TheScienceofSabinoAug07.pdf

Is your horse a sabino? Geneticists are

working to answer that complex question.

Find out what they’ve discovered and what

it means for your breeding program.

by IRENE STAMATELAKYS

ike many Paint Horse owners, I

like to play “name that coat pat-

tern.” At the novice level of this

game, you have to correctly sort the

tobianos from the overos. At the in-

termediate level, toveros are thrown in

to increase the difficulty. At the ad-

vanced level, you divide the overos

into frame overos, sabinos and

splashed whites. Get bonus points if

you correctly guess the color, too.

Just when I thought I had mastered

the coat-pattern game, I learned that

a genetic test recently became avail-

able to determine whether a horse car-

ried the Sabino 1 gene—the cause of

one of the sabino patterns.

Now, I was pretty sure that my

chestnut overo mare was a sabino, so

my curiosity was piqued. Wouldn’t it

be cool to get her tested and find out

for sure? What is a Sabino 1 horse?

How many sabino patterns are there?

What would the test results tell me?

What if my mare wasn’t a sabino after

all? And most important of all—was

it worth spending the money?

Curiosity finally got the best of me,

and I pulled 30 mane hairs, sent them

off to a laboratory, crossed my fingers

and determined to find out as much as

possible about the Sabino 1 gene. Here’s

what researchers know about sabinos

and whether the Sabino 1 test should be

a part of your breeding program.

An unexpected find

That the Sabino 1 gene mutation

was discovered at all is amazing. Dr.

Samantha Brooks of the University

of Kentucky was actually researching

the tobiano pattern in 2005 when

some unusual foals caught her at-

tention.

“I was on a site visit with a local

Tennessee Walking Horse breeder

to collect some blood from one of

her tobiano horses,” explained

Brooks. “She had two all-white

foals that year, both healthy and

sound, out of rather plainly marked

mares. Both mares had four socks

and a blaze. Since I was there, I col-

lected blood from both foals and

their relatives.

“Once I got back to my office and

did a little research, it became appar-

ent that the pattern on the mares, and

the white coat of the foals, closely re-

sembled patterns produced by simple

Homozygous SB1

horses are mostly

white, at least 90

percent depigmented

at birth.

The Sabino 1 test allows breeders to differentiate between a horse that is

homozygous for the Sabino 1 gene and one that is heterozygous for the

dominant white gene.

62 u PAINT HORSE JOURNAL u AUGUST 2007

Dr. Brooks and Dr. Bailey defined sabino in the horse as a white spotting pattern characterized by white patches with

irregular borders on the face, lower legs or belly and interspersed white hairs on the midsection.

mutations of the KIT gene, a gene I

was already working on because it was

known to be linked to tobiano.”

In the article that resulted from the

research Brooks and her colleague Dr.

Ernest Bailey conducted, it is ex-

plained that the KIT gene is responsi-

ble for causing spotting in mice, pigs

and humans, similar to the sabino

phenotype found in horses. In the

mouse, the characteristics for het-

erozygotes are white markings along

the mid-ventral line often extending

to the extremities, white head spots

and some dilution of the remaining

body color. Homozygotes are com-

pletely white with black eyes.

Brooks and Bailey defined sabino in

the horse as a white spotting pattern

characterized by white patches with

irregular borders on the face, lower

legs or belly and interspersed white

hairs on the midsection. The white

areas lack pigment, both in the hair

and the skin.

They hypothesized that the KIT

gene—which had already been linked

to two spotting patterns in the horse,

roan and tobiano—was also responsi-

ble for sabino spotting.

“As it turned out, I included the

samples from the white foals with my

tobiano samples when I sequenced

the KIT gene and ‘eureka’—found a

large piece of the gene missing in the

white foals,” said Brooks. “This miss-

ing piece is due to a mutation in the

gene that causes it to be abnormally

processed as it is prepared to be used

to make the final protein.”

That “eureka” moment lead to a re-

search project involving three families

of Tennessee Walking Horses and in-

dividuals from 12 other breeds, with a

total of 320 horses tested. Blood sam-

64 u PAINT HORSE JOURNAL u AUGUST 2007

ples, pedigree information and pho-

tographs showing coat color patterns

were analyzed. In all three Tennessee

Walking Horse families, the only

spotting pattern present was sabino.

Why were Tennessee Walkers the

focus of the study, when so many

sabino-type Paint Horses are avail-

able? The answer is simple.

“The problem with studying spot-

ting patterns in the APHA industry is

that there are many, many horses with

more than one pattern,” said Brooks.

“This really causes problems because it

interferes with our ability to define a

phenotype.

“For a while it was more fashionable

to have a solid-colored Tennessee

Walker, so now the variety of spotting

genes is limited in that population. It is

easier to study one spotting pattern at a

time without worrying about other pat-

terns interfering. So I did not use any

APHA families for the initial inheri-

tance studies in that paper.”

In the study, the Tennessee Walking

Horses were divided into three

groups: those without the sabino

spotting pattern, those with the

sabino pattern and those with mostly

white body color who were the off-

spring of two sabino parents.

During the course of the project,

another interesting question arose.

Were those white foals possibly ho-

mozygous for this sabino gene?

The researchers hypothesized that

homozygosity would result in a pheno-

type with extensive white coat color, at

least 90 percent depigmented from

birth. If the foals were white at birth,

this would exclude the possibility that

they later turned white due to the gray

gene, which eliminates a horse’s normal

coat color as it ages. (See “Gray isn’t a

‘color’” in the August 2006 issue of the

Paint Horse Journal .)

All the white-colored horses that

participated in the study were tested

for other genes that might imitate or

conceal the effects of a sabino gene,

such as overo lethal white foal syn-

drome causing frame overo spotting

patterns, cream and tobiano.

DNA sequencing showed that horses

expressing this particular sabino phe-

notype, both spotted and the more ex-

tensive white type, had a mutation of

the KIT gene—exon 17 was missing.

Brooks and Bailey were able to prove a

complete link between this mutation,

which they designated as SB1, and this

coat pattern in the Tennessee Walking

Horse families in the study, which they

named Sabino 1.

Five horses were homozygous for

SB1, and all five were white, a pheno-

type they called sabino-white. The 68

horses with one copy of SB1 all ex-

pressed the Sabino 1 phenotype or were

multi-patterned. Some of the multi-

patterned horses appeared to be all

white, but they also carried genes for

frame overo and tobiano, which

demonstrates an additive effect of white

spotting patterns.

But also of major importance was

the fact that 13 horses expressing

sabino-type patterns did not have

the mutation. Presumably, other ge-

netic factors can also produce what

we describe as sabino.

Sabino 1 gene’ (SB1) or ‘the sabino

genes.’”

In the case of Brooks and Bailey’s

study, they defined Sabino 1 horses as

those with three of the four following

characteristics:

• two or more white feet or legs

• a blaze or white patch extending the

length of the face

• jagged margins around white areas

• spots or roaning in the midsection

Beyond Sabino 1, it is difficult to

assign definitive names and de-

scriptions to the other patterns in

this group. Common terms used in

the past include “minimal sabino,”

“maximum sabino,” “sabino white,”

“white sabino,” “roaned sabino,”

“sabino roan” and “Clyde-type

sabino.” From a scientist’s point of

view, these are rather imprecise.

“Many people who contact me are

very caught up in specific characteris-

tics—lip spots, for example, or ‘light-

ning strikes,’” said Brooks. “I’ve had

many an owner tell me that their

horse must be sabino because it has

this one leg with a sock that is pointy

at the top, and that this is a sabino

characteristic. Well, yes and no. Many

sabinos do have pointy socks, but

there are many different biological ex-

planations for pointy socks, only one

of which is Sabino 1.

“When I look at a pattern, I want

to know what may be discernible

about the genetics,” Brooks explained.

Define “sabino”

That last point brings up one of

Brooks’ pet peeves.

“You can’t say ‘the sabino gene,’”

Brooks explained. “It doesn’t work

that way. There are many genes

that cause patterns that are com-

monly described as sabino, at least

five [patterns] that I have seen. All

of these have fundamentally differ-

ent genetic causes, potentially dif-

ferent genes, so you really have to

specify. It’s correct to say ‘the

Sabino 1 characteristics

Heterozygous SB1 horses generally exhibit three of the four following

characteristics:

• two or more white feet or legs

• a blaze or white patch extending the length of the face

• jagged margins around white areas

• spots or roaning in the midsection

Homozygous SB1 horses are mostly white, at least 90 percent depig-

mented at birth.

An in-depth description of the characteristics and range of expres-

sion commonly associated with sabino patterns is available on the

APHA’s Web site at apha.com/breed/geneticeq4.html. You can also

download APHA’s “Guide to Coat Color Genetics” at apha.com/

forms/guidebooks.html.

PAINT HORSE JOURNAL u AUGUST 2007 u 65

Dr. Brooks and Dr. Bailey hypothesized

that the KIT gene—which had already

been linked to two spotting patterns in

the horse, roan and tobiano—was also

responsible for sabino spotting.

“I want to know what population of

horses it exists in and what mode of

inheritance it is transmitted by.

“The draft-type sabino, for exam-

ple, is present in heavy horses, Shires,

Clydesdales, etc. This population,

until the recent popularity of draft-

crosses and spotted drafts, was not in-

termingled much with the light

horses. And, though it seems to be

dominantly inherited, it does not

produce a white phenotype.”

While there is discussion that some

sabino patterns may be polygenic

(coming from more than one gene),

this is not the case with Sabino 1,

which is caused by a single gene—

SB1. The Sabino 1 has a semi-domi-

nant mode of inheritance. This means

that heterozygotes do not look identi-

cal to homozygotes.

Although the presence of the sin-

gle, dominant allele, in this case SB1,

is clearly visible, it’s effect is not as

strong as when two dominant alleles

are present.

Compare this to the tobiano pat-

tern, which is caused by the action of

a single, dominant gene. It is impos-

sible to tell with absolute certainty if

a tobiano is heterozygous or ho-

mozygous just by looking. In the

case of a Sabino 1 horse, the ho-

mozygous horse is clearly different

from the heterozygous horse. The

first is completely white or nearly all

white. The second always expresses

the Sabino 1, but is easily distin-

guishable from the first.

Even though we classify the Sabino

1 horse as an “overo” pattern, the SB1

gene is not associated with overo

lethal white syndrome. In homozy-

gotes, Sabino 1 produces a perfectly

viable white, or nearly all-white,

horse. Of course, if a horse carried

genes for SB1 and OLWS, it could

potentially produce a lethal white

foal.

How does a mutation in the

KIT gene create the sabino

pattern?

According to Brooks and Bailey’s article, the KIT gene encodes the

growth factor receptors of mast/stem cells. Melanoblasts—early forms

of melanocytes or the specialized cells containing melanin—come from

either side of the neural crest in the embryo. These melanoblast cells mi-

grate away from the neural crest toward the extremities, finally becom-

ing part of the epidermis, the outer layer of skin.

Although several factors control the development of melanoblasts into

mature skin cells that produce melanin, KIT signaling is necessary for

that process to happen normally. The alteration of the KIT gene affects

the way it works. Brooks and Bailey provide evidence that shows that

because the KIT gene is missing a piece—exon 17—it causes the white

spotting pattern they named Sabino 1.

A widespread phenomenon?

At this point, no one knows how

prevalent the Sabino 1 gene mutation

is in a specific breed or in the general

horse population.

Brooks and Bailey’s study included

320 individuals from 13 different

breeds. Tennessee Walking Horses

made up the majority of subjects,

with 110 tested. Among those, 67

were not carriers, 39 were heterozy-

gous and had one copy of the SB1

gene, and four were homozygous with

two copies of the gene.

66 u PAINT HORSE JOURNAL u AUGUST 2007

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