MRL/MpJ-Atp6v1b1^vtx/Kjn

Stock number: 021771
Product name: vortex
Research areas: Cell Biology Research, Internal/Organ Research, Metabolism Research, Sensorineural Research

Description

This mutant is valuable for the study of fluid homeostasis in the inner ear and kidney. Mutations in ATP6V1B1 cause the human genetic disease distal renal tubular acidosis with progressive nerve deafness. On the MRL/MpJ inbred background vortex homozygotes have inner ear defects that are not found in C57BL/6J congenic vortex homozygotes. On both backgrounds this mutation causes hypocalciuria, with greater severity on the MRL/MpJ background, but increased urine pH is only found on the C57BL/6J background, possibly due to the inherently high urine pH of the MRL/MpJ inbred strain.

Detailed description

The vortex mutation is a spontaneous point mutation causing a G78D amino acid substitution in Atp6v1b1, a component of the vacuolar H+ ATPase pump that is active in the kidneys and inner ear. Mutations in the human ortholog, including two separate instances of G78D substitutions, have been shown to cause distal renal tubular acidosis with progressive nerve deafness (OMIM #267300). On the MRL/MpJ background the vortex mutation causes developmental defects of the inner ear with profound hearing and vestibular impairment in homozygotes, likely the result of failed fluid homeostasis. Although they are generally healthy and have normal fertility, vortex homozygotes on the MRL/MpJ background can be identified before wean age by their circling and head tilting. They are deaf, with ABR thresholds essentially absent across a broad range of frequencies and measurements of the endocochlear potential are nearly absent. At E15 the membranous labyrinth is dilated relative to controls, with the endolymphatic sac, duct, utricle, saccule, and cochlear duct showing swelling; at P1 the cochlear duct is enlarged and what little there is of utriclular or saccular otoconia is dispersed; and in adults histology finds lateral expansion of cochlear fluid spaces, basal displacement of the organ of Corti, small scala tympani, expanded scala media, displacement of the Reissner’s membrane, elongated yet stubby spiral limbus, very thin spiral ligament, very few type I or III fibrocytes, and the boney interscalar septum between the lower cochlear base and lower apical turn is often absent. The inner hair cells are reasonably normal, but the outer hair cells are lacking in the cochlear base and variably present in the apex. While the vestibular organs show normal cristae and no distinct hair cell or neural loss, there are very few otoconia but there are some very large otoconia, mostly in the saccule, that have triangular shapes and in 5 of 9 homozygotes assessed these large, misshapen otoconia were also found in the cochlear scala media.

While vortex homozygotes on either the MRL/MpJ or C57BL/6J background have normal potassium, sodium, and chloride concentrations in blood and urine, both show decreased calcium in the urine, with only 45.9 +/- 6.01 mg/g in MRL/MpJ homozygotes relative to 163.3 +/- 17.4 mg/g in MRL/MpJ controls, a reduction of approximately 70%, and 99 +/- 9.6 mg/g in C57BL/6J congenic homozygotes versus 139.2 +/- 9.1 in C57BL/6J controls, a reduction of approximately 30%. No elevation in urine pH was found on the MRL background, but the MRL/MpJ inbred strain has a naturally high urine pH relative to other inbred strains. On the C57BL/6J congenic background (see Stock No. 021772) an elevation in urine pH is found, but the inner ear develops normally and ABR thresholds and endocochlear potential measurements are normal. Vortex homozygotes on an (MRL x B6)F1 hybrid background have ABR thresholds much closer to normal than do homozygotes on the MRL/MpJ background. An MRL/MpJ modifier that contributes to the vortex hearing loss phenotype was mapped to Chromosome 13 between rs3719347 and rs3023390. No abnormal phenotypes have been identified in heterozygotes on either inbred genetic background. (Tian et al., 2017.)

Development

The vortex mutation arose spontaneously in a subline of MRL/MpJ-Fas^lpr/J that carried the witch nails mutation. Backcrossing to MRL/MpJ via a backcross-intercross breeding scheme removed the lymphoproliferation allele and witch nails phenotype and produced this coisogenic mutant subline of MRL/MpJ. Sperm was cryopreserved from homozygous males at generation N6F8. The last time the witch nails phenotype was seen in this strain was at generation N4F3, but there is a small possibility that this recessive allele of keratin 90 might still be segregating in the cryopreserved bankstock.

Allele

Symbol: Atp6v1b1^vtx
Name: vortex
MGI accession ID: MGI:5469980
Generation method: Spontaneous
Attributes: Not Specified
Marker symbol: Atp6v1b1
Marker name: ATPase, H+ transporting, lysosomal V1 subunit B1
Chromosome: 6
Strain of origin: MRL/MpJ-Fas^lpr/J
Molecular note: This spontaneous G to A transition in exon 3 (c.233G>A) replaces a highly conserved glycine with an aspartic acid at amino acid 78 in the amino terminal domain.