Several years ago we showed that the spontaneous mouse mutation dilute (d) was caused by the integration of a retrovirus into the mouse genome. This observation was very exciting because it was the first demonstration of a retrovirus-induced mutation in mammals. dilute mice make normal amounts of pigment but the melanosomes, the pigment-containing vesicles in the melanocyte, are inefficiently transported from the perinuclear region of cell to the dendritic tips. This results in an uneven release of pigment and a dilution of coat color. Subsequently, we showed that d encodes an actin-based molecular motor, MYO5A. Yeast two hybrid studies suggested that MYOVA functions as part of a complex with a microtubule-based motor, KIF5B. This was the first evidence for an integrated motor complex that can move on both microtubule and actin tracks and we are now testing this hypothesis by generating mice lacking KIF5B in melanocytes. We used positional cloning to identify the products of ashen (ash) and leaden (ln), two mutations that phenocopy dilute. In subsequent collaborative studies we showed that the ash, which encodes RAB27A, and the ln, which encodes a RAB effector designated Melanophilin (MLPH), proteins form a complex on the melanosome that functions as the receptor for MYOVA. This was the first molecular description of a receptor for a molecular motor on a vesicle in mammals. Importantly, all three proteins are mutated in human disease. All patients with Griscelli Syndrome, a rare autosomal recessive disorder, display hypopigmentation of the skin and hair. Some also show neurological impairment of varying severity, which correlates with mutations in MYO5A; others show immune dysfunction in addition to hypopigmentation and these phenotypes correlate with mutations in RAB27A. Mice carrying various alleles of dilute, which can also display neurological abnormalities, and of ashen, which also display immune abnormalities, are now excellent models for the further study of these human diseases. Patients with mutations in MLPH display only a hypopigmentation phenotype, consistent with the observation that no other phenotypes are detected in mice homozygous for the ln mutation. In studies that have not yet been submitted for publication we succeeded in positionally cloning dilute suppressor (dsu), the first unlinked suppressor mutation to be identified in mammals. dsu is a semidominant suppressor that rescues, at least partially, the coat color dilution of d, ash, and ln mice. Contrary to our expectations, dsu does not encode a motor protein, or a protein (i.e. a transcription factor) that could regulate a motor protein and substitute for the loss of MYO5A. Instead, we found that dsu encodes a small protein with no striking functional motifs. dsu may therefore represent the founding member of a new pathway that regulates directional vesicle transport. We also uncovered surprising effects of dsu, d, ash, and ln in the eye, a subject of future studies.In collaborative studies, we positionally cloned ruby eye-2 (ru2) and ruby-eye (ru), two other pigment diluting mutations that are partially suppressed by dsu. We showed that the proteins encoded by ru2 and ru form a protein complex and are mutated in human Hermansky-Pudlak syndrome (HPS) type 5 and 6 patients, respectively. HPS is a multigenic disorder that affects melanocytes, platelets, and lysosomes, organelles that share a common biosynthetic pathway. Very recently, collaborative studies revealed that the gene encoded by the sandy pigment diluting mutation is mutated in HPS type 7 patients. Again, mice carrying these various diluting mutations are very important models for further studying human disease.
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