Murine mucopolysaccharidosis type VII. Characterization of a mouse with beta-glucuronidase deficiency.

We have characterized a new mutant mouse that has virtually no beta-glucuronidase activity. This biochemical defect causes a murine lysosomal storage disease that has many interesting similarities to human mucopolysaccharidosis type VII (MPS VII; Sly syndrome; beta-glucuronidase deficiency). Genetic analysis showed that the mutation is inherited as an autosomal recessive that maps to the beta-glucuronidase gene complex, [Gus], on the distal end of chromosome 5. Although there is a greater than 200-fold reduction in the beta-glucuronidase mRNA concentration in mutant tissues, Southern blot analysis failed to detect any abnormalities in the structural gene, Gus-sb, or in 17 kb of 5' flanking and 4 kb of 3' flanking sequences. Surprisingly, a sensitive S1 nuclease assay indicated that the relative level of kidney gusmps mRNA responded normally to androgen induction by increasing approximately 11-fold. Analysis of this mutant mouse may offer valuable information on the pathogenesis of human MPS VII and provide a useful system in which to study bone marrow transplantation and gene transfer methods of therapy.

DNA determinants of structural and regulatory variation within the murine beta-glucuronidase gene complex.

The murine beta-glucuronidase (GUS) gene complex, [Gus], encompasses the GUS structural element, Gus-s, and a set of regulatory elements which serve to modulate Gus-s expression. Three common GUS haplotypes representing virtually all inbred strains of laboratory mice have been compared with respect to GUS mRNA sequence. Results of such comparisons revealed sequence variations which target the location of one of the GUS regulatory elements to sequences within Gus-s and which account for known electrophoretic and heat stability differences among GUS allozymes of the three common GUS haplotypes.

Two genetic elements regulate murine beta-glucuronidase synthesis following transcript accumulation.

Mutant alleles of two genetic regulatory elements, which underlie a three- to sixfold reduction in beta-glucuronidase (GUS) activity levels, distinguish mice of the H haplotype from those of the other two common GUS haplotypes, A and B. Both elements are tightly linked to the GUS structural gene over which they exert control. One (Gus-u) exerts a cis-active effect upon GUS activity levels in all tissues at all times while the other (Gus-t) regulates GUS activity in trans after the 12th postnatal day in certain tissues. While previous studies show that differences in the rate of GUS synthesis account for the combined effects of these two elements in liver of adult mice, we demonstrate the separate effects of each on GUS synthesis at times during early postnatal development when their individual expressions can be distinguished. Assessments of the relative levels of S1 nuclease protection of a radiolabeled GUS antisense RNA probe after hybridization with total liver RNA preparations from adult mice of A and H haplotypes reveal no differences. These results argue that Gus-u and Gus-t exert their control of GUS expression subsequent to the accumulation of processed GUS transcripts.

A comparison of stem cell populations and hemoglobin switching in normal versus beta-thalassemic mice.

The hematopoietic stem cell concentrations in tissues of homozygous beta-thalassemic and non-thalassemic fetuses and neonates were compared by using the spleen colony-forming units (CFU-S) assay. The relative quantities of embryonic and adult hemoglobins were also determined for fetuses. Beta-thalassemic fetuses had a reduced incidence of CFU-S in the liver throughout gestation, but after birth the beta-thalassemic neonates maintained a greater number of CFU-S in the liver for an extended period. The incidence of CFU-S in the bone marrow was not different for the two groups. The beta-thalassemic mice exhibited a significant expansion of CFU-S in the spleen beyond 11 days after birth. The switch from the synthesis of primarily embryonic to primarily adult hemoglobins in circulating erythrocytes in beta-thalassemic fetuses appeared later than the switch in normal fetuses. These observations establish that the developmental timing and expansion of hematopoiesis are perturbed in beta-thalassemic mice.

Expression of the two adult beta-globin genes in mouse yolk sac and fetal liver erythrocytes.

The two adult beta-globin genes (beta 1s2major and beta 2sminor) of the Hbbs2 haplotype are differentially expressed during development. Centrifugal elutriation was used to separate the two populations of erythrocytes present in developing fetuses. Hemoglobin analysis showed that the larger, nucleated erythrocytes (yolk sac-derived) have relatively larger amounts of beta-sminor hemoglobin than do smaller, nonnucleated cells (fetal liver-, spleen-, and bone marrow-derived) at the same stage of development.

Independent expression of the two mouse adult beta-globin genes.

Dot blot hybridization was used to determine the relative amounts of the beta-major and beta-minor globin RNAs present in reticulocytes of mice at 14.5 and 17.5 days of gestation, newborns, and adults of the Hbab/Hbab;Hbbs2/Hbbs2 globin genotype. RNAs isolated from embryonic yolk sac, fetal liver, and adult reticulocytes were hybridized with the following labeled DNA probes: alpha-1, beta-minor specific, and beta-major specific. The level of beta-sminor RNA in reticulocytes at 14.5 and 17.5 days of gestation is nearly the same as in induced reticulocytes of adult mice. In contrast, the level of beta-s2major RNA in reticulocytes at 14.5 days of gestation is 0.23 X and at 17.5 days of gestation is 0.66 X the amount found in reticulocytes of adult mice. These results correlate well with earlier observations that the beta-sminor globin gene approaches its normal adult level of expression by 14.5 days of gestation, whereas the beta-s2major globin gene expression increases between 14.5 days of gestation and 6 days postnatally. They indicate that the differential expression of beta-sminor and beta-s2major globins during development is regulated at the level of transcription. Expression of the beta-minor globin gene in reticulocytes of adult normal mice is not maximal, however, because the levels of the beta-minor globin and its RNA are increased further in reticulocytes of thalassemic mice.

Use of a new mouse beta-globin haplotype (Hbbs2) to study hemoglobin expression during development.

Mice of the mutant haplotype (Hbbs2) produce a variant beta-s globin (beta-s2major) which can be distinguished from beta-smajor and beta-sminor by cellulose acetate electrophoresis and ion exchange chromatography. Mice homozygous for this mutation were used to study the relative quantities of the mutant beta-s2major and normal beta-sminor globins specified by the two adult beta-globin genes of the Hbbs2 haplotype during development. At 11.5 days of gestation, beta-s2major comprises under 20% and beta-sminor over 80% of the adult beta-globin. The relative level of beta-sminor decreases through fetal development; at birth beta-sminor represents 33.7% of the beta-globin. The adult value of 71.0% beta-s2major and 29.0% beta-sminor globin is expressed in mice 6 days after birth. In mildly anemia alpha-thalassemic heterozygotes (Hbab2(th)/Hbab;Hbbs2/Hbbs2) the level of beta-sminor globin increases from 29.0 to 37.9%, but beta-sminor is elevated only slightly (29.0 to 33.9%) in asymptomatic beta-thalassemic heterozygotes (Hbab/Hbab;Hbbd3(th)Hbbs2). The relative quantity of beta-sminor is increased significantly (29.0 to 41.4%) in doubly heterozygous alpha-thalassemic, beta-thalassemic mice (Hbab2(th)/Hbab;Hbbd3(th)/Hbbs2). The relative levels of expression of the beta 1s2major and beta 2sminor globin genes of Hbbs2/Hbbs2 mice correlates well with the expression of the beta 1dmajor and beta 2dminor globin genes of Hbbd/Hbbd mice during development and in response to hematological stress caused by thalassemia. Expression of the beta 1sminor globin gene should not have been affected by the ENU-induced base substitution in the beta 1smajor gene. Therefore, we propose that the beta 1sminor gene is also expressed in mice of the Hbbs haplotype. The results also indicated that the two adult beta-globin genes of the Hbbs2 and, presumably, of the Hbbs haplotypes are regulated independently as are the beta 1dmajor and beta 2dminor genes of the Hbbd and Hbbp haplotypes.

Detection of an unstable murine hemoglobin.

3H-leucine was used in vitro to label newly synthesized adult alpha and beta globins in reticulocytes removed from normal (Hba-b/Hba-b;Hbb-s2/Hbb-s2 and alpha-thalassemic (Hba-b2(th)/Hba-b;Hbb-s2/Hbb-s2) mice. The ratio of synthesis of beta-s2major: beta-sminor globins was 71:29 in reticulocytes from normal mice and 55:45 in reticulocytes from alpha-thalassemic mice. The two beta-globins are structurally identical except for a Val----Glu substitution at position 60. Denaturation of these mouse hemoglobins in isopropanol indicated that the tetramer containing the beta-s2major globin is unstable.

Differential expression of murine adult hemoglobins in early ontogeny.

Mice with a mutation at the Hbbs, beta-globin locus, were used to study the relative levels of beta-s2major (mutant) and beta-sminor globins specified by the mutant Hbbs2 haplotype during development. At 11.5 days of gestation, beta-sminor comprised over 80 percent and beta-s2major under 20 percent of the adult beta-globin. The relative level of beta-sminor decreased through fetal development; at birth beta-sminor represented 33.7 percent of the beta-globin. The adult values of 29.0 percent beta-sminor and 71.0 percent beta-s2major globin are expressed in mice six days after birth. Because the two beta-globin genes are expressed in mice of the Hbbs2 haplotype, both the beta-smajor and beta-sminor genes are presumably expressed in mice of the Hbbs haplotype. Expression of the beta-sminor gene is elevated to 35.6 percent in adult Hbbs2/Hbbs2 mice that have been bled repeatedly. Thus, the 5' beta-s2major and 3' beta-sminor genes of the Hbbs2 haplotype and, presumably the 5' beta-smajor and 3' beta-sminor genes of the Hbbs haplotype, are regulated independently and are homologous to the 5' beta-dmajor and 3' beta-dminor genes of the Hbbd haplotype. Mice of the Hbbs2 haplotype are better suited than mice of the Hbbd haplotype for studying the mechanisms of hemoglobin switching because with Hbbs2 each of the three embryonic and two adult hemoglobins can be separated by electrophoresis.