have been recovered over the years in germ-cell mutagenesis experiments performed at the Oak Ridge National Laboratory. A large series of radiation- and chemical-induced b mutations known to be chromosomal deletions, and also known to be prenatally lethal when homozygous, were analyzed by pairwise complementation crosses as well as by pseudodominance tests involving flanking loci defined by externally visible phenotypes. These crosses were designed to determine the extent of each deletion on the genetic and phenotype map of the chromosomal region surrounding the b locus; the crosses also provided basic data that assigned deletions to complementation groups and defined four new loci associated with aberrancies in normal development. Specifically, the pseudodominance tests identified deletions that include the proximally mapping whirler (wi) and the distally mapping depilated (dep) genes, thereby bracketing these loci defined by visible developmental abnormalities with landmarks (deletion breakpoints) that are easily identified on the physical map. Furthermore, the complementation crosses, which were supplemented with additional crosses that allowed determination of the gross time of lethality of selected deletions, defined four new loci required for normal development. Homozygous deletion of one of these loci (b-associated fitness, baf) results in a runting syndrome evident during postnatal development; deletion of one locus [l(4)2Rn] causes death in the late gestation/neonatal period; and deletion of either of two loci [l(4)1Rn or l(4)3Rn] results in embryonic death, most likely in pre-, peri- or postimplantation stages. The placement of these new functionally defined loci on the evolving molecular map of the b region should be useful for continuing the analysis of the roles played in development by genes in this segment of chromosome 4.

Molecular genetics of the brown (b)-locus region of mouse chromosome 4. I. Origin and molecular mapping of radiation- and chemical-induced lethal brown deletions.

“…This was done by crossing the appropriate +/b' GI male (or G, carrier female) to +/+ mice and identifymg, in each subsequent generation, those that were +/ b' (as opposed to +/+) by a progeny testcross to b / b animals. In some cases, brown animals from these progeny testcrosses ( i. e., b/b') were used for additional crosses in this study as well as for crosses outlined in a companion study (RINCHIK 1994) D4Rck4,150 bp;D4Rck52,190 bp;and D4Rck140,123 bp) were derived by cloning (into hgtl0) EcoRI fragments prepared from chromosome fragments microdissected from the mid-region of chromosome 4 (BAHARY et al 1993). Hybridization probes were prepared from these three microclones by polymerase chain reaction (PCR) amplification using AgtlO primers that flank the EcoRI cloning site (5'-AGG AAGTTCACCCTGGTTAAG and 5'6TTATGAGTATTTCITG CACGGTA) .…”

“…We have also been able to determine that D4Rck140 maps distal to b ( T y r p l ) by deletion mapping, whereas it always segregated with b in the relatively small number of backcross segregants analyzed (BAHARY et al 1993). As will be described in the accompanying report (RINCHIK 1994), the D4Rckl40 locus provides a useful point of molecular access to a locus required for late-gestation/neonatal development as well as to a locus (dep; depilated; MAYER et al 1976) required for normal hair development.…”

“…The whirler ( w i ) locus has been reported to map between l and 5 cM proximal to b (DAVISSON et al 1989), and the I f a locus maps approximately 4 cM distal to b (BLANK et al 1991). Only one of these 25 deletions, b'IR3OM, extends proximally far enough to include the wi locus (RINCHIK et RINCHIK 1994), but 17 other deletions include the D4Rck4 locus, which maps between wi and b ( T y r p l ) . D4Rck4 was previously reported to map approximately 0.8 cM proximal to b on the basis of interspecific backcross analysis ( B w y e t al.…”

Complementation analyses for 45 mutations encompassing the pink-eyed dilution (p) locus of the mouse.

“…1986JENKINS et al 1989;JOHNSON et al 1989;NISWANDER et al 1989;KLEBIC et al 1992a; NICHOLLS et al. 1993;RINCHIK 1994). Additionally, recent saturation-mutagenesis studies, utilizing the powerful point-mutation inducer N-ethyl-N nitrosourea (ENU), have refined the functional maps of genomic regions associated with some of the deletion complexes, have identified single-gene components of certain complex phenotypes, and have provided information on the density of "essential" genes per unit length of genome (RINCHIK et al , 1993aRINCHIK and CARPENTER 1993).…”

“…Not only have the deletion complexes identified loci of considerable developmental interest (RINCHIK et al , 1994MERCER et al 1991;KINGSLEY et al 1992; KLEBIG et al. 1992b;RUPPERT et al 1992;CULIAT et al 1993;RINCHIK 1994;AVRAHAM et al 1995), but they are providing the genetic and physical reagents for positional cloning of functionally significant genes within the region, both newly identified and previously known ones. Several of these genes have major significance for understanding human genetic disorders (e.g., congenital cleft palate) (CULIAT et al 1994).…”