In December 2011 we reported an increased incidence of polymelia (notomelia, cephalomelia and pygomelia) in Angus cattle which appeared to be transmitted as a heritable disorder. ( See Flock and Herd article )
Here we present an update on our more recent work in this heritable syndrome.
In August 2013 a new autosomal (Chromosome 26) recessive mutation was identified in Angus cattle with polymelia by Prof. Jonathan Beever of the University of Illinois at Urbana, USA, using samples from Australian calves with polymelia that we sent to him in 2011 and 2013. The mutation causes a single amino-acid substitution at a locus that has been conserved without change throughout the evolution of vertebrate and non-vertebrate animals.
As a result of the research described below which demonstrated that polymelia is just one of several forms of developmental duplication caused by this same mutation, the name of this syndrome was changed to "Developmental Duplications" (DD).
Angus Australia, the American Angus Association and Angus breed societies in other countries have declared DD to be a recognized genetic condition in the Angus breed.
The DD allele is transmitted in Angus and Angus infused breeds (Brangus, Black Simmental, etc) by descendents of a 1977 born US AI sire, Ken Caryl Mr Angus 8017. Almost all DD carriers in Australia today are however descendants of a fifth generation descendent of this 1977 bull, the influential US Angus sire, B/R New Design 036 born in 1990.
Sporadic cases of polymelia have been reported in other breeds including Bos indicus breeds but it is not yet known whether these cases result from the same mutation. One Angus calf with notomelia that does not carry the DD mutation has been identified in NSW, so phenocopies do occur, as is expected.
The economic significance of DD lies partly in the high incidence of dystocia with polymelic and conjoined calves, the high mortality rate in these calves and the cost of the surgical amputations that are necessary in many surviving cases. However, the DD genotyping data also suggests that a significant proportion of DD homozygotes are "missing" from the population - more than would be expected from the reporting rate for DD phenotypes in neonatal calves. This suggests there may be significant embryonic mortality in DD, as reported with similar syndromes in other species.
An analysis of the Angus Australia database has shown that more than 15% of currently registered Angus cattle carry the DD mutation (GeneProb). This analysis also showed that the carrier frequency has been rising rapidly in recent years with the widespread use of AI sires that are sons and grandsons of B/R New Design 036. Without detection of the mutation it is estimated that the DD carrier frequency would have reached 25% in the next decade. Based on the genotyping data, the mortality in Australia from all of the DD phenotypes described below (including embryonic mortality) was estimated to be about 3,000 per annum and rising rapidly towards 10,000 per annum over the next decade. Application of the DNA diagnostic test for identification of DD carriers will however now counter this increase.
With more than 36,000 DNA tests for DD conducted since September 2013, it is now well established that the abnormal phenotypes caused by the DD mutation are transmitted with incomplete penetrance and variable expressivity. Penetrance of the DD phenotype is estimated to be 54%, with 46% of the expected number of homozygotes actually present in the population as apparently normal individuals, although some of these animals are likely to have sub-clinical DD lesions and develop clinical signs at a later time.
Commercial DNA diagnostic testing for the DD mutation is now available through the University of Queensland Animal Genetics Laboratory and Zoetis Australia. DNA testing can be done on tail hair, EDTA blood or tissue samples. Tail hair is preferred.
The following genotype nomenclature has been adopted in Australia.
DDF Free - Animal is a non-carrier based on a DNA test result
DDFU Free Untested - Animal is expected to be a non-carrier based on its recorded pedigree
DDC Carrier - Animal is heterozygous for DD based on a DNA test result
DDA Affected - Animal is homozygous for DD based on a DNA test result. However, due to incomplete penetrance of the DD phenotype, a DDA animal may or may not express clinical signs of DD. Many DDA animals appear to be normal.
DD__% - Probability% that the animal is a DD carrier, calculated by "GeneProb" software from the animal's BreedPlan recorded pedigree
Although the DD mutation was identified initially in calves with polymelia, we had suspected some association between polymelia and conjoined twins and had included a DNA sample from a calf with a partial conjoined "twin" (caudal axial duplication) with the polymelia DNA samples sent to Prof. Beever in 2013. This conjoined calf tested homozygous for the DD mutation.
Based on a literature review of limb duplications and conjoining, we began to suspect that the phenotypes associated with the DD mutation would be much wider than just polymelia and conjoining. With support from Angus Australia and using the DNA test for DD which became available at the Animal Genetics Laboratory of the University of Queensland in September 2013 under license from Prof. Beever, we began a survey of abnormal calves with high DD risk pedigrees and likely phenotypes. Within a few weeks a wide range of new DD phenotypes was identified.
Based on this work, it is now clear that DD is a fundamental defect of embryonic neurulation involving both cranial and spinal dysraphism. (In cattle, neurulation commences before implantation of the embryo in the uterine wall and is complete by the fourth week.)
The wide range of spinal and cranial dysraphism related abnormalities that has now been identified in DDA animals, either together in the same animal or as so-called "isolated phenotypes", has also been reported in dysraphic syndromes of other species.
Nothing is currently known about the function of the protein coded by the DD gene but it is likely to be a regulator of gene transcription in cell-cell signalling during neurulation. The DD gene is not a vertebrate homolog of the slimb gene that is involved in limb duplication in Drosophila melanogaster. The DD and slimb proteins do however have some common features.
The clinical presentations of the DD phenotype identified to date include:
However, these teratomas can also occur near the thoraco-lumbar junction (in close proximity to the site in the embryo of the organising node of the primitive streak). Some thoraclumbar teratomas are embedded in the dorsal process of a host vertebra. All of these teratomas have a substantial neural connection to the host CNS. This neural connection can be a duplication/division in the brainstem, a duplication/division of the terminal spinal cord in the sacrum or an abnormal nerve root arising from the dorsal surface of the spinal cord, often associated with an area of diastematomyelia or diplomyelia and syringomyelia in the host cord. Teratomas linked to the mid-brain can be entirely intracranial, partly intracranial through an abnormal opening in the occipital bone or entirely extracranial and linked to the mid-brain by a nerve passing through an abnormal foramen in the occipital bone. In one DDA calf we observed an intracranial neural tumour that did not appear to be a mature teratoma, This solid glistening white tumour was located in the arachnoid layer of the meninges overlying the occipital lobes and cerebellum. It had a strong neural connection to the brainstem, as seen with the teratomas.
Other presenting signs are expected in DD based on the comparative pathology but have not yet been detected in DDA calves. It is possible that some of the duplications observed externally in DDA cases can also occur internally. An intra-abdominal leg was reported in one human dysraphic case. Based on observations from human medicine and laboratory animals, it is also expected that a proportion of congenital teratomas will become active later in life, with any expanding intracranial or intramedullary teratoma quickly becoming clinically significant. Dysraphia has also been associated with thymic aplasia and aortic root anomalies. We are continuing to survey Angus calves for further DD phenotypes.
Suspect DD cases that match the clinical descriptions above are needed for this research - live animals or fresh cadavers. Brains, spinal cords and teratomas are the tissues of most interest, but they are difficult to remove for fixation without destroying some of the significant abnormalities. Histopathology has been limited to date by significant autolysis in the case material received.
DNA testing from EDTA blood or tail hair samples of abnormal calves that meet the case criteria above and are submitted through Dr Denholm (see contact details below) with a full history and a photograph of any external abnormalities or necropsy findings will be undertaken free of charge to the submitter until further notice.
We suggest that EDTA blood from any newborn Angus calf or aborted foetus which meets any of the phenotype descriptions above should be routinely submitted for DNA testing for DD. Prof. Beever is also seeking ongoing submission of DNA samples from all polymelia and conjoined twin cases to continue this research. DNA samples from DDA calves will be routinely forwarded to his laboratory in Illinois by Australian laboratories.
Photographs of DD cases and the anatomic pathology in this heritable disease can be viewed online at Dr Denholm's Photobucket site.
Field veterinarians who identify suspected cases of DD are asked to contact Dr Denholm in the first instance on 0418 641957, any time of day or evening including weekends.
Dr Laurence Denholm
BVSc(Hons) LLB(Hons) DipAgSc GradDipLegPrac PhD(Cornell)
Principal Policy Analyst
Strategic Policy and Economics Division
NSW Trade and Investment
Locked Bag 21 ORANGE NSW 2800 Australia
T 61 2 6391 3634 F 61 2 6391 3650 M 0418 641957
|| Posted 4 December 2013 ||