General Description for Dwarfism Genetics

Dwarfism is characterized by abnormal growth, leading to either a disproportionate body type or overall smaller body size. There are many variants for Dwarfism including, but not limited to, ACAN D1-D5 (originally found in miniature horses and Shetland ponies). Dwarfism can be caused by mutations in the aggrecan (ACAN) gene resulting in a dwarf foal with a wide range of physical ailments and limitations. Dwarf horses often have breathing problems, eating difficulties, and other health issues related to their abnormal bone growth. Other examples of unrelated causes of Dwarfism are skeletal atavism and osteochondrodysplasia which are not a result of an ACAN mutation. Recent studies also indicate that horses with only one copy of the ACAN variants (heterozygotes) may be slightly smaller than their ACAN negative relatives. One such study (Graves, Eberth & Bailey, 2020) indicated heterozygotes for ACAN variants D1-D4 displayed an average size reduction of 1.4 inches in height as compared to their like-bred non- ACAN counterparts.

ACAN variants D1 through D4 have been shown to be causatives for dwarfism in any combination of the following 4 variants, e.g., D2/D2,D2/D3, D3/D4 etc. In some cases, as in D1/D1, D1/D2, D1/D3, or D1/D4, the results indicate likely foal death or severe abnormality.

A Brazilian study (de Andrade et al.2020), conducted in miniature horses only, revealed another variant associated with dwarf stature. This variant, D5, (herein called D5bR- D5 Brazil Risk) shows evidence as a risk or association only and does not yet support evidence as a causative. It is hypothesized that this variant, found in many breeds of horse, was associated with the dwarfism phenotype in this population of miniature horses, possibly due to linkage or correlation to an unknown causative variant or in combination with one of the established/known variants D1, D2, D3, or D4. In its “lone” form, occurring by itself without another known dwarfism variant, it is currently reported as a “risk” variant. D5bR will require further study in multiple breeds, and as such is reported as “discovery” allowing more data collection for further research.

We previously reported the Brazilian variant simply as “ACAN _D5” (D5). However, given that there is now a second publication (Eberth J.E., 2023) out of University of Kentucky, also identifying a dwarfism risk variant and also named D5, we will be reporting this second variant as D5kyR (D5, Kentucky, Risk) and the first will be modified to “D5bR” (D5 Brazil, Risk). In miniature horses the Brazil variant seems to correlate with dwarfism in combination with another known variant. This same variant, alone, in other breeds, such as Arabians and Gypsy horses has not been observed to affect size or health. Therefore in miniature horses this variant may be linked, or recombined, to another unknown dwarfism variant, and is likely not a direct cause of Dwarfism.

Based on the PhD thesis by Dr Eberth in 2023, we now are reporting out ACAN _D5kyR (D5kyR) as a discovery mutation for increased risk found in miniature horses in the USA which seems to demonstrate activity when combined with ACAN _D1-D4 variants resulting in the dwarfism phenotype.

The ACAN genetic region is essential for proper bone growth in horses. When certain variations (called mutations) occur in this gene, they can cause dwarfism in miniature horses or other effects not yet fully understood. Scientists have identified 4 different mutations (D1 to D4) that, when paired together in any combination (like D2/D2, D1/D2, or D1/D3), can lead to dwarfism, or foal death (like D1/D1). The aforementioned additional two variants have been discovered, D5br and D5kyr, as potential risk associated variants within the miniature horse population although these variants exist in many horse breeds outside of Miniatures.

Recap of the "Brazilian Variant" Discovery: ACAN _D5bR

In Brazil, researchers found “D5” (D5bR - D5 Brazil Risk) another novel variant on the ACAN gene in local miniature horses. At first, it seemed to be linked to dwarfism, so it was labeled ACAN _D5. However, further research showed that it might not actually cause dwarfism on its own. Instead, it could be linked to another, still-unknown mutation in miniature horses. Because the data was unclear, scientists stopped reporting this variant as a dwarfism-causing mutation and it has been assessed as a “risk” or correlative variant until further research is completed in multiple populations and breeds.

Recap of the Newest Dwarfism Discovery: ACAN _D5kyR

More recent research, derived from a 2023 PhD study by Dr. Eberth uncovered another mutation in miniature horses in the U.S.herein referred to as ACAN _D5kyR (D5 Kentucky Risk) to separate it from the previous variation ACAN_D5bR (Brazilian variant). This mutation appears to explain some cases of dwarfism that didn’t quite fit with the previously known mutations. Essentially, it provides a clearer picture of why some miniature horses with only one known dwarfism mutation still showed signs of the dwarfism phenotype (reduced size, abnormal proportions, health issues).

The Bottom Line

ACAN **mutations (D1-D4) have been shown to cause **or increase the risk for dwarfism. To date, miniature horses and Shetland ponies are the only breeds depicted in the research. Although these variants exist in other breeds, no evidence for any effect has been documented in the scientific journals.

There are currently 2 different ACAN variants in discovery, one from University of Kentucky (Eberth, et al., 2023) and an earlier version out of Brazil (de Andrade, et al., 2020) Both are named D5.

The Brazilian variant (D5bR) shows an association or increased risk for dwarfism in miniature horses, however more studies are needed.

D5bR in its homozygous form (D5bR/D5bR) has been found in other [standard sized] breeds, such as Arabians, Quarter Horses, and Gypsies without D1,2,3, or 4 and no dwarfism phenotypes have been observed or documented in those homozygous D5bR horses.

The new U.S. mutation (D5kyR) also shows an association or increased risk in dwarfism cases that weren’t previously well-understood.

It is estimated that there are still many additional unknown/unidentified variants for dwarfism in existence. There are confirmed cases of dwarf foals being born to parents who have tested negative (n/n) for all currently identified forms of dwarfism - more research is needed.

Gene Information

The aggrecan (ACAN) gene encodes the large proteoglycan aggrecan, which provides a hydrated gel structure for the proper functioning of the articular cartilage. In humans, multiple pathological ACAN mutations have been identified as the cause of short stature. Spondyloepiphyseal dysplasia type Kimberley, characterized by shortened limbs and trunk, is one of these disorders and is caused by a single base pair (bp) insertion in exon 12 of the ACAN gene. Other human disorders caused by ACAN are spondyloepimetaphyseal dysplasia and familial osteochondritis dissecans (OCD).

References

Eberth, J.E., "Chondrodysplasia-Like Dwarfism in the Miniature Horse" (2013). Theses and Dissertations--Veterinary Science. 11

Eberth, J., Graves K. MacLeod J. & Bailey, E. (2018). Multiple alleles of ACAN associated with chondrodysplastic dwarfism in Miniature horses. Animal Genetics. 49. 10.1111/age.12682. https://pubmed.ncbi.nlm.nih.gov/30058072/

Eberth, J.E., "MOLECULAR GENETIC STUDIES OF HORSES, ESPECIALLY WITH REFERENCE TO AGGRECAN AND DWARFISM" (2023). Theses and Dissertations--Veterinary Science. 59. https://doi.org/10.13023/etd.2023.276

Graves K.T., Eberth J.E. & Bailey E. (2020). Heterozygotes for ACAN dwarfism alleles in horses have reduced stature. Animal Genetics, 51, 420–422. https://pubmed.ncbi.nlm.nih.gov/32065671/

de Andrade, D.G.A., Basso, R.M., Magro, A.J. et al. Evaluation of a new variant in the aggrecan gene potentially associated with chondrodysplastic dwarfism in Miniature horses. Sci Rep 10, 15238 (2020). https://doi.org/10.1038/s41598-020-72192-3

Dwarfism Variants in B4GALT7

Dwarfism is characterized by abnormal growth, leading to either a disproportionate body type or overall smaller body size. Within the Friesian horse, a mutation was found in the B4GALT7 gene on chromosome 14 (ECA14:g.4535550C > T) that is strongly associated with the dwarfism phenotype in Friesian horses. Specifically, the limbs and ribs are mainly affected, leading to a normal sized head, disproportionate back, and abnormally short legs. Research has indicated that it is inherited in an autosomal recessive manner.

Gene Information

The gene B4GALT7 or beta 1,4-galactosyltransferase 7 encodes a protein that is one of the enzymes synthesizing the tetrasaccharide linker between protein and glycosaminoglycan moieties of proteoglycans of the extracellular matrix. The mutation affects a conserved region which impedes splicing of the primary transcript in cultured fibroblasts from a heterozygous horse. The level of B4GALT7 mRNA in fibroblasts from a dwarf is only 2% compared to normal levels. Mutations in B4GALT7 in humans are associated with Spondylodysplastic Ehlers-Danlos syndrome and Larsen of Reunion Island syndrome.

References

Leegwater, P.A., Vos-Loohuis, M., Ducro, B.J. et al. Dwarfism with joint laxity in Friesian horses is associated with a splice site mutation in B4GALT7 . BMC Genomics 17, 839 (2016). https://doi.org/10.1186/s12864-016-3186-0