Whilst sporadic cases of polymelia in cattle have been reported all around the world in both Bos taurus and Bos indicus breeds, in recent years the incidence of this congenital defect seems to be increasing in Angus calves in Australia above the background sporadic level, suggesting the possibility of an emerging heritable defect in this breed.
At least fifteen cases of the notomelic form of polymelia were reported in newborn Angus calves in NSW in the last two years. One purebred commercial Angus herd with a high incidence of two recently recognised but unrelated heritable defects in Angus (AM and NH) as a result of intense linebreeding to high marbling American Angus sires has also reported several cases of notomelia and one case of the dipygus/pygomelic form of polymelia in the last two years. Other recent polymelia cases in NSW were however more sporadic. Several identical polymelia cases have been reported in regional newspapers in other Australian states and on national television, all in the Angus breed (Geelong Advertiser article) and (Yahoo news article). Wikipedia uses a notomelic Angus steer from northern NSW to illustrate its webpage on polymelia. There are also numerous press reports on the internet of polymelia in US beef cattle.
Pedigree investigations of these recent Angus polymelia cases in NSW are underway to identify the closest common ancestor. Trial breeding of affected animals is proposed. Some DNA samples were sent to the USA for SNP chip analysis earlier this year but additional DNA samples are now required for this research, from cases of polymelia in purebred Angus with at least some pedigree data available.
Although the current evidence suggests an underlying primary genetic basis to polymelia in these Angus calves, no clear pattern of inheritance has been identified to date. Trial breeding can identify patterns of inheritance and hence obtaining further polymelia cases for trial breeding would be a useful strategy to determine the heritability of this congenital defect, as proved so useful with contractural arachnodactyly (CA) in Angus calves. Further polymelia cases are also required for the SNP analysis of DNA now in progress to identify any causal mutation present in this syndrome.
The project to date has involved a field collaboration of veterinarians from DTIRIS (Dr Laurence Denholm), New England LHPA (Lisa Martin) and Blayney Veterinary Hospital (Andrew Denman). The collaborating genomics laboratory, led by Assoc. Prof. Jonathan Beever of the University of Illinois and AgriGenomics Inc., has an excellent track record in identifying causal mutations and developing commercial DNA diagnostic tests for carriers of heritable defects in cattle including arthrogryposis multiplex (AM), neuropathic hydrocephalus (NH), contractural arachnodactyly (CA), osteopetrosis (OP), tibial hemimelia (TH), pulmonary hypoplasia with anasarca (PHA), idiopathic epilepsy (IE) and alpha-mannosidosis (MA) - all in the last few years.
NSW veterinarians who see polymelia cases in Angus calves where the owners have at least some pedigree details and are prepared to cooperate in case investigation are asked to contact their local LHPA veterinarian or Dr Laurence Denholm (see below). Polymelia cases in other breeds should also be reported.
Variants of polymelia are classified according to the point of attachment to the body - notomelia where the attachment is in the region of the embryonic notochord, cephalomelia where the attachment is on the head, thoracomelia where the attachment is on the thorax below the dorsal midline (for example, where the attachment is to the margin of the scapula of the normal limb) and dipygus or pygomelia where the attachment is to the pelvis.
The supernumerary limb will have either a left or a right sided anatomy and may develop as a forelimb (cephalomelia, notomelia and thoracomelia) or as a hindlimb (usually as pygomelia). Pygomelia usually results from a caudal bifurcation of the long axis of the body. More rarely, a form of pseudopolymelia occurs in ischiopagus conjoined twins, where the parasitic twin is acephalic.
The majority of bovine polymelia cases are notomelic, with one or more supernumerary forelimbs attached along the dorsal midline anywhere from the occiput to the thoraco-lumbar junction. The foot of the supernumerary limb may be normal or syndactyl. Minor osseous anomalies in these extra limbs are common, including absence of the proximal diarthrodial joints with fusion of the adjoining leg bones (see Fig. 2). Thin bands of muscle tissue may or may not be seen on dissection of the limb, embedded in large amounts of adipose tissue. Some cases have more than one complete supernumerary limb. Others have a single additional limb with abnormal bifurcation at a level below the shoulder joint, creating the appearance of multiple limbs. The limbs may lack innervation.
Losses from dystocia and the significant costs of veterinary amputation under anaesthesia in affected calves surviving birth constitute the main economic impacts of the syndrome. After birth, many surviving calves appear to grow and breed normally regardless of whether the extra limbs are surgically removed.
Developmental anomalies of the limbs are amongst the most common congenital defects in man and domestic animals and more frequent in the distal parts of the limbs. True polymelia apparently results from a failure in normal intercellular inductive signalling during early embryonic development, within the lateral plate mesoderm or in the reciprocal inductive signalling between lateral plate mesoderm and overlying apical ridge ectoderm, leading to a failure of the normal regression of supernumerary primordial limb buds within the lateral mesoderm and hence the development of one or more supernumerary limbs (or parts thereof) from ongoing growth of these persistent vestigial limb buds.
The molecular pathogenesis of supernumerary limbs in mammals remains uncertain, although a genetic basis to this type of teratogenicity is likely, as clearly demonstrated in Drosophila spp. where loss of function mutations of the slimb gene cause supernumerary limbs as a result of ectopic activation of pathways regulating transcription of genes involved in control of the cell cycle during body patterning. The F-Box/WD40 repeat protein slimb is a subunit of a multi-protein complex that targets proteins for degradation by the ubiquitin-proteasome pathway, an important mechanism regulating the abundance of several proteins in eukaryotes involved in embryonic cell cycle regulation. Slimb is thus an important regulator of body pattern development in Drosophila.
An evolutionarily conserved homolog of the slimb gene called Β-TrCP in mammals is also now known to be the specificity determinant for signal-induced ubiquitination pathways in mammals, but no congenital abnormality of limb development has yet been attributed to loss of function mutation in the Β-TrCP gene or to mutation of any related gene in these developmental ubiquitination regulatory pathways. Nonetheless, supernumerary limbs in mammals could well result from mutations of one or more of the many genes involved in the signal-induced phosphorylation pathways that regulate ubiquitination and subsequent degradation of proteins controlling the nuclear entry of proteins required for activating the transcription of target genes involved in cell cycle regulation and apoptosis in limb bud regression. There is certainly some potentially interesting developmental molecular biology in these bovine polymelia cases.
Despite evidence suggesting the possibility of a heritable basis to these recent bovine polymelia cases in NSW, alternative mechanisms of teratogenicity are possible but seem unlikely on epidemiological grounds - particularly given the apparent breed predisposition and the wide geographic distribution of these recent Angus polymelia cases. Nonetheless, it is known that pesticide immunocompromised tadpoles of several frog species develop supernumerary limbs when infected with any of several parasitic species of the trematode Riberoiia.Moreover, retinol toxicity and chromosomal defects have also been identified as causes of polymelia in amphibians.
Karyotype abnormalities have also been described in polymelic calves, but whether these arise from an environmental teratogen or from a heritable mutation causing chromosomal instability at a particular locus is unknown.
The same cellular signalling pathways in early embryonic development can be impacted by environmental toxins or early embryonic infection as well as by heritable mutations of genes encoding proteins in the pathway, thereby producing identical morphological abnormalities from quite different primary causes.
Dr Laurence Denholm BVSc(Hons) PhD,
Principal Officer, Trade and Investment NSW
61 2 6365 5482, 0418 641957