New genetic defect affects Angus cattle
By Jennifer Bremer
Editor's note: This is the first in a series of stories which will run over the next several weeks discussing genetic defects in cattle and how they have affected the industry in recent years, as well as how to manage affected herds.
Recessive traits are common in all species, including cattle. In recent years it appears that lethal genetic defects in cattle are being reported more frequently. While affected calves are often born dead, carriers of these defects can survive and continue to pass the defective gene to their offspring, resulting in an increased prevalence of the mutation within the population.
In late August 2008, the American Angus Association became aware of a genetic defect affecting the Angus breed and Angus cross cattle. Even though, according to the association, they were contacted about the existence of a small number of calves born dead with bent and twisted spines back in March 2007, it wasn't identified as a genetic defect until August 2008.
Dr. David Steffen of the University of Nebraska has been a contact for many different breed associations when it comes to identifying abnormalities in calves, whether it is from a genetic defect, environmental factors or viral infection.
In a notice sent to the association in August, Dr. Steffen said, "The widespread use of reproductive technologies has the potential to reduce the effective cattle gene pool and can result in the more frequent recognition of recessive defects. Those same technologies that have benefited--and continue to benefit--a breed may also increase the potential for economic harm when recognition of problems is delayed or ignored."
The disorder, dubbed "curly calf syndrome" is now known with the scientific name--arthrogryposis multiplex (AM).
AM is caused by a simple recessive gene, according to University of Illinois researcher Dr. Jon Beever, who has developed a DNA test for AM and other genetic defects.
The disorder, which is lethal, causes calves to have a spine which is bent and twisted, according to Dr. Steffen. "The calves are small and appear thin due to limited muscle development. Legs are often rigid and may be hyper extended (especially in the rear legs) or contracted," he said. "In some cases the rigid limbs result in calving difficulties. Additional unique features are recognized during laboratory examination."
Once Drs. Beever and Steffen had received enough samples from calves afflicted with these problems, the search started for the gene causing the problem.
"I had to sequence the DNA of animals which were non-carriers of the gene and then sequence animals which were known carriers, along with the offspring of those animals and the actual affected calves," said Dr. Beever.
"Non-carriers are homozygous for the gene and carriers are heterozygous for the gene," he added.
As described by the Angus association, if an animal is homozygous for the normal variant (called an allele), they are referred to as AM-free (AMF) indicating that they have been tested for the causative mutation and been found "free" of the mutation.
If an individual is found to be heterozygous, or a "carrier" for the mutation--meaning they possess one normal allele and one mutant allele--they are referred to as AM-carrier (AMC).
Although affected calves are rarely tested, they would be homozygous for the mutation and referred to as AM-affected (AMA). Using the known genetic information from two animals, a breeder can determine the chances of having an affected calf, according to Dr. Beever.
He explained, if the normal gene is "A" and the defective gene is "a," mating a carrier bull with the genotype "Aa" for the AM gene to a carrier cow also with the genotype "Aa" for the AM gene will result in three calves that look normal at birth, but two of the three will be carriers for AM (Aa). The fourth calf will be born with AM (aa). Thus, mating two carriers gives a breeder a 25 percent chance of having an AM calf every time this mating is repeated.
Mating a carrier bull or cow to a non-carrier cow or bull will result in 50 percent of the calves being AM carriers (Aa). A non-carrier would have the genotype AA.
Dr. Beever has developed a test for AM, although it is not commercially available as of press time.
"Getting closer to testing and commercialization of the test, we want to reassure commercial cattlemen that mutations are naturally occurring and we are thankful we currently have the technology that can help eliminate these problem genes from the gene pool, regardless of what breed you are dealing with," said Bryce Schumann, CEO of the American Angus Association.
Dr. Beever said the process of developing a test includes having a routine lab method for each test and then determining the accuracy of the test.
While the other tests he has developed have been used in smaller numbers, which could be handled by his lab in Illinois, Dr. Beever expects the AM test to be handled differently.
"Since there are so many animals that will need to be tested all across the country, as well as in Australia and around the world, I would guess the test will eventually be available by multiple labs," he said. "The Angus association will play a role in figuring out all the logistics concerning testing."
Tracing the gene
After the original notice the first of September, the American Angus Association received written and verbal reports between Sept. 6 and Sept. 15, 2008, of 48 calves previously born dead with physical characteristics consistent with AM. Each report also included the recorded identities of the sire and dam of the abnormal calf.
The association provided Drs. Beever and Steffen with this information as it was available and continues to do so.
Based on those reports, Dr. Beever came to the tentative conclusions that the simple recessive gene tentatively traced from the most recent common ancestor, GAR Precision 1680, and with further research, his sire Rito 9J9 of B156 7T26, a bull born in 1979, is also a known carrier of AM.
Forty-seven of those 48 calves identified to the association contained "1680" genetics on both sides of the abnormal calves' pedigrees.
"With the assumption of a simple recessive inheritance pattern, the risk for producing a calf affected with this syndrome can manifest itself when the recessive gene is found in both the sire's and dam's pedigree," said the association's statement.
A breeder's perspective
Mark Gardiner, of Gardiner Angus Ranch in Ashland, Kan., has seen AM calves on the ranch in southwest Kansas.
"We have seen about 11 calves similar to the AM calves since 1991," said Gardiner, breeder and owner of GAR Precision 1680. "That's 11 in about 27,000 calves born."
While that is a small population on their ranch, Gardiner wants to keep his customers confident in GAR genetics.
The Gardiners have raised Angus cattle at the ranch since the 1930s, with their first Angus being registered in 1947. They have used total artificial insemination in their herd since 1947, collecting carcass data and performance data for many years.
"We are in the business of selling bulls to cattlemen. If there is a problem, we will do everything we can to resolve it. We stand behind our cattle completely," he said. "Our buyers know we will take care of them and that hasn't changed."
Gardiner said once a test is available for AM, they will test their herd for carriers and then make the necessary management decisions.
"Once we know the carriers, we will, in all reality, use bulls that to the best of our knowledge are free of the gene," he said. "If we have bulls we plan to sell that come back as carriers, they will be cut and fed out in the feedlot. Females will likely be used as recipients for our embryo program.
"Knowledge is power with this issue. We have to be aware of these things and make decisions accordingly. The key is to not overreact," Gardiner added. "We need to keep the good and get rid of the bad and move forward to breed good Angus cattle."
"There appear to be lots of look-alikes to this defect. Determining the difference between virally infected calves versus AM calves takes a well trained eye," said Dr. Beever. "I cannot do all this testing and research without the help of Dr. Steffen. I do the genetic mapping and he does the pathological diagnosis."
Dr. Beever noted the AM gene may be present in other breeds besides Angus because of crossbreeding programs and cautions breeders to realize that.
"Genetic mutations are present in all generations and populations of cattle. These mutations may spike in a population when popular animals are carriers of the gene," he explained. "We have to be aware of what is out there and prevent propagating the mutated genes in the future."
Research continues with hopes of commercialization of an AM test soon. As part of the test development, the genotype and AM status of 736 widely used Angus AI sires has been released. The complete list is available on the American Angus Association website. The Angus association also plans to continually update that list as more animals are tested.
"Commercial cattlemen need to work with the seedstock producers they buy their bulls from when making purchasing decisions to better their herds," said Schumann. "It is important for the seedstock producers to manage the situation properly so commercial cattlemen continue to have confidence in Angus genetics.
"Being proactive is very important. I feel the association has been proactive with this issue and I stand behind the decisions our board makes because they are breeders as well," concluded Schumann.
The American Angus Association board of directors will be meeting in the coming week in hopes of making policy regarding AM and AM carrier animals. For the most current information, visit the American Angus Association's website at www.angus.org.
Jennifer Bremer can be reached by phone at 515-833-2120 or by e-mail at email@example.com.