Several reviews on inherited disorders in cattle have been published,^1–3 and a regularly updated electronic database, Online Mendelian Inheritance in Animals (OMIA), is available at http://omia.angis.org.au/.4

This chapter describes the morphologic characteristics, mode of inheritance, breeding lines affected, and availability of testing for selected heritable bovine fetal abnormalities. A comprehensive summary of all known heritable congenital abnormalities would be extensive and is beyond the scope of this chapter. The purpose of this review is to discuss those heritable bovine fetal abnormalities recently described for which the mutation has been identified and a test is available.

Arthrogryposis multiplex

Arthrogryposis multiplex (AM) is a lethal autosomal recessive genetic defect that originated in Angus cattle. Beginning in 2008, researchers in collaboration with the American Angus Association (AAA) investigated abnormal calves believed to fit the description of what was then called AM and commonly referred to as “curly calf syndrome” in Angus cattle. Within 2 months, researchers obtained samples and pedigrees from affected calves and their parents, the mutation was identified, the DNA test was developed and validated, and the status of over 700 AI bulls was determined.⁵

Calves with AM are born dead or die shortly after birth. They are small-for-gestational age (15–25 kg) and have markedly diminished muscle mass, but dystocia is common as a result of the congenital arthrogryposis, scoliosis, torticollis, and possibly hydroamnion. It appears that in AM an essential protein that allows communication between nerves and muscle tissue is absent, so the calf (which fails to move in utero) is born with the joints of all four limbs fixed and the legs twisted. There are several characteristics of AM, including arthrogryposis (fixed twisted joints), kyphoscoliosis (twisted spine), and decreased muscling⁵ (Figure 66.1).

The mutation is a deletion that involves three genes; one of these genes is involved in the development of nerve and muscle. Affected calves are missing about 23 kb. These missing base pairs result in complete loss of function of all three genes in homozygous calves.⁶

Bovine arachnomelia syndrome

Bovine arachnomelia syndrome (AS) is an inherited monogenetic autosomal recessive trait with complete penetrance. Affected calves are usually stillborn with skeletal abnormalities including a “spidery” appearance of the limbs (dolichostenomelia) with marked thinning of the diaphysis and an abnormally shaped skull⁷ (Figure 66.2). In cattle there are two virtually identical AS phenotypes in Brown Swiss cattle and German Fleckvieh/Simmental cattle.

In Brown Swiss cattle, AS is the result of a single base (G) insertion in exon 4, leading to a frameshift and a premature stop codon in the coding sequence of the bovine sulfite oxidase (SUOX) gene, interfering with the expression of the SUOX protein (molybdohemoprotein sulfite oxidase, a terminal enzyme in the oxidative degradation pathway of sulfur-containing amino acids).⁷ Deficiencies of the SUOX enzyme in humans are characterized by major neurological abnormalities and early death.^8–11 Cattle that are heterozygous for the mutation do not exhibit any clinical signs. Of the 302 unaffected Brown Swiss cattle tested, 10 (3.3%) were identified as carriers of the mutation in the SUOX gene.⁷

More recently, there were more than 150 confirmed cases of AS in German Fleckvieh/Simmental cattle by 2008, with an estimated 6% prevalence in the cow population.¹² The mutation in German Fleckvieh/Simmental cattle is a 2-bp deletion in the bovine gene encoding molybdenum cofactor synthesis 1 (MOCS1) resulting in a frameshift and premature termination of the bovine MOCS1 protein (73 vs. 633 amino acids). When cattle were sampled randomly from the Bavarian Simmental population, excluding first-degree relatives of known carriers, 17 of 616 (2.8%) were heterozygous for the genetic mutation (2-bp deletion in MOCS1) for AS in Simmental cattle.¹³ The MOCS1 protein is required for the synthesis of the molybdoprotein cofactor, which forms the active site in SUOX (the defective protein in Brown Swiss cattle affected with AS).⁷ The involvement of SUOX and MOCS1 in a common biochemical pathway mutually supports the causality of these mutations in Brown Swiss and German Fleckvieh/Simmental cattle.

Bovine citrullinemia

Bovine citrullinemia is an autosomal recessively inherited disease of Holstein cattle that was first described in Australia.¹⁴ Bovine citrullinemia was disseminated throughout the Australian Holstein population following importation of semen from the North American sire Linmack Kriss King.^15,16 Affected cattle are clinically normal at birth but within 24 hours become depressed, wander aimlessly, head press, appear blind, teeth grind and, within 4–5 days, develop proprioceptive deficits and ataxia, become recumbent, convulse, collapse, and die. Lesions include mild to moderate diffuse astroglial swelling in the cerebrocortical gray matter and mild to severe hepatocellular hydropic degeneration.¹⁷ Bovine citrullinemia is caused by transition of cytosine (CGA/arginine) to thymine (TGA/Stop codon) at codon 86 of the gene coding for argininosuccinate synthase, leading to impairment of the urea cycle and extreme elevation of citrulline and ammonia in plasma of affected cattle.^17,18

Bovine dwarfism

Dwarfism as a heritable condition has been reported in many mammals, including multiple breeds of cattle.^1,19–25 Several types of inherited chondrodysplasia have been reported in the bovine. Although the phenotypic expression is variable, all are characterized by systemic skeletal disorders, including shortness and deformity of limbs, head, and vertebrae. Dwarfism has been recognized in the Angus, Brown Swiss, Danish Red, Dexter, Hereford, Holstein, Japanese Brown, and Shorthorn breeds.²⁵ No single gene or mutation is responsible for all reported cases of bovine dwarfism. Here we describe two unique forms of bovine dwarfism.

Dexter bulldog dwarfism has been a major problem for the Dexter breed since its description in the early nineteenth century.²⁶ Homozygosity for the Dexter dwarf mutation is lethal and fetuses are generally aborted at approximately 7 months of gestation. Clinically, the defective fetuses are characterized by an extreme disproportionate dwarfism, a short vertebral column, marked micromelia, large abdominal hernia, relatively large head with a retruded muzzle, cleft palate, and protruding tongue.²⁷

Bulldog dwarfism of Dexter cattle is caused by two discrete mutations in the aggrecan (ACAN) gene, and DNA testing is available for the mutation.²⁷ When defined strictly on the lethal bulldog phenotype, the inheritance pattern of Dexter dwarfism is described as autosomal recessive (i.e., the bulldog dwarf is homozygous for the genetic mutation). However, because heterozygotes have an intermediate dwarf phenotype, the trait can also be described as having an incomplete dominance mode of inheritance (8% expression of ACAN in heterozygotes compared with homozygous normal).²⁷ This has presented a major problem for Dexter breeders, because the favored Dexter phenotype, featuring short legs (a mild form of disproportionate dwarfism), tends to be heterozygous for the bulldog mutation. Selecting for the favored Dexter phenotype maintains a lethal allele at a high frequency.²⁷

Dwarfism in Aberdeen Angus calves was described in 1951, and determined to be autosomal recessive.²¹ The AAA handled the issue by the virtual annihilation of all animals associated with the primary source herd.²⁸ Following the efforts of the breed association to eliminate dwarfism, there were no certified reports to the AAA of dwarfism in registered Angus cattle from the 1970s until 2002. At the start of the twenty-first century, abnormal Angus calves were reported in several herds in the western United States. Unlike the previous form of dwarfism, these calves appeared normal at birth, but failed to grow, and after several months appeared to have abnormally short legs and thick bodies. Gross and histopathological examination of these calves indicated evidence for diminished endochondral ossification and other features consistent with dwarfism, including the protrusion of the alar wing of the basisphenoid bone into the cranial cavity, abnormalities of the ventral vertebral bodies, and curving of the transverse vertebral processes (“long-nosed” dwarfism).²⁹ The genetic defect of the long-nosed Angus dwarf has not been definitively identified but is not the same as the defect in Dexter (mutations in ACAN) or Japanese brown cattle (LIMBIN mutations).^29,30

Brachyspina syndrome

Brachyspina syndrome is a recently reported lethal malformation in the Holstein breed.^31–33 The syndrome is characterized by calves born dead following a slightly prolonged gestation. The gross morphology of brachyspina syndrome shares many features with short spinal lethal syndrome in cattle, first described in Old Norwegian Mountain calves born in 1930.³⁴ Brachyspina calves are growth retarded, with severe shortening of the entire vertebral column and limbs. Most vertebral segments have some lack of organization, with irregular ossification separated by cores of cartilage that prevent identification of individual vertebra. Multiple defects of the internal organs, including renal dysplasia and intestinal atresia, are consistently present, whereas brachygnathism and caudal dislocation and compression of the brain are inconsistently present. The appendicular skeleton may or may not be affected.

Although variation in morphology caused by a common syndrome is not unusual, the six recently reported cases of brachyspina syndrome may have had more than one etiology, since they were morphologically diverse. In two reports of brachyspina syndrome, genealogical examination showed that the cases occurred in familial patterns,^32,33 a common ancestor to all parents was found, and the occurrence of the abnormal calves could potentially be explained by transmission of recessive alleles. Although these observations supported the hypothesis of a genetic basis of brachyspina syndrome, association does not prove causation. Widely used sires may occur as common ancestors in the pedigree of malformed calves without that individual being a carrier, or without the defect having an inherited etiology. If brachyspina syndrome is inherited as an autosomal recessive anomaly, it may be an emerging worldwide disease in the Holstein breed.

Complex vertebral malformation

Complex vertebral malformation (CVM) syndrome is a recessively inherited lethal disorder in the Holstein breed that increases embryonic death, abortion, and perinatal death. CVM fetuses have a composite of phenotypic abnormalities, including axial skeletal deformities (e.g., hemi- and mis-shaped vertebra, ankylosis of mainly the cervicothoracic vertebra, and scoliosis), symmetric arthrogryposis of the lower limbs, craniofacial dysmorphism, and cardiac anomalies.^35–37 Clinical heterogeneity among affected calves may make it difficult to make a diagnosis of CVM; however, a presumptive diagnosis can be made at necropsy (if pedigree information is available).

Genealogical research identified Carlin-M Ivanhoe Bell (registration number 1667366), an elite Holstein-Friesian bull born in 1974, to be the main ancestor of cattle carrying this mutation.³⁶ Because of the superior lactation performance of his daughters, Ivanhoe Bell was extensively used for two decades. Carriers of the CVM mutation exist in Holstein cattle populations worldwide and the frequency of CVM carriers among Holstein sires has reached an alarming level.^35,38–41

By comparing DNA sequences of unaffected and affected calves, recent research⁴² has uncovered a point mutation in the form of a transversion (G → T) in the SLC35A3 gene, causing a valine at position 180 to be replaced with phenylalanine. The SLC35A3 gene product is a Golgi-resident transporter critical for the formation of glycoproteins and, ultimately, axial skeleton development. A screening test (DNA) for CVM is now available and has been widely performed on Holstein sires. Pedigree information now includes the designation “TV” for tested animals free from CVM and “CV” for carriers of CVM.

Congenital contractural arachnodactyly

Congenital contractural arachnodactyly (CA), or “fawn calf syndrome,” is a nonlethal autosomal recessive genetic defect of Angus cattle. CA calves are normally born alive and most can walk, suckle, and survive. The birthweight of CA calves is “normal.” The phenotype is subtle, so that CA may not initially be recognized as a defect. CA is a developmental defect involving reduced elasticity of the connective tissue of muscles first identified in Victoria, Australia in 1998 but now reported in many countries.⁴³ Although CA is a less severe disease than lethal genetic defects of Angus calves, without human intervention up to 20% of CA calves die soon after birth because they are unable to stand and suckle.⁴³ CA manifests in newborn calves as elongated limbs, congenital proximal limb contracture, congenital distal limb hyperextension, and congenital kyphosis with significant postnatal improvement in these clinical signs as the calf grows and matures.⁴³

Researchers have identified the genetic defect that causes CA and have partially characterized the specific mutation responsible for CA as a deletion of at least 38 kb, severely compromising function.⁴⁴ The complete sequence of the deleted DNA segment is not known, making it difficult to develop a diagnostic test that is 100% accurate.⁴⁴ Until recently the breed associations (AAA and Angus Australia) have avoided identifying any animal as a CA carrier because the current diagnostic test is less than 100% accurate.⁴⁵ However, some specifically identified animals have been named as either carriers or “highly likely” to be carriers of the CA mutation by Angus Australia.⁴⁴ The current assay generates some false positives in a number of pedigrees, creating a significant danger of misinterpretation of test results. The current test does allow an overall estimation of frequency of the AM mutation in the population. With more than 500 animals genotyped with several of the genetic markers for AM, the maximum frequency of AM in the AI sire population is approximately 3–4%.⁴⁴

Crooked tail syndrome

The Belgian Blue breed is well known for its exceptional muscular development. This phenotype is due in part to an 11-bp loss-of-function deletion in the myostatin gene that has been fixed in the breed. Intense selection in the Belgian Blue has substantially reduced the effective population size, resulting in greater inbreeding and therefore recurrent outbreaks of recessive defects. Most recently a novel defect referred to as crooked tail syndrome (CTS) has been described in the Belgian Blue.^46,47 In addition to the deviation of the tail, cattle with CTS have general growth retardation manifested at approximately 1 month of age, abnormal skull shape manifested as a shortened broad head, and extreme muscular hypertrophy.⁴⁶ Although the defect is not lethal itself, the most severe cases (~25%) are euthanized on welfare grounds.⁴⁶ The surviving 75% nevertheless cause important economic losses as a result of growth retardation and carcass depreciation.⁴⁶

CTS is caused by mutations in the mannose receptor C type 2 (MRC2) gene. MRC2 encodes the 180-kDa endocytic transmembrane glycoprotein (Endo180), a recycling endocytic receptor that is expressed in mesenchymal cells such as stromal fibroblasts and in the chondrocytes, osteoblasts and osteocytes in developing bones, and is thought to play a role in regulating extracellular matrix degradation and remodeling. Two mutations in MRC2 have been identified as causing CTS. The first mutation is a 2-bp deletion in the open reading frame of MRC2 resulting in a frameshift and a premature stop codon that causes a nonsense-mediated decay of the mutant mRNA; the second identified mutation is a T → C substitution in MRC2. Both defects result in a virtual absence of functional Endo180.^46,47

When 1899 healthy Belgian Blue cattle were tested, unexpectedly 24.7% were identified as carriers of the first mutation (2-bp deletion in MRC2).⁴⁶ The unusually high frequency of the first CTS mutation in Belgian Blue cattle suggests that it might confer heterozygotes an advantage in this highly selected population. In fact, CTS carrier animals are smaller, stockier, and more heavily muscled, and they have a thinner skeleton and more rounded ribs. Moreover, the MRC2 genotype accounts for 3.6, 3.6, and 2.6% genetic variance of height, muscularity, and general appearance, respectively within 519 Belgian Blue pedigree bulls.⁴⁶ Enhanced muscularity of CTS carriers may contribute greatly to the rapid increase in the CTS mutation in Belgian Blue cattle. Indeed, carrier animals are approximately twice as likely to be selected as elite sires than their noncarrier siblings.⁴⁶

Deficiency of uridine monophosphate synthase

Deficiency of uridine monophosphate synthase (DUMPS) is a hereditary lethal autosomal recessive disorder in Holstein cattle causing early embryonic mortality during implantation in the uterus and possibly fetal mummification.⁴⁸ About 2% of the Holstein cattle in the United States possess an autosomal recessive form of the gene for DUMPS.⁴⁹ DUMPS interferes with de novo biosynthesis of pyrimidine (a nucleotide constituent of DNA and RNA). It is inherited as a single autosomal locus with two alleles.^50,51 In mammalian cells, the last step of pyrimidine nucleotide synthesis involves the conversion of orotic acid to uridine monophosphate (UMP) and is catalyzed by UMP synthase.⁵² Growth and development of homozygous recessive embryos is arrested, leading to mortality around 40 days after conception.⁴⁹ DUMPS is caused by a single point mutation (C → T) at codon 405 within exon 5.⁵³

Developmental duplication

Initially observed in Australia, developmental duplication (DD) is an autosomal recessive genetic defect of Angus cattle. Affected DD calves are born with additional limbs (polymelia). Duplication of the front legs is typical, with the limbs originating from the neck or shoulder region. Variations of DD are classified according to the point of attachment to the body. The foot of the supernumerary limb may be normal or syndactyl. Affected DD calves may also present as conjoined twins. There has been one case of conjoined twins in Australia that were reported to be homozygous for the DD mutation.

The main economic impacts of DD were thought to be from losses related to dystocia and costs associated with supernumerary limb amputation. However, the frequency of reported DD cases is unexpectedly low, especially with carrier frequency among US sires being moderately high at approximately 6%. These data indicate that calves presenting with polymelia at birth are rare events that survive embryonic death. Early DD events may prevent many embryos from developing to term, resulting in embryonic death and the reduced frequency of live births that are being observed. This may be a more significant economic impact of DD than losses related to dystocia and limb amputation. Genetic testing for DD is available through Angus Genetics Inc. and Zoetis Genetics. Either association archived or newly collected samples may be used for testing.

Inherited congenital myoclonus

Inherited congenital myoclonus (ICM), also known as idiopathic epilepsy, is a seizure disorder observed in Hereford cattle and their crosses caused by an autosomal recessive genetic defect incompatible with life. ICM is characterized by spontaneous and stimulus-responsive myoclonic spasms that are prenatal and prevent calves from rising at birth. Affected calves have a “normal” phenotype when they are not experiencing seizures. Environmental stressors (heat, cold, weaning) can trigger the seizures, and the seizures can last from minutes to more than 1 hour. Bovine ICM has been attributed to a severe disturbance of glycine-mediated neurotransmission in the spinal cord.^54,55 The observed phenotype is due to a nonsense mutation in codon 24 of the glycine receptor polypeptide, which results in truncation of the α₁-subunit and subsequent loss of cell-surface expression.⁵⁶

Microdeletion in the maternally imprinted Peg3 domain

A deletion in the maternally imprinted Peg3 domain has been identified in Finnish Ayrshire cattle that results in loss of paternal MIMT1 expression and causes late-term abortion and stillbirth has been identified. The stillborn calves weigh approximately 20 kg, or half the average normal birthweight for the breed, and the lungs are not inflated. No other visible abnormalities are detected (Figure 66.3). Fetal and placental development in mammals are both affected by imprinted genes for which either the paternally or maternally inherited allele has become epigenetically inactivated, leading to monoallelic expression. Approximately half of the fetuses can be adversely affected when a male transmits the allele for the defective fully penetrant imprinted gene that is silenced on the maternal allele. In mammalian cloning experiments, the loss of epigenetic control of imprinted genes has led to low success rates and birth of animals with health problems.⁵⁷ Defects in maternally imprinted genes will be transmitted silently from females to their progeny, whereas phenotypically normal males will transmit these mutations to their progeny as though they were heterozygous for a dominant mutation. The mutation in Peg3 was associated with late fetal death and stillbirth in at least 42.6% of the offspring of one Finnish Ayrshire bull (YN51). The mutation, when inherited from the sire, is semi-lethal for his progeny, with an observed mortality rate of 85%. The survival of 15% is presumably due to the incomplete silencing of maternally inherited MIMT1 alleles, which is a common phenomenon for imprinted loci.⁵⁸ Crossbreeding with Holstein heifers and cows did not impact the survival rate of fetuses, consistent with the imprinting model of inheritance. The surviving female calves with the deletion should transmit the mutation to 50% of their offspring without any impact on fetal death, and bull calves inheriting this mutation from these dams could regenerate the problem.⁵⁹ Although the biological role of MIMT1 is unknown, “natural” and experimentally induced (mice) knockouts suggest an important regulatory mechanism of Peg3 affecting late prenatal development. Moreover, this inherited disorder of cattle stresses the importance of defects that cause stillbirth and abortion as well as the potentially drastic effects of mutations in imprinted genes.