The vortex mutation is a spontaneous point mutation causing a G78D amino acid substitution in <i>Atp6v1b1</i>, 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). 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 (see Stock No. <a href="https://www.jax.org/strain/021771"">021771</a>), but the MRL/MpJ inbred strain has a naturally high urine pH relative to other inbred strains. On the C57BL/6J congenic background an elevation in urine pH is found, but the inner ear develops normally and ABR thresholds and measurements of endocochlear potential are normal. However, on the MRL/MpJ background vortex homozygotes developmental defects of the inner ear with profound hearing and vestibular impairment, likely the result of failed fluid homeostasis. No abnormal phenotypes have been identified in heterozygotes on either inbred genetic background. (Tian et al., 2017.)

This mutant is valuable for the study of fluid homeostasis in the inner ear and kidney. Mutations in <i>ATP6V1B1</i> 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.

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