Stalking the Lethal White Syndrome

University of Minnesota researchers track down
the gene responsible for lethal white foals

By PAUL D. VROTSOS RVT and ELIZABETH M. SANTSCHI DVM
from the July 1998 Paint Horse Journal

In order for a lethal white foal to be produced, each parent must contribute a copy of the lethal allele. It is important to note that the genetic makeup of the lethal foal will not affect that of subsequent foals.

This condition, referred to as Overo Lethal White Syndrome (OLWS) is always fatal, and results in both emotional and economic loss to breeders.

In 1996, the University of Minnesota Genetics Group sent a grant proposal to the American Paint Horse Association. We requested funds to study OLWS using newly developed techniques that allow scientists to decode genes. In September of that year, APHA gave the Group $7,500 to begin the investigation.

The Equine Genetics Group is comprised of faculty and staff who seek to improve equine health and breeding by investigating the genetic basis of diseases found in horses. The OLWS group includes veterinarians Dr. Elizabeth Santschi and Dr. Stephanie Valberg, biochemist Dr. James Mickelson, and Paint Horse breeder Paul Vrotsos, who manages the university's large-animal hospital.

The focus of the Equine Genetics Group is to find the genetic basis for OLWS. If a causative gene could be found, Paint breeders could make better-informed breeding decisions and minimize the occurrence of OLWS.

The body of all living things is made of protein, mineral and water. The proteins comprise a large proportion of the body's structure, and also regulate body functions by acting as hormones and enzymes.

The building blocks of proteins are amino acids, and the function of the protein is dependent on the order of the amino acids. The order of the amino acids is determined by DNA, which codes for the composition of the body, its growth and function.

The code for inherited conditions such as OLWS is in the DNA, and therefore its composition must be determined before it can be known how it causes disease.

Similar to how a protein is made of amino acids, DNA is made of bases. These bases are arranged in triplets that code for the amino acids.

When the base pairs change (through mutation) and cause an amino acid substitution, the function of the protein coded is altered. The alteration of protein function causes genetic traits, including disease.

The University of Minnesota has been involved in research on OLWS for many years. The first anatomic description of OLWS in 1982 was by Dr. Bruce Hultgren, who also noted the similarity of OLWS to certain conditions in humans and lab animals.

Paul Vrotsos was motivated to pursue the cause of OLWS because his mares had delivered OLWS foals. He was aware, based on his own experience and scientific training, that there was misinformation circulating about OLWS. Some breeders were basing breeding decisions on this erroneous information.

He also knew that small breeders such as he need the best information available to be successful.

Vrotsos joined the Equine Genetics Group to help find a molecular cause of OLWS, so we could begin debunking the myths and replace them with scientific fact.

The work began, as many projects do, in the library.

This overo mare has had three lethal white foals. The only way to know if her tobiano foal is a carrier of the lethal gene is to bloodtest it.

We knew that causative genes responsible for similar conditions in other species had been detected. We elected to sequence (determine the order of base pairs and the resulting amino acids) for two "candidate genes."

During the summer of 1996 (mid-point in Minnesota's breeding season) we collected tissue from four OLWS-affected foals. During the winter and spring of 1997, sequencing of tissue from normal horses and those displaying OLWS was performed.

By early summer, we thought we had detected a mutation; two base pairs in OLWS foals differed from those found in normal horses in a gene that controls an important receptor. This changed an amino acid in a critical section.

The next step was to develop a test for the normal and lethal sequences, and test samples from OLWS-affected foals, the parents of those foals, and unaffected horses. This would determine the correlation between the gene sequence and the disease.

Each gene has two units (alleles), and foals inherit one allele from each parent. We expected OLWS foals to have two lethal alleles (L), their parents to have one normal (N) and one lethal allele, and solid-colored horses of other breeds to have two copies of the normal allele.

To test the theory, we asked for and received the support of the Minnesota Paint Horse Association. They provided approximately 100 blood samples for analysis.

We were gratified when test results were as expected; all OLWS foals were L/L, their parents N/L, and solid horses N/N.

This is strong evidence that the gene mutation detected is responsible for OLWS. To date, we have tested almost 1,000 horses, and the results are consistent.

The next step to prove that this mutation causes OLWS is to demonstrate a change in the function in the receptor in tissue, and that work is progressing.

There are some Paint Horses born completely white with blue eyes that survive. These are often termed "living lethals," a misnomer as they are not homozygous for the lethal allele.

Concurrently, we are testing Paints from all over the country to determine the association of the Lethal allele with coat color. While the results are not yet final, it appears that all overo horses are not the same, at least for this gene. Overos can carry either (N/N) or (N/L). We have not found a living adult horse that has two copies of the lethal sequence (L/L), and we have tested several all-white Paints.

We have found carrier horses in overos, tobianos, toveros, Solid-colored Horses, crop-out Quarter Horses and Pintos. The discovery of the lethal allele in Pintos is important in limiting the spread of this mutation, because many Pinto breeders are unfamiliar with OLWS and the gene is now making its way into other breeds that are crossing onto Paints for color production.

By taking the lead in the investigation of OLWS, the APHA has provided valuable information to all breeders. At the University of Minnesota we are excited about these discoveries and the avenues of investigation they open.

Our success is due to the collaborative efforts of scientists, the Minnesota Paint Horse Association and the APHA. We are now anxious to move forward in the discovery of new information about the inheritance of coat color, coat patterns and equine disease.

Don't Believe It

Following are nine common myths regarding lethal white syndrome. The correct information is provided by the University of Minnesota Equine Genetics Group.

Myth #1: All overo horses are carriers of the lethal allele.

Fact: There are many overos that do not carry the lethal allele.

Myth #2: Twenty-five percent of foals from two overo parents will be lethal whites.

Fact: Because there are overos that do not carry the allele, the incidence of lethal white syndrome is less than 25 percent in overo-to-overo matings.

Myth #3: Registered tobianos, Solid-colored Horses, or Paint crosses cannot carry the lethal allele.

Fact: There are tobianos that have overo bloodlines, and these horses can be carriers of the lethal allele. Solid-colored Horses and Paint crosses can carry the lethal allele.

Myth #4: Totally white Paints are not carriers of the lethal allele.

Fact: These white horses are often carriers of the lethal allele.

Myth #5: All totally white foals born to two overo parents are lethal whites.

Fact: There are totally white Paints that are not affected by the lethal white syndrome.

Myth #6: Mares cannot produce lethal foals in consecutive years.

Fact: The genetic make-up of one foal does not affect subsequent births.

Myth #7: Only one parent determines if a foal will be a lethal white.

Fact: Both sire and dam contribute a copy of the lethal allele.

Myth #8: Crop-out Quarter Horses cannot carry the lethal allele.

Fact: A small number of crop-outs have been tested and found to be carriers of the lethal allele.

Myth #9: You can reliably tell the carrier status of a Paint by their color pattern.

Fact: This is false.