A mutation in Rab27a causes the vesicle transport defects observed in ashen mice Scott M. Wilson*, Richard Yip*, Deborah A. Swing*, T. Norene O’Sullivan*, Yuke Zhang†, Edward K. Novak†, Richard T. Swank†, Liane B. Russell‡, Neal G. Copeland*, and Nancy A. Jenkins*§ *Mouse Cancer Genetics Program, National Cancer Institute–Frederick Cancer Research and Development Center, Frederick, MD 21702; †Department of Molecular and Cell Biology, Roswell Park Cancer Institute, Buffalo, NY 14263; and ‡Biology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 Contributed by Liane B. Russell, May 11, 2000 The dilute (d), leaden (ln), and ashen (ash) mutations provide a cells (9, 10). The absence of SER in spines probably disrupts unique model system for studying vesicle transport in mammals. the regulation of intracellular Ca2ϩ and postsynaptic signal All three mutations produce a lightened coat color because of transduction. defects in pigment granule transport. In addition, all three muta- A retroviral integration into the d gene in mice carrying the tions are suppressed by the semidominant dilute-suppressor (dsu), dilute-viral (dv) allele allowed the cloning of the d gene and the providing genetic evidence that these mutations function in the demonstration that d encodes an unconventional myosin heavy- same or overlapping transport pathways. Previous studies showed chain, myosin Va (MyoVa) (11, 12). MyoVa is expressed at high that d encodes a major vesicle transport motor, myosin-VA, which levels in nearly all tissues in the adult (12) and has the expected is mutated in Griscelli syndrome patients. Here, using positional structure for a member of this family—i.e., a globular head cloning and bacterial artificial chromosome rescue, we show that domain containing the ATP- and actin-binding sites, a ‘‘neck’’ ash encodes Rab27a. Rab GTPases represent the largest branch of domain, which is the site of calmodulin (or light-chain) binding, the p21 Ras superfamily and are recognized as key players in and a tail domain, which is thought to represent the cargo- vesicular transport and organelle dynamics in eukaryotic cells. We binding domain. MyoVa acts as a dimer and moves along actin also show that ash mice have platelet defects resulting in increased filaments in a plus-end-directed manner (13). Human MYOVA bleeding times and a reduction in the number of platelet dense mutations have also been identified and shown to produce GENETICS granules. These defects have not been reported for d and ln mice. Griscelli syndrome, a rare autosomal recessive disorder charac- Collectively, our studies identify Rab27a as a critical gene for terized by pigment dilution, variable cellular immunodeficiency, organelle-specific protein trafficking in melanocytes and platelets neurological defects, and acute phases of uncontrolled lympho- and suggest that Rab27a functions in both MyoVa dependent and cyte and macrophage activation (14). independent pathways. Yeast two-hybrid studies have shown that the tail domain of MyoVa can interact with a microtubule-based transport motor, ore than 50 loci in the mouse have been identified that ubiquitous kinesin heavy chain (KhcU) (15). Considerable bio- Maffect pigmentation of the coat (1). Three of these loci (d, logical and biochemical evidence indicates that long-range trans- ln, and ash) have been grouped together because they have an port in animal cells occurs on microtubules, whereas short-range identical phenotype: Pigment granule (melanosome) synthesis is transport occurs on actin filaments (16). An important question normal in these mice, but melanosome transport is abnormal (2). raised by these studies is how transport along these pathways is In wild-type mice, melanins are synthesized in melanosomes, coordinated. The yeast two-hybrid studies suggest that one way specialized organelles derived from the Golgi. The melanosomes this transport is coordinated is through the direct interaction of are then transported along the dendrites and exported to neigh- the different motor molecules. These data are consistent with boring keratinocytes in the hair bulb and subsequently to the hair previous studies, which showed that MyoVa and kinesin both can shaft (2, 3). In mice homozygous for these three recessive bind synaptic vesicles in the rat (17, 18) and that MyoVa is mutations, the melanosomes are localized mainly in the perinu- required for short-range, but not long-range, transport of smooth clear region of the melanocyte (3–5) resulting in the uneven endoplasmic reticulum in Purkinje cells (9, 10). release of pigment into the hair bulb and a lightened coat color. MyoVa has also been shown to bind to a novel RING finger Genetic evidence suggests that these loci encode proteins that protein BERP (19). It thus appears that MyoVa transports cargo function in the same or overlapping pathway. For example, mice as part of a complex involving several different proteins includ- homozygous for any double, or the triple, combinations of d, ln, ing KhcU and BERP. An intriguing possibility is that ash and ln and ash on the nonagouti background all have the same pheno- also encode proteins that function as part of this motor complex. type as mice homozygous for any of the individual mutations To determine whether this is the case, we have initiated the (N.G.C. and N.A.J., unpublished observations). In addition, all positional cloning of ash and ln. In studies described here, we three mutations are suppressed by the semidominant suppressor, present evidence to show that ash encodes Rab27a. Rab proteins dsu (6, 7). dsu was first recognized by its ability to suppress the are recognized as key players in vesicular transport and organelle coat color phenotype of d mice and in subsequent experiments dynamics in eukaryotic cells. was also shown to partially suppress the coat color phenotype of ash and ln mice (6). In all cases, the level of suppression could be correlated with the extent of the restoration of normal Abbreviations: MyoVa, myosin-VA; YAC, yeast artificial chromosome; SSLP, simple se- melanosome distribution. dsu has no phenotype on its own, is cell quence length polymorphism; BAC, bacterial artificial chromosome; HPS, Hermansky Pud- lak syndrome; CHM, choroideremia. autonomous (8), and can be seen only in combination with d, ash, Data deposition: The sequence reported in this paper has been deposited in the GenBank or ln. database (accession no. bankit332438). In addition to the melanosome transport defect, d null mice §To whom reprint requests should be addressed. E-mail: [email protected]. also show a neurological defect, characterized by opisthotonus The publication costs of this article were defrayed in part by page charge payment. This and ataxia, which results in the death of the animal 2–3 weeks article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. after birth. The neurological defect is thought to result from the §1734 solely to indicate this fact. failure to transport smooth endoplasmic reticulum (SER) from Article published online before print: Proc. Natl. Acad. Sci. USA, 10.1073͞pnas.140212797. the dendritic shaft to the dendritic spines of cerebellar Purkinje Article and publication date are at www.pnas.org͞cgi͞doi͞10.1073͞pnas.140212797 PNAS ͉ July 5, 2000 ͉ vol. 97 ͉ no. 14 ͉ 7933–7938 Downloaded by guest on September 28, 2021 Methods primers, which flank the fourth intron (nucleotides 259–280 and Mice. C3H͞HeSn-ash͞ash and Mus castaneus (CAST͞Ei) mice 449–426). PCR products were gel purified by using a GeneClean were purchased from The Jackson Laboratory and maintained III kit (BIO 101, Vista, CA) and then subjected to direct DNA and propagated at the National Cancer Institute–Frederick sequencing. In addition, aberrantly spliced Rab27a transcripts Cancer Research and Development Center, Frederick, MD. from the mutant strain were cloned and subjected to DNA Mice carrying the d-se deficiency chromosomes 13SaSd, sequencing. Sequencing was performed by using the Big Dye 11FHAFo, 19Zb, 1DTD, 9R250M, 37FBFo, 17Zb, and 10FD- Terminator Cycle Sequencing Ready Reaction Kit (Perkin– FoD were obtained from the Oak Ridge National Laboratory, Elmer), and data were acquired on an ABI Model 377 DNA Oak Ridge, TN. sequencer (Applied Biosystems). Genetic and Physical Maps. Genetic and yeast artificial chromo- Hematology. Bleeding times were determined by tail bleeding as some (YAC)-based physical maps of the ash critical region were described (26). Platelets were lysed in 1 ml distilled water and generated as described (20). A 129͞Sv bacterial artificial chro- assayed fluorimetrically for serotonin (27). Platelet-rich plasma was collected from citrated undiluted blood by centrifugation at mosome (BAC) library (Research Genetics, Huntsville, AL) was ϫ screened with the simple sequence length polymorphism (SSLP) 150 g for 10 min as described (26). Unfixed and unstained markers and YAC end probes derived from the ash critical platelets were rapidly air dried on carbon-coated grids before region. Positive BACs were end sequenced by using SP6 and T7 electron microscopy (28). promoter primers. PCR primer pairs, designed from the BAC Melanocyte Cell Culture. end sequence, were then used to construct a BAC contig of the Melanocyte cell lines were isolated from ash critical region. skin of mutant and wild-type mice as described (29). Results and Discussion Deficiency Mapping. To further refine the ash critical region, eight A Genetic and Physical Map of the ash-Critical Region. To initiate the deficiency chromosomes (21, 22) were tested for their ability to positional cloning of ash, we generated 897 (C3H͞HeSn-ash͞ complement ash by crossing mice carrying the deficiencies with ash ϫ Mus castaneus) ϫ C3H͞HeSn-ash͞ash N progeny, which C3H͞HeSn-ash͞ash mice. Three deficiency chromosomes 2 were typed at weaning for ash by visual inspection. Tail DNAs (13SaSd, 11FHAFo, 19Zb) complemented ash function, and from the mice were then typed for SSLP markers predicted to the remaining five deficiency chromosomes (1DTD, 9R250M, map near ash, defining an ash-critical interval of Ϸ0.3 cM or 37FBFo, 17Zb, 10FDFoD) did not.