In other species: dog

Possibly relevant human trait(s) and/or gene(s) (MIM number): 305100

Mendelian trait/disorder: yes

Mode of inheritance: X-linked

Considered a defect: yes

Key variant known: yes

Year key variant first reported: 2001

Species-specific name: Congenital hypotrichosis and anodontia defect; Ectodermal dysplasia; X-linked hypohidrotic ectodermal dysplasia

Species-specific symbol: HAD; HED; XLHED

Species-specific description: Because of the obvious homology of this disorder with the homologous human disorder, Drögemüller et al. (2001) proposed that the bovine disorder be called by the name of its human homologue, which is now done in this catalogue. The earlier names are listed here as species-specific names [Frank Nicholas 20 June 2002].

Mapping: Xq22

Molecular basis: By cloning and sequencing a very likely comparative candidate gene (based on the homologous human disorder), Drögemüller et al. (2001) demonstrated that this bovine disorder is due to a large deletion including exon 3 in the gene for ectodysplasin (ED1; now called EDA), in black-and-white German Holstein cattle.

The following year, a different mutation was discovered in red-and-white German Holstein cattle: "a single G >T transversion was located at the second position of intron 8 (IVS8 +2T>G). The mutation changed the canonical GT dinucleotide at the beginning of the 5′ splice site sequence into GG" (Drögemüller et al., 2002).

Ogino et al. (2011) reported yet another EDA mutation: "a 19 bp deletion in exon 1 in male Holstein calves".

Yet another mutation was reported by Karlskov-Mortensen et al. (2011) in Danish Red Holstein cattle, namely "a LINE1-derived pseudoexon between EDA exons 1 and 2. The 161-bp-long pseudoexon introduces a shift in reading frame and a premature stop codon early in EDA exon 2".

A fifth mutation was reported by Ogino et al. (2012), this time in Japanese Black cattle: "an insertion of 4 bp at nucleotide 280 (c.280_281insAGGG) in exon 1. This insertion is predicted to result in a frameshift beginning with amino acid residue 94, with a termination codon occurring at position 143 (p.Gly94GlufsX49), compared to the normal bovine ectodysplasin A protein sequence of 391 amino acids".

Escouflaire et al. (2019) reported a most interesting new likely causal variant that appears to have arisen de novo in a female French Holstein, namely "a 3.8-Mb inversion on chromosome X of a heterozygous female calf that causes a dominant and generalized form of HED via skewed X-inactivation and truncation of the EDA protein." The breakpoints are "located in the first intron of EDA and the first intron of XIST" [the latter being the gene that initiates X-inactivation]. As the authors explain, the inversion "leads to the separation of the XIST exon 1 from the rest of the gene. Thus, if transcribed, the mutant XIST RNA would lack the main evolutionary constrained element among mammals, and contain only 48% of the normal transcript. In addition, this mutation does not affect the integrity of TSIX, which encodes the main repressor of XIST. . . . Although we could not conduct expression studies to validate our hypothesis, these arguments combined with the observation of a generalized HED phenotype in a female heterozygous for a mutation that truncates EDA, suggest that XCI is impaired on the X chromosome carrying the mutation and results in the skewed inactivation of the normal X."

Breeds: Holstein, Japanese Black.

Associated gene: