Foal Immunodeficiency Syndrome
Foal immunodeficiency syndrome (FIS) is an inherited condition causing severe anemia and B-lymphocyte deficiency in affected individuals. The condition was first reported in 1996 as a syndrome believed to be unique to the Fell Pony. Affected foals are usually clinically normal at birth, but from the age of 2 to 8 weeks, they progressively lose condition and become severely anemic. Signs of immunodeficiency become apparent at about 3 to 4 weeks of age, when foals develop signs of opportunistic infections, including diarrhea, weight loss, nasal discharge, and coughing. The mortality rate is 100% despite intensive treatment. Affected foals die or are euthanized usually before they reach the age of 3 months.
The susceptibility to infection has been shown to be a result of primary B-lymphocyte deficiency and reduced antibody production. Concentrations of immunoglobulin isotypes immunoglobulin M (IgM), IgGa, IgGb, and IgG(T) are low. Affected foals are unable to generate an adaptive immune response, resulting in immunodeficiency once colostrum-derived immunoglobulin titers have waned at 3 to 6 weeks of age. The loss of maternally derived antibodies correlates with the onset of FIS signs. At the same time, affected foals develop a nonhemolytic, nonregenerative, progressive, profound anemia, severe enough to cause death. This anemia is the primary finding used by the veterinarian in making the decision to euthanize.
Although FIS was initially believed to be unique to the Fell Pony breed, in 2009 the first case was reported in a Dales Pony. The prevalence of FIS in the U.K. Fell Pony population, before development of a DNA test, was reported to be 10%, with a 1% prevalence estimated in the Dales Pony population. Foal immunodeficiency syndrome was also reported in Fell Pony populations in the Netherlands, Germany, the Czech Republic, and the United States.
Elucidating the Genetic Basis of the Syndrome
The clinical and pathologic findings for FIS suggest a primary defect of genetic origin. The pattern of inheritance, with apparently unaffected individuals producing affected foals, is typical of an autosomal recessive condition. Pedigree analysis of affected individuals has identified common ancestry and a likely founder animal, which features in both the Fell and Dales Pony studbooks.
The Fell and Dales Pony are both rare breeds and are recognized by the Rare Breeds Survival Trust. The Dales Pony is classified as endangered, with 300 to 500 breeding individuals, and the Fell Pony is also at risk, with only 900 to 1500 breeding individuals. The degree of inbreeding in rare breeds is often high because of small population size and the small number of animals used for breeding. Inbreeding is also exacerbated by overuse of a few popular sires, a common occurrence in many animal breeds. Inbred populations are at higher risk for inherited disease than non-inbred populations because a higher proportion of their genome will be homozygous as a consequence of the inbreeding. Deleterious mutations can spread through the population and, when they are present in the homozygous state (two copies of the mutated gene being inherited by affected individuals from their carrier parents), lead to manifestation of the disease.
After a condition has been identified as having a genetic cause, the next step is to identify the gene and mutation so that a DNA test can be developed. Such a test can be used both for diagnostic purposes and to assist breeders in identifying carriers and planning matings to avoid production of affected animals. Single-gene autosomal recessive mutations are typically mapped to a position in the genome with a technique called homozygosity mapping, the aim being to identify a region of the genome shared by all affected individuals. Since 2007, when the horse genome was sequenced, the tools available for mapping genes in the horse have enabled major advances and now rival those available for livestock species. The availability of high-density single-nucleotide polymorphism (SNP) genotyping chips and next-generation sequencing technology has led to acceleration in the discovery of disease genes, significantly reducing the time between a condition being reported and successful development of a DNA test.
A mutation associated with FIS in both the Fell and Dales Pony was identified in 2011. An association mapping study, in which 18 FIS-affected and 31 control Fell Ponies were genotyped for 54,602 SNP markers, identified a region associated with FIS between 29.6 Mb and 32.2 Mb on chromosome 26. The region was fine-mapped by adding an additional 62 SNP markers, and a homozygous segment of 992 kb that was shared by all affected individuals was identified. The region contained 14 genes, including several known to be involved in regulation of immune function. The critical region was resequenced in five selected individuals, including one affected foal, the two obligate carrier parents, one apparently clear animal, and one obligate carrier chosen for maximal homozygosity across the region, based on previous genotyping. The region of interest was captured by use of sequence capture arrays and was sequenced. The causal variant was identified by the process of looking for variants that fit the expected segregation pattern among the 5 selected individuals.
The FIS-associated SNP was found within the single exon of the sodium/myo-inositol cotransporter gene (SLC5A3), a cell membrane transporter protein responsible for the cotransport of sodium ions and myo-inositol. The SNP is nonsynonymous, causing a proline-to-leucine substitution in the amino acid sequence at residue 446. Alignment of the sequence with other protein sequences of the SLC5 family suggests this amino acid is located in a transmembrane helix that is involved in forming the substrate cavity and that tilts during substrate transfer. SLC5A3 is an osmotic stress response gene, which acts to prevent dehydration caused by increased osmotic pressure in the extracellular environment and consequent disruption of cellular functions. Osmotic response mechanisms have been demonstrated to be critical for lymphocyte development and function; however, a functional link between SLC5A3 and the deficiency of B cells in FIS is yet to be determined.