This article is from the Summer 1999 AFRMA Rat & Mouse Tales news-magazine.
By Debra Mauzy Melitz, September 1998
Over 100 genes have been identified that effect coat color. A large number of the genetic loci have their roots in the European and Asian mouse fancy communities of the 18th century. Fancy mice provided some of the initial tools used to understand the process of mammalian coat color development. The loci described below have been well studied in the mouse and then sometimes studied in other mammals (Searle 1979, Silvers 1979, Jackson 1991, Jackson 1994, Barsh 1996).
Albino locus: c
The albino series of alleles are concerned with a step wise reduction in the intensity of pigmentation, first the phaeomelanin and the eumelanin. Effects are also seen in eye color. The c locus encodes the enzyme tyrosinase (Jackson 1991). Tyrosinase is the first enzyme needed to produce either eumelanin or phaeomelanin. Some mutations in this locus do not eliminate the enzyme but effect its efficiency. The allele Himalayan produces an enzyme that will not work at normal body temperatures but will work at slightly lower temperatures (temperature sensitive). The Himalayan tyrosinase produces melanin only at the cooler extremes of the animal (tip of tail, nose, feet).
Brown locus: b
The brown series of alleles seems to be mainly concerned with a change from a oval black eumelanin granule to a round brown one. The brown allele has no effect on phaeomelanin. Eye color will be effected. Heterozygotes (Bb) have darker hair than the homozygotes (bb). One common allele is the cordovan allele (bc) which gives a leather brown color. The brown locus encodes TRP1 (DHICA oxidase) involved the eumelanin polymer formation. A mutation in this locus eliminates one of the more common monomers or subunits formed from Dopaquinone. This probably effects the overall structure of the eumelanin polymer which may in turn effect the packaging of the eumelanin into the melanosomes.
Agouti locus: a
Named for a South American rodent. This locus is concerned with the regional distribution of eumelanin and phaeomelanin in the developing hair only. The melanocytes of every agouti genotype can produce both pigments. The locus is concerned with regulating the switch from one type to the next, not with the melanin production itself. Only a few mammalian orders do not appear to have this genetic locus. Those orders are; Cetacean (whales, porpoises, and dolphins), Proboscides (elephants), and Edentate (sloth, armadillos, and anteaters) (Searle, 1968).
Dominant alleles (A) cause a switch to phaeomelanin production. The results are an all yellow to orange color where color is produced. The agouti loci encodes a protein that acts as an antagonist to the hormone MSH that stimulates melanocytes to decrease glutathione activity and increase tyrosinase activity (Lu et. al. 1994, Bultman et. al.) It acts by binding to the same receptor as MSH and not allowing MSH to bind. When the agouti protein is present, MSH cannot decrease the level of glutathione and more Dopaquione becomes Cysdopas (see Figure 3). Dominant agouti mutation causes the glutathion activity to be high and hence only Cysdopas is made. There are several lethal or developmental problems that are associated with some of the dominant agouti alleles. This is because the agouti protein also effects a related MSH receptor in the brain (Lu et. al. 1994). Recessive mutations cause an all black or eumelanin phenotype because all the Dopaquione is converted to Dopachrome.
The number of alleles described for the agouti loci in the mouse are 19. Some of the allele cause drastic changes in pigmentation in different regions of the animal (Bultman et. a1.1992). Examples of some of the alleles described in the mouse are: lethal yellow (Ay): The hair is a rich yellow or orange but the homozygotes (Ay/Ay) die before the embryo is a few days old. Heterozygotes survive but have multiple associated problems (obesity, sterility and diabetes-like syndrome and increased pulmonary tumors, hepatomas in males, mammary tumors in females); viable yellow (Avy): Like lethal yellow but the only problems associated is potential problems with obesity. There can be variable effects in mottling or pattern of coloration in different backgrounds; non-agouti (a) The hairs are exclusively black, a/a;b/b hairs are brown. Two agouti alleles, at and Aw can cause regional pigmental differences by selectively inactivating the expression of different forms of the agouti mRNA (Buitman et.al., 1994). Another combination of agouti alleles can also lead to a variable expression of this locus (Perry et. al., 1994). This could lead to adjacent hairs being two different colors.
Extension series: E
The various alleles at the e locus either increase or diminish the amount of eumelanin with the opposite effect on the phaeomelanin. The relative amounts of eumelanin and phaeomelanin in hair are controlled by signals at the surface of the melanocyte. The signal received is usually from the hormone MSH and the protein that relays this signal inside the follicular melanocyte is the product of the extension locus (Robbins et. a1.,1993). MSH can induce a 5–50 fold increase in tyrosinase activity. If the extension protein is mutate such that it sends a signal whether there is MSH or not, that mutation produces a dominant effect and the hair will be black. On the other hand, if the extension protein is mutate so it does not work at all, no signal is relayed and the hair is yellow. There can be degrees in between. Brindling or tortoiseshell coat patterns produced by some mammals are the result of a partially recessive extension locus allele (Robbins et. al., 1994).
Silver locus: si
The silver locus encodes a melanosome matrix protein which is somehow involved in the packaging of the melanin in the melanosome. Silver can cause a reduction in the number of melanin granules and even then a complete absence in certain areas and in certain hairs. There can be selective premature death of melanocytes in the hair follicle. Silver animals have a mixture of white tipped, all white, and lighter shades of the background or normal color. The belly is always lighter than the back. Males also tend to show the effect more even though the locus is not on the X chromosome (sex linked). Light silvers have more white hairs and larger unpigmented zones than dark silvers. Probably the effect of different alleles of the silver locus. The age of the animal also has an effect on the phenotype. Animals become increasingly “silvered” with age except in the agouti mutation background. In the case of A/-;si/si older animals have less “silvering” than younger animals. There is an altered dominance relationship of brown in the presence of silver. B/b;si/si mice are paler than b/b;si/si mice which is the opposite expected.
Pink-eyed locus: p
The pink-eyed locus is also responsible for the organization of the pigment granules with the end result being a lighter animal. The protein encoded by the pink-eyed locus is responsible for transport of tyrosine into the melanosome where it is used by tryosinase to begin melanin production (Jackson 1994). Mutations in the locus cause the granules to form flocculent clumps. There is also a marked reduction in the number of granules, especially in the cortical cells. The eye color is usually effected more than hair color. The effect is greatest in the eumelanin’s granules because they are the most dependent on the melanin organization. In backgrounds where phaeomelanin synthesis does not occur, the animals are gray. This genetic locus may be restricted to only a few mammalian groups. The wild type allele is dominant with the mutated recessive alleles causing a reduction or spotting of coat color. Eye color is usually effected more than the coat colors with the homozygous recessive animals having pink eyes.
The pigment in the hair and skin is secreted as the melanosome granules into the growing hair or the skin’s keratinocytes. The melanocytes have a highly dendritic or branched form that is crucial to this process. Several mutations effect the morphology of the melanocytes, specifically the dendritic nature which in turn affects the pigment transport into the hair.
Dilute locus: d
Mutation in the dilution locus lead to melanocytes without dendrites (Provance et. al., 1996). The loss of dendrites cause the granules to clump into larger group rather than be uniformly spread out. This causes the overall color to appear lighter. Dilution is not due to a decrease in pigment, in fact animals with a dilution mutation often have slightly more overall pigment. Clumping of the melanin granules occur in the medullary compartment of the hair and there is a reduction in the deposit of the granules in the cortex of the hair. The density of the eye pigments of both the iris and the retina will be affected. And there appears to be a decrease in body size if the locus is mutated. The wild type allele is dominant, so the effects of any mutation in this genetic locus is seen in homozygote only. Two color effects are seen depending on the recessive allele, a dilution of the normal color or a bluing of the normal color. Some mutations (particularly the one that gives a blue color) can develop severe convulsions at a young age. The mutation acts biochemically like human phenyketonuria (Jackson 1991). Several other genetic loci, leaden and ashen, also have similar effects. Unlike dilute, leaden and ashen do not appear to have development problems. Also, the eye color does not appear to be effected in mutations in these other genetic loci.
Spotting or Piebald locus: s
Only epidermal and follicular melanocytes are effected by mutations in the spotting locus. Eye pigment in the retina is not effected. Mutations cause irregular white spotting which tends to be more common ventrally (belly) and on the extremes. No melanocytes can be detected in the white areas. The wild type allele is dominant with the mutated recessive alleles causing spotting of coat color. Several recessive alleles cause what is referred to as a piebald phenotype. The spotting locus has an associated complex called the “k” complex that controls the extent of spotting. The “k” complex has been analyzed and consists of numerous other genetic loci which have small effects individually. But they have additive effects and can also influence spotting to specific regions. As described before, there are 14 primary regions of the body that were populated from clones of one of the 14 primordial melanocytes. The “k” complex can influence specific spotting within one of these regions only or multiple regions (Jackson 1994). It is believed that the Spotting locus together with the “k” complex inhibits the evelopment of melanocytes
Microphthalmia locus: MI
Most of the numerous alleles of Mi, when homozygous, result in no pigmentation in the hair or skin. The effects can also be seen in the eyes and can mimic albino animals. However, unlike albinos, the lack of pigment is due not to a lack of enzyme but to the complete lack of melanocytes. The Mi alleles frequently produce white spotting when heterozygous. Eye color will also be effected usually reducing a black eye to a dark ruby. Again where the spotting occurs, no melanocytes are present. The protein product encoded by the microphthalmia or Mi locus is a transcription factor that controls melanocyte development (Bentley et al., 1994). Without this protein neural crest cells do not become melanocytes. It also effects two other cell types, mast cells and osteoclasts, so extensive loss of this protein as is the case in homozygotes can cause problems in bone development.
Steel locus: SI
Most steel mutations are loss of function mutation with less or no protein being produced. The protein encoded by the steel locus is the signal for the receptor protein encoded by the W locus (Jackson 1940, Williams et. al., 1992). The steel signal is needed to keep melanocytes from undergoing a programmed cell death (Murphy et. al., 1992). What happens in the animals with a steel mutation is that the neural crest cells migrate correctly and develop into melanoctyes but then die (Murphy et. al., 1992).
Dominant White Spotting or W locus: W
The protein encoded by the W loci is called Kit (Besmer et. a1., 1993). As mentioned above, it is the receptor for the steel protein signal. The Kit protein translates the steel signal inside the melanocytes and the melanocyte remains alive. W/+ (the heterozygote) mice generally have variable amounts of white hairs interspersed among pigmented hairs and a well defined white belly spot. The amount of white spotting is dependent on another whole group of genetic loci m(W) which modify the effects of W (Silvers 1979). W usually behaves as a semidominant but can appear acting as a recessive because of the multiple affects of the m(W) modifiers. In all cases though, the white spots are cause by the absence of melanocytes in those areas.