Categories: Embryo phene

Links to possible relevant human trait(s) and/or gene(s) in OMIM: 618277 (gene)

Mendelian trait/disorder: yes

Mode of inheritance: Autosomal

Disease-related: yes

Key variant known: yes

Year key variant first reported: 2014

Species-specific symbol: DD

Species-specific description: Much useful information on this disorder, including a video, is available from http://www.angus.org/pub/DD/DDInfo.aspx

History: Initial reports of this disorder, which first presented as cases of polymelia (supernumerary limbs) in NSW Australia, were summarised by Denholm and Martin (2011). The name "Developmental Duplications" was given to this disorder by Dr Laurence Denholm and Professor Jon Beever in August 2013. In that same month, Prof. Beever identified a likely causal variant (see Molecular section). In December 2013, Denholm and Martin (2013) provided a more detailed account of the disorder. Following development of a diagnostic PCR test for heterozygous and homozygous carriers, based on the above-mentioned discovery of a likely causal variant, Dr Laurence Denholm tested a consecutive series of Angus calves in Australia which presented with any visible anatomic malformation and demonstrated that the majority of Angus calves (but not all) with lesions that have previously been associated with neural tube defects (NTD) in other species were homozygous for the mutation identified in the polymelia cases. Parnell et al. (2014) reported that the frequency of the variant when it was identified in 2013 was approx. 8% and 6% in the Australian and U.S. registered Angus populations respectively, with approx. 50% of the expected number of homozygotes present in these populations as apparently normal individuals. The incidence of malformed newborn calves could not explain the expected number of homozygotes missing from these populations and high early embryonic mortality was suspected. Subsequently, Delhotel et al. (2016) reported three separate mouse strains produced by the TALEN technique with 4, 12 and 19 base pair deletions in the NHLRC2 gene adjacent to the locus of the DD mutation in Angus cattle. There was 100% embryonic mortality in the mutant homozygotes of all 3 strains. Investigation of embryonic mortality in the 12 bp deletion strain indicated that embryonic death occurred after implantation but before embryonic day E8.5. More recently, polymelia and other lesions associated with neural tube defects have been reported in children from a small tribal group in Niger, some from consanguineous parents (Kelani et al., 2016). (Text provided jointly by FN and Laurence Denholm)

Inheritance: A GWAS conducted by Prof. Beever showed clear evidence of a recessive mutation in a single autosomal gene. However, the likely causal variant shows incomplete penetrance and variable expressivity: not all homozygotes for the likely causal variant show signs of polymelia (at least externally) or any other NTD-related lesion, and the extent and location of supernumerary limbs varies considerably. In effect, this evidence points to DD being a polygenic/multifactorial developmental disorder with one (known) gene of large effect contributing to the phenotypic variation. (Text provided jointly by FN and Laurence Denholm)

Molecular basis: Starting with a GWAS on genetically related polymelia affected and normal Angus calves under an assumption of recessive inheritance, Prof. Beever identified a likely causal mutation for this disorder in August 2013. The mutation was disclosed by Beever et al. (2014): a "non-synonymous substitution was identified within exon 5 of the bovine NHL repeat containing 2 (NHLRC2) gene causing a valine to alanine substitution at residue 311 (V311A) of the encoded protein. Comparative analysis using the protein sequence from diverse taxa indicates the amino acid valine is invariable among the 53 species with sequence available.” (Text provided jointly by FN and Laurence Denholm)

Clinical features: Many informative photographs of this disorder are provided by Dr Laurence Denholm at http://s107.photobucket.com/user/denholml/library/Developmental%20Duplications%20DD%20-%20formerly%20Polymelia?sort=6&page=1 Dr Denholm has also provided the following notes: "The many different DD phenotypes are apparently the sequelae of an underlying neural tube defect (NTD). The most common DD phenotype is occipital cranioschisis with meningocoele, with or without lipoma, myolipoma or teratoma. A few of these cases present as cephalomelia, but most do not. Meningocoele is also the most common phenotype in murine NTD syndromes and is common in human NTD. Meningocoele and other lesions of cranial dysraphism are more common than polymelia in DD calves and also more common than lesions related to spinal dysraphism. This is consistent with our observation that, in DD calves with polymelia, supernumerary forelimbs are more common than supernumerary hindlimbs. Nonetheless, by far the most common outcome for DD homozygotes is early embryonic death and resorption or undetected abortion, as demonstrated by the population genotype ratios in DD. High early embryonic mortality is also reported in murine NTD syndromes - both genetic and toxicological NTD syndromes. Some DD homozygous calves have isolated lesions, but the majority have multiple lesions from the list described below. The occurrence of polymelia with other NTD associated lesions supports the idea that supernumerary limbs are dysraphic appendages. The main phenotypes in homozygous DD calves are described below, with the more common at the top and less common at the bottom. Cranioschisis (usually occipital) with meningocoele. The larger meningocoeles are usually associated with a lipoma, myolipoma or teratoma, often quite large and directly attached to the outside of the meninges lining the meningocoele. Dermoid cysts and/or dermoid sinuses are also common in the tissue surrounding these large meningocoeles. In some of the smaller menigocoeles with a small cranioschisis, there is stricture and subsequent fibrosis at the base of the meningocoele (within the cranioschisis) and subsequent resorption of the CSF trapped in the meningocoele. The resulting "rudimentary meningocoele" then looks like an elongated skin flap or pouch on the midline between the ears. Some DD calves have more than one of these rudimentary meningocoele skin flaps, each associated with its own small midline cranioschisis.. Exencephaly occurs when the meningeal membranes covering the brain tissue in a meningoencephalocoele are lost. Cranial and vertebral clefts on the dorsal midline often accompany these cranial dysraphism related lesions. Chiari malformation Cerebral dysmorphogenesis, sometimes with anatomic duplication or neural hamartoma in the midbrain. Diprosopus (partial duplication of the skull associated with duplication of the brain anterior to the diencephalon) Microphthalmia (with or without cleft palate) Cleft palate Polymelia (cephalomelia, notomelia, pygomelia) Heteropagus conjoined twins (including rachipagus conjoined twins) Spinal teratoma (embryogenic teratoma a.k.a. mature cell teratoma) Spina bifida (with or without myelomeningocoele) Split cord syndrome (diastematomyelia) Caudal duplication syndrome (duplication of the caudal vertebrae, pelvis or parts of the pelvis, with additional hindlimbs) Gastroschisis with intestinal eventration (seen with other lesions of DD in three cases) We have also had a report of dicephalus in the progeny of two known DD carriers, but homozygosity of the affected calf was not confirmed by DNA testing for the NHLRC2 mutation. (Dicephalus is likely to occur in DD given that diprosopus is unusually common in DD.) The phenotypes described above are well described in association with cranial dysraphism and/or spinal dysraphism in the medical and veterinary literature. For example, most of these lesions are reported in neonatal mice exposed to carefully timed retinoic acid toxicity during early embryonic development. Importantly, this NHLRC2 mutation in Angus calves is the first mutation described at the molecular level in any species that is causally associated with a high incidence of conjoined twins.” Hiltunen et al. (2022) reported that Nhlrc2 knockout mouse "embryos showed limited embryonic or extraembryonic tissue differentiation and remained at the cylinder stage. Nhlrc2 expression was ubiquitous but strongest in the epiblast/ectoderm and extraembryonic ectoderm on E6.5 and E7.5. NHLRC2 is essential for early postimplantation development, and its loss leads to failed gastrulation and amniotic folding in mice", all of which is consistent with the numerous clinical signs of this disorder in cattle.

Genetic testing: A DNA test for the causal allele is available from the Animal Genetics Laboratory in the School of Veterinary Science at the University of Queensland; from Neogen Corporation (GeneSeek), Lincoln, Nebraska; and from Zoetis Inc. (Text provided jointly by FN and Laurence Denholm)

Breed: Angus (Cattle) (VBO_0000104).
Breeds in which the phene has been documented. (If a likely causal variant has been documented for the phene, see the variant table breeds in which the variant has been reported).

Associated gene: