Mice with mutations of Dock7 have generalized hypopigmentation and white-spotting but show normal neurological function

Abstract

The classical recessive coat color mutation misty (m) arose spontaneously on the DBA/J background and causes generalized hypopigmentation and localized white-spotting in mice, with a lack of pigment on the belly, tail tip, and paws. Here we describe moonlight (mnlt), a second hypopigmentation and white-spotting mutation identified on the C57BL/6J background, which yields a phenotypic copy of m/m coat color traits. We demonstrate that the 2 mutations are allelic. m/m and mnlt/mnlt phenotypes both result from mutations that truncate the dedicator of cytokinesis 7 protein (DOCK7), a widely expressed Rho family guanine nucleotide exchange factor. Although Dock7 is transcribed at high levels in the developing brain and has been implicated in both axon development and myelination by in vitro studies, we find no requirement for DOCK7 in neurobehavioral function in vivo. However, DOCK7 has non-redundant role(s) related to the distribution and function of dermal and follicular melanocytes.

Fig. 1.

Moonlight coat color phenotype. (A) mnlt/mnlt mice have an overall reduction in pigmentation (mnlt/mnlt, Left; C57BL/6J, Right). mnlt/mnlt mice additionally display (B) a white tail tip (mnlt/mnlt, Top; C57BL/6J, Bottom), (C) paws, (D and E) genitalia (D) mnlt/mnlt ; (E) C57BL/6J and (F) belly spot.

Fig. 2.

Moonlight bleeding time and cytotoxicity. Bleeding time for mnlt/mnlt and wild-type C57BL/6J mice. Data shown from one of two experiments with similar results, n = 5 (Left). NK cell cytotoxicity was assessed by in vivo killing of TAP^−/− cells, mnlt/mnlt n = 3, C57BL/6J n = 6 (Middle). Following immunization, in vivo T cell killing was assayed on peptide pulsed target cells, mnlt/mnlt n = 3, C57BL/6J n = 6 (Right). All panels depict means ± SEMs.

Fig. 3.

Identification of the mnlt and m mutations. (A) The mnlt mutation was confined to chromosome 4 using a panel of 128 informative markers. The peak LOD score is 5.4 on chromosome 4. (B) Effect of the mnlt mutation on Dock7 mRNA and protein. Exons 22–28 are deleted, resulting in splicing of exon 21 directly to exon 29. Unbolded text indicates wild-type Dock7 sequence. An aberrant splice joins nt 2596 to nt 3519. Nucleotides of exon 29 are italicized, and the frame-shifted protein product is both highlighted and bolded. (C) Effect of the m mutation on Dock7 mRNA and protein. An anomalous insertion occurs within exon 18. Unbolded text indicates wild-type Dock7 sequence; following the insertion, aberrant nucleotides and the translation product are bolded and highlighted. For exon diagrams in (B) and (C), coding regions are shown in black; non-coding regions in gray.

Fig. 4.

Neurobehavioral studies of Dock7^mnlt/mnlt mice. (A) Tail suspension test. Mice are suspended by the tail and monitored for movement. Shown is the cumulative time spent immobile over the course of the trial. (B) Habituation of object exploration. Mice are placed into a field containing 3 plastic objects. Contacts with each object are measured over 5 min of time. Contacts for the 3 objects are averaged for each mouse. Three consecutive trials are performed. (C) Y maze test. The percentage of spontaneous alterations is the number of consecutive triplets of different arm choices out of the total number of arm choices. (D) Light/dark transfer test. Total time spent in the brightly lit compartment was measured (Left) and the total number of transitions between light and dark compartments were counted (Right). (E) Social interaction test. Sociability is measured as the number of entries into a chamber containing a stranger mouse. Differences between Dock7^mnlt/mnlt and C57BL/6J controls are not significant (females P = 0.0619). Social novelty measures the number of entries into a chamber containing a stranger mouse as opposed to a chamber containing a familiar mouse. C57BL/6J is depicted by open bars; Dock7^mnlt/mnlt is depicted by filled bars. Data are represented as means ± SEMs, n = 8.