Semi-Dominant Lethal Spotting

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A Mutation Investigation Group Report

Lethal spotting in the UK

 
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In 2008, a thread started on the eGerbil forum telling members that Pets@Home petshop chains were then selling what looked like Extreme White and White paws gerbils at a few of their outlets.  To explain further, White paws gerbils are carrying a single dominant lethal spotting gene, so in effect this type of lethal spotting is said to be in its heterozygous state, if it were homozygous, then the effects would be lethal and these pups die, usually any time after birth until just after being weaned. These gerbils have very little spotting on them, usually it is just an extra wide bib, maybe a small neck spot, and on their bellies they may have a "bikini-line" spot.  As the name suggests, their front paws are white, and hind feet usually have two or three toes that are white, but usually not all are white.  Their toenails are often odd; some nails being pigmented while others are not.  These gerbils are sometimes hard to classify and they can look very similar to the white-socked solid gerbils that come out of Dominant spotting litters.  By contrast, the Extreme White gerbils are produced by combining one of these lethal spotting genes with the well known Dominant spotting gene, and the end result is a gerbil that has very little pigment left in its coat, with just the odd diluted streak or patch around the head and over the rump area. In general, their markings are often usually much greater than those produced by Dominant spotting alone.

With this amount of extreme depigmentation, the end result is that many Extreme White gerbils can also suffer from several health problems.  The reason is very simple; In the growing embryo, pigment cells not only migrate to the base of the hair follicles in the coat but they are also migrating to important areas inside the body, such as the eyes, ears and the  brain, and when they fail to do so, it can cause great impairment to their health.  In the growing embryo, a small fold develops along the dorsal region known as the neural tube, and this contains an active area known as the neural crest, which in turn, supplies the pigment cells which will eventually migrate all over the body.  Not only this, but this same region also supplies the neural cells which eventally supply the nerves to make the intestines function correctly.  If these cells fail to migrate, the end result is known as megacolon.  This renders the animal unable to pass any faeces, which in turn leads to their death.

After getting in touch with the threads author, I managed to acquire some of these "suspect" gerbils to investigate whether this was indeed the case (The gerbils kindly supplied came from Halifax Pets@Home). Although the gene had become common in some parts of Continental Europe, it was still being regarded as being exceptionally rare in the UK and the practice by breeders of intentionally combining lethal spotting with Dominant spotting had already  been condemned in places such as Austria and Germany by many of the experienced breeders there, because of the adverse health effects that had already been well documented previosly  (see this report here).

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Initially I test bred the gerbils to see if this was possible, as Pets@Home supposedly have a no import policy in place, however it was highly probable that either their current commercial suppliers were either importing themselves from Central Europe to meet rising quotas or they had intentionally acquired the gene to enable them to mass produce Extreme White coated gerbils for selling on to pet shops.  In the past the only way to produce gerbils with a great deal of white markings was to selectively breed Dominant spot gerbils over many generations, and the whole process took a lot of hard work and dedication.  The resulting gerbils are often exceedingly tame and usually blessed with good health.  With the appearance of Lethal spotting, suddenly this wasn't the case and any 'breeder' could produce a high white gerbil in just one single generation. The practice for commercial breeders doing this though is quite alarming as these "quick-fix" high white gerbils make exceedingly poor pets and have numerous adverse health problems attached to them due to the practice of combining these two spotting genes together.

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Both of the Extreme White gerbils I received were females, and initial tests indicated that both were deaf.  However, their white markings weren't too excessive, and both had patches over their ears, so the deafness could easily have been the result of poor breeding practices, rather than due to the combination of two spotting genes.  Initially in my breeding programme I allowed a single litter off one of the females to see if the animals were capable of producing very white animals, and this turned out to be the case. The resulting pups were extremely white. These pups also seemed bigger than the solid agoutis in the litter. However the vigour of the pups were extremely poor. The agouti pups failed to grow and died at approx 5 weeks. The Extreme White pups also died at around the ten week stage.

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The Extreme White Agouti female also came with a littermate, a Slate male,  which was suspected to be a White paws, and this too was outcrossed to my own stock.  I had a few goals in mind if the resulting pups were healthy, so I began by crossing the suspected White paws male to a spotted DEH.  The DEH in question came from a spotted line that carried no modifiers for pied or mottled, but was carrying several other recessive colour genes which in turn helped me fathom the suspected White paws males genetic make-up quite quickly. 

 

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The resulting pups in this litter were Agouti, Grey Agouti, black, PEW, and Cream spot.  However, out of the two litters I allowed, no Extreme Whites appeared, but all pups were very healthy. I could of allowed more litters to see if any did subsequently appear but from the two litters there were two individuals that looked very much like White paws gerbils. So the safer route to take was to see if any of these Whitepaws could produce a Rumpblack when bred together, which would then quickly confirm the presence of the Lethal spotting gene. 

 

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 Taking things a step further, I selected a black female from the litters to backcross to the White paws male.The Black I chose did resemble a White paws phenotype, but the amount of white was less than her father.  Incidentally, she also lacked the small "bikini-line" spot too that is common on both White paws types and also the solid types from the US "high-white" coat variety that carry the modifiers for extreme spotting.  In all respects, apart from her odd coloured toe nails she could of easily been mistaken for a normal solid that appears in a mottled litter.  Her main difference though was that she had started fading from a very young age and had a few odd coloured toenails.  This particularly unusual effect of fading in the coat had been noticed previously in some, but not all White paws types.
 

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Other test crosses at this time included the Cream spot backcrossed to the spotted line. All the pups produced were either solid or just had the typical tri-spots (nose, forehead and nape of the neck) that were the hallmark of this particular spotted breeding line.  No White paws or Extreme white coats were observed in the two litters I allowed.  Also the second female from Pets @Home, a black Extreme White,  was outcrossed to a solid coloured gerbil that had no history of dominant spotting in its ancestry.  This particular cross also produced several White paws which were later used in the breeding programme.

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In the first litter from the backcross I was very surprised to see a Rumpblack appear.  This particular Rumpblack differed from all other Rumpblacks that I had previously looked at from the many examples  that had been bred, documented and photographed in Continental Europe.  This particular Rumpblack had patches of dark pigment around the eye and ear areas, as well has having the familiar pigmented rump area.   I maybe wrong but I can't recall any previously being bred in Germany /Austria with any other markings other than the dark rump.

 

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Similar to all Rumpblacks, the one produced in this litter eventually succumbed to megacolon. It did show though, that the gene was capable of delivering some extra pigment to the important migration sites around the head region. However, the necessary neural cells that innervate the intestine had obviously failed to migrate from the neural crest along with the pigment cells while in the developing embryo. It's probably a failure of these neural cells migrating from the neural crest to the end of the intestine that leads to the Rumpblack gerbil succumbing to megacolon in early life. One other thing that was surprising was that it lived until 6 weeks, which at the time was around a week longer than the eldest recorded raised from German breeding lines.

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Whether the animal was variant of Rumpblack or was due to the fact that I selected for less white on the female parent, as opposed to selecting for more white, (if it's down to variants in the spotting modifiers that give rise to rumpblacks with no head pigment) remained unknown at this time.

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Later on In the same year I received a pair of White paws from German/ Austrian breeding lines, who to my surprise, and also the breeding group's surprise,  produced Rumpblack offspring with pigment around the ears and face.  This breeding line though differed from the UK Whitepaws line I was developing as it had much more obvious neck spots, very wide bibs and the socks were also whiter and better defined.  So it seemed that despite the obvious differences in their coat markings, both the German and UK lines were capable of producing homozygous Rumpblacks with a good deal of head pigment, and for all intents and purposes, in both cases it was the same gene at work.

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Rather than test breed further for Extreme Whites being produced, I decided to outcross the UK Whitepaws line to a breeding line of Slates that had been very stable for several generations with no apparent health or seizure problems.   My decision to do this, and move them away from their Dominant spot background was because there was a real risk of polygenes that adversely affect health migrating from one line to the other.  In previous Austrian breeding lines it had been noticed that deaf gerbils were being produced with very little spotting, but their ancestors were all Extreme Whites.

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The fact that using Spotted Lethal as a short-cut to produce high white gerbils has also in the past decimated the health of many breeding lines of Dominant spot in Germany, and indicated that unless extreme caution with selection is practiced it can easily ruin any Dominant spot line. Not all "Sp/Ls"animals are highly mottled, but many are carrying ear problems that become more apparent as they age.  If Dominant spotting co-operates so readily with the new lethal spotting gene, maybe it is also allowing minor polygenes that influence deafness and megacolon to pass over to their Dominant spot offspring, and in so doing, polluting the Dominant spot genepool with unhealthy genes.

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At the time, the UK Whitepaws line still had some obvious faults, the most notable being tail barbering, but females were good parents, never barbering tails, however the problem occurred when they reached adulthood, and not in all but a high proportion of them.  In succeeding generations which are now being line bred, selection has removed this fault.  The other thing noticed is that in some individuals they can fade greatly with age, with a great deal of white hairs appearing in the coat and tail.  This effect had also been noticed in some individuals in German breeding lines, but isn't apparant in the one that I'm currently maintaining who all remain as undiluted black as they age. The German/Austrian line is currently on a non-agouti background and has been linebred for several generations without any outcrossing.  Fertility and health remain good in both lines and breeding gerbils are selected in a simple manner;  the UK Whitepaws line has been selected by only using offspring from litters whos parents give rise to Rumpblacks living over 5 weeks.  Where as the German line has been selected on their white markings, using only those that have the maximum expression for this trait. Interestingly, the UK line has recently produced a Rumpblack that lived to just beyond 12 weeks of age and most live well beyond 6 weeks, but the German line still has a high proportion of Rumpblacks dying before 18 days (before their eyes open).  In all cases the death is due to megacolon.

The Rumpblack gerbil

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As mentioned above the Rumpblack gerbil's coat is created by the Semi-dominant Lethal Spotting gene being in its homozygous state.  When the lethal spotting gene is homozygous and combined with Dominant spotting the two genes readily co-operate  together to remove the last of the remaining rump pigment, which then in turn produces a Black-Eyed White gerbil that also eventually succumbs to megacolon.

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Essentially with the UK breeding line, I was moving further and further away from any Dominant spot ancestry with each generation of WPxWP cross.   Any parents where a RB dies before weaning isn't bred from any further.  Those where they get past weaning, I am then using their WP  offspring to breed further.  Using this basic selection, we are now getting some RB's that are living long enough to examine more closely.  It's still a huge shame knowing at some point they will die from megacolon, and I seriously doubt we  will remove the problem of megacolon,but these limited breeding experiments are showing that there are certain polygenes that are governing the severity of its effect.

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The breeding studies conducted so far have established that there are polygenes at work which can extend the lifespan of the homozygous Rumpblacks, and at present I've personally noticed four modes in the genes spectrum of lethality. Type 1 get megacolon from the first week onwards, which then kills them around 11-12 days. Type two lives till around the eye- open stage (approx 18 days), type 3 seem to do fine until 4 weeks or weaning age, then die around 5 weeks. Type 4 live live beyond 6 weeks.

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In the past some breeders working with this gene offered the Rumpblacks only wet foods but not water where others have tried high water diets plus access to water, which presumably is to soften the faecal matter, but very little helps once the process starts, and their demise is usually swift.

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Those that die  are probably as a result of toxic megacolon and appearing very similar to Hirschsprung Disease in humans.  As such, a high water, soft food diet would do very little to alleviate their condition.  Those that do live beyond 6 weeks seem to have a functioning colon (to what extent we don't know) and seem to defecate normally, but when they do show symptoms we know that basically there are other factors along with lethal spotting that are triggering megacolon at various stages of their early life.  The thing that is key here, is can we select for these polygenes and increase longevity even further, or should we even try to, as it certainly isn't pleasant seeing the Rumpblacks die.

Behaviour

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While the adverse health behaviours have been well documented in Extreme White gerbils, there appears to be no such behaviours present in either the Whitepaws heterozygotes and Rumpblack homozygotes.   While I observed tail barbering in the UK line this was probably due to poor selection rather than the gene itself, as it was easily bred out of the line plus the German line didn't adopt this behaviour at all.  It has been reported in the past that in some German lines, the White paws were extremely aggressive, but the two breeding lines I'm currently maintaining show no overt aggressive behaviour, and can be kept in reasonably large clans (both males and females) but both lines still appear  to be quite nervous and skittish when compared to other breeding lines until they are tamed properly on an individual basis.  However neither lines are prone to seizures even though they both initially display this nervous behaviour.

Types of Lethal Spotting, "spotted lethal" in the gerbil, and the nature of Spotting mutations

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In rats, the spotting lethal (sl) gene is a recessive mutation of the Endothelin-B receptor gene,   which causes depigmentation of the forehead which leads to them having a blaze.  This  is also sometimes accompanied by a lack of neural connections to the colon, which then eventually leads to megacolon and is fatal. There is also a comparable endothelin- B receptor mutation in horses which leads to the condition known as Lethal White Foal Syndrome (LWFS).

 

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Mice too also have a comparable mutation to the spotted lethal rat that affects the Endothelin-B receptor, which is known as piebald spotting (s) .  Piebald (s) spotted mice can vary from showing a small amount of depigmentation in their coats, right up to the "broken" and "even" coat colour varieties that are extensively depigmented. The placement and extent of the markings on Piebald mice are largely governed by other minor polygenes. The gene also has the potential to cause a reduction of melanocytes to the choroid layer of the eye and cause structural damage to the iris. Homozygotes may also develop megacolon, and megacolon itself can also be affected by minor modifying genes.  Also known as another recessive allele on this locus is piebald lethal. Piebald lethal homozygotes are usually almost black-eyed whites with just some occasional small patches of pigmented hairs on the head and/or rump. Some Piebald lehal (sl) homozygotes die as early as 1 or 2 days after birth while others can survive for more than a year. The usual age at death is about 15 days, but regardless of how long they live, they all eventually suffer from and usually succumb to megacolon.

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Although I have shown that modifying genes are at work which can increase a Rumpblack gerbils lifespan to some extent, it remains to be seen if Lethal spotting in the gerbil also has accompanying modifying genes that can affect whether the gerbil succumbs to megacolon or remains free from this condition through out its life.  As noted above with the piebald lethal gene in the mouse, even those with the longest lifespans eventually suffered and died from megacolon.

However, the mouse also has another similar recessive spotting mutation that knocks out Endothelin 3, and which also results in similar effects to the above mentioned spotting genes (depigmentation and megacolon) This gene is known as lethal spotting (ls) and shouldn’t be confused with spotting lethal (sl) in rats which is a variant of the piebald gene.
 

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Because lethal spotting is inherited recessively the heterozygotes are fully pigmented. The homozygotes show variable degrees of white spotting on the back, and their belly is usually white. Phenotypically the homozygous mice resemble s/s mice except that their ears and tail are less pigmented than is usually the case with piebald mice. on some genetic backgrounds lethal spotting homozygotes usually die with megacolon at between 2 and 3 weeks of age (hence their name!), and the few which lived to breed eventually succumb to this condition, however, it was found that by placing the gene on other genetic backgrounds these homozygotes survive quite well, and are about as prone to megacolon as are piebald mice.

In the gerbil, the name for the recent lethal spotting gene has changed a few times from "semi-lethal" spotting and is currently known as "Spotted lethal" presumably after the rat gene of the same name. However, phenotypically it bears a much  closer resemblence to the Lethal spotting alleles in the mouse than it does to the Spotted Lethal (sl) gene in the rat or Piebald (s) in the mouse.  Therefore, throughout the article I've used the term Lethal spotting for this particular mutation in the gerbil to denote its close similarities to the same gene in the mouse.

With regard to spotting mutations, the process of pigment cell migration is extremely complex and it has many steps to follow, so there's also many ways in which it is possible for that process to go wrong, and usually it is due to a mutation.  Migrating pigment cells from the neural crest in the developing embryo all follow  specific molecular signposts and in turn, these signposts then tell the pigment cells where to go in the body. It is important that they get onto the right route as these pigment cells are needed not just for colouring the hair, but for other vital functions elsewhere inside the body.    As well as these signposts having the potential to mutate, a  mutation could just as easily occur in the migrating cells receptors which in turn read these molecular signposts.  A mutation here could cause them to read the molecular sign post differently, or they may be unable to read them at all. As well as pigment cells migrating, neural cells also migrate from the neural crest, and as you can see, this pathway can be easily be modified by any mutation that can cause disruptions anywhere along this particularly complex route, and in so doing delaying the migration of these cells, sometimes severely, so that not all of them make it to their desired destination, and these neural cells are vital for the survival of the animal.

There are many known mutations that affect the migration of cells from the neural crest, some similar in their actions, some quite different different, and many of these mutations can be further modified by other genes. All of these neural crest mutations have the potential to have pleiotropic effects, and as such,can not just affect the coat colour, but also behaviour and sensory function.  This is often why you will see that in many domestic species that white coat colour markings, eye colour anomalies, deafness, and megacolon are often found grouped together in many of the spotted breeds. These traits are all the result of mutations that cause a delay in cell migration from the neural crest.

Historically, the process of domestication and spotting mutations tended to go hand-in-hand as there is a distinct connection between a depigmented coat and subsequent behaviour.  As mentioned earlier, the pigment cells migrate to areas in the brain, and some of these areas are associated with the regulation of mood and also the stress response.  So as early breeders selected for tameness they were also inadvertantly selecting for a subtle alteration in cell migration in the developing nervous system which then led to calmer animals.  A side effect of this type of selection also leads to an alteration in pigment cell distribution in the skin and the result is a piebald coat. Spotted markings, or piebald animals, are known in many differing domestic species and are well known for their docile behaviour.  However breeders also found out in many of these domestic species that selecting for excessive white markings can end up having the opposite effect, and cause neurological impairments.

One of the most noted studies into piebaldism and behaviour was concerning foxes and is currently still running after 50 plus years of experiments conducted in Russia, and was founded by the scientist Dmitri Belyaev.  The study still continues to some extent today, and was under the supervision of Lyudmila Trut*.  The animals in question, the Silver Fox, which is a colour form of the Red fox, are distributed as pets internationally, but at a very high price.

This domestic fox, marketed as the Siberian fox are nothing like their wild cousins and exhibit both behavioural and physiological changes from their wild ancestors. They are friendly to humans, wag their tails, fold their ears down like dogs, and even vocalise like a domestic dog.  They also developed many colour patterns and lost their natural musky odour.

Belyaev believed that a key factor when selecting for the domestication of dogs wasn't size or reproduction, but behaviour, and as such it's their amenability towards tameness that brought about their domestication.

 So Belyaev used wild silver foxes and after bringing them into captivity he set about breeding them specifically for their tame behaviour using various simple techniques to assess each generations tameness.  Over the generations, the results were quite surprising, and the descendents of the wild foxes eagerly approached humans, licked their hands and faces and even tried to attract their attention by whining or wagging their tails.

As well as behavioural differences another side effect was that they began differing morphologically and physiologically from their wild ancesters.  The tame foxes had floppy ears, curly tails and domed skulls.  Females began going into heat twice a year instead of annually, and their coats changed too, and instead of having the usual solid silvery black coat, many had patches of depigmented fur, and the percentage of these piebald foxes rose dramatically in the population.

By simply selecting for tameness and against aggressiveness meant also selecting for physiological changes in the systems that are governed by the body's hormones and neurochemicals.  This quickly lead to many changes in the animal, and one of the most noticeable was mottling or spotting in their coats.

At the same time, the study also conducted breeding experiments selecting only the fiercest, most aggressive animals.  In subsequent generations these animals would snap and attempt to bite any human coming near their cages and showed no fear.

Trut also conducted similar experiments in the Norway rat.  Using wild agouti rats, and selecting for tameness for over 30 generations, she had very similar results to Belyaev's fox studies regarding depigmentation, with the solid coloured rats eventually dissapearing from the population and being eventually replaced by piebald rats.

Lethal Spotting with a Fading coat

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The fading aspect of the Lethal spotting gene in the gerbil is an example of variation in their coat phenotype because not all lines of this spotting gene display it.  Some animals remain very dark and undiluted and dont have a trace of fading as they age, while some individuals have a lot of fading  appearing in their coats.  This unusual effect can probably be explained by haploinsufficiency, or variations in it, between the different breeding lines of this spotting gene. 

With all forms of dominant spotting genes we are in essence seeing that a single wild allele at the locus isn't enough to "repair" the pigment pathway and depigmentation still occurs, so a single copy of a wild gene isn't enough to produce enough of the gene product (which is typically a protein) to bring about the wild type state and we are left with a spotted animal.  However with the fading effect we can see with Lethal spotting, we are seeing that over time an increase in depigmentation of the hair is occuring. So we can see that a single functional wild allele (or its protein dosage) isnt enough to prevent the depigmentation worsening over time and this is being visually displayed in the animals coat.  So maybe variations in haploinsufficiency is causing this? 

A good example of this is the condition retinitis pigmentosa in humans.  There are two wild-type alleles of this gene, one is a high-expressivity allele while the other is known to be a low-expressivity allele. When the mutant gene is inherited with a high-expressivity wild allele, there is no disease phenotype. However, if a mutant allele and a low-expressivity wild allele are inherited, the residual protein levels falls below that required for normal function and the disease phenotype appears.

Retinitis pigmentosa

Candidate genes for Semi Dominant Lethal Spotting in the Gerbil

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In the mouse and rat there are a few candidate genes which as well as causing spotting also cause megacolon in the homozygotes, however until recently these candidates such as piebald lethal, lethal spotting, dom(megacolon) in mice and spotting lethal in rats all differed in some respects from the White paws gene in the gerbil.

This seems to have changed with a very recent discovery and subsequent documentation of a semi-dominant gene in the mouse that is most likely an allele of recessive lethal spotting.  Although Lethal spotting was a close candidate gene for the "spotted Lethal" White Paw gene in the gerbil, it did differ in several respects. With the Lethal spotting gene in the mouse, although mutant homozygotes die at around weaning time, the gene is inherited in a recessive manner.  Not only this, but the phenotype of the homozygous Lethal spotting mouse mutant displays far more pigment than the homozygous Rumpblack coat does in the gerbil.

The phenotype of the homozygous semi-dominant Lethal Spotting mouse is particular severe when compared to that of the recessive allele at this locus. and the pigmentation looks like it is limited to just the rump and the head region.  With the appearance of the UK Rumpblacks (and now also the European line) with head pigmentation along with a pigmented rump, the similarities of these two phenotypes are quite close, as is their pathogenic phenotype.

The appearance of this mutant gene in the mouse  may have made it possible to shed some more light on a similar mutation in the gerbil, and also it may help to pinpoint a little closer as to which gene maybe responsible.

Below is the current research on this new semi-dominant spotting gene in the mouse;

A new mouse mutation named semidominant lethal spotting is mapped to Chromosome 2

-Belinda S Harris, Patricia F Ward Bailey, Roderick T Bronson, Kenneth R Johnson and Muriel Davisson

 Source of Support: This research was supported by grants RR01183 to the Mouse Mutant Resource (M.T. Davisson, PI) and Cancer Core Grant CA34196.

Mutation (allele) symbol: Sls

Mutation (allele) name: Semidominant lethal spotting

Gene symbol: Sls  (may be an allele of Edn3)

Strain of origin: C57BL/6J

Current strain name: C57BL/6J-Sls/GrsrJ

Phenotype categories: skin and hair, homozygous lethal at weaning

Abstract

 

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A new mutation named semidominant lethal spotting (Sls) arose on the C57BL/6J strain in the Mouse Mutant Resource at the Jackson Laboratory. Mice homozygous for  this semidominant mutation may exhibit megacolon ending in lethality. Coat color varies from  large area of white spotting in the homozygote at weaning age, small white spots on the belly in the heterozygotes, and normal black coat color in the wild type. This mutation has been mapped to the same chromosomal region of Chromosome 2 as Edn3<ls> (lethal spotting)[MGD].

Origin and Description

The Sls mutation arose on C57BL/6J and is maintained by heterozygote x wild type matings. When heterozygotes are mated together, 3 distinct phenotypes are produced. At weaning age, a large area of white spotting in the homozygote is seen (See Photo above), small white spots on the belly in the heterozygotes, and normal black coat color in the wild type. When two wild type animals are mated together they produce only wild type animals.

Genetic Analysis

Using our standard mapping protocols a C57BL/6J-Sls/+ mouse was mated to a CAST/Ei mouse to produce F1s. The F1 progeny were then intercrossed and produced the F2 progeny used to map this mutation. The F1 progeny produced by this mapping cross to CAST/Ei were all normal appearing, but when mated together produced 3 distinct phenotypes. The Sls mutation maps between D2Mit113 and D2Mit148 and is non-recombinant with D2Mit213. Genetic linkage and marker order were analyzed with the Map Manager software program (Manley, 1993). The recombination estimates with standard errors and best gene order are: centromere-[D2Mit51] -17.3 +/- 7.2 - [D2Mit113] - 2.4 +/- 2.4 - [D2Mit213, Sls ] - 2.4 +/- 2.4 - [D2Mit148] -  4.8 +/- 3.4 -  [D2Mit200]. Based on the Ensembl assembly Build 32 for Chromosome 2, our non-recombinant marker D2Mit213 is at 175.1 mb and Edn3<ls> (lethal spotting) is at 175.3mb.

Pathology

Our standard pathological screen showed  hydronephrosis and hydrocephalus in one mutant, but other heterozygotes showed no lesions. Some homozygotes had megacolon. Hearing assessed by ABR testing and opthalmascopic eye tests showed both mutants and controls are normal.

Discussion

This mutation was named semidominant lethal spotting to indicate similarity with but differences from the original lethal spotting. In comparison with lethal spotting, Sls is semidominant, has a severe, sometimes lethal phenotype in the homozygote, a mild spotting phenotype in the heterozygote, a normal black coat in the wild type. Sls is probably allelic with the lethal spotting mutation (Edn3<ls>) because of its' chromosomal position and similar phenotype.  A direct test for allelism was not possible because Edn3<ls> is now available only as cryopreserved embryos.

Acknowledgements

The authors wish to thank Coleen Marden, Jane Maynard, Heping Yu, Qing Yin Zheng, and Norm Hawes for their technical expertise.

References

Manley KF (1993) A MacIntosh program for storage and analysis of experimental mapping data. Mamm Genome 4,303-313

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