Autoimmune lymphoproliferative syndrome in Boerboel dogs: clinicopathologic, diagnostic, and genetic characterization.

Autoimmune lymphoproliferative syndrome (ALPS) is a rare genetic disorder characterized by Fas cell surface death receptor (FAS)-mediated defective lymphocyte apoptosis, leading to chronic lymphadenomegaly, splenomegaly, and autoimmune cytopenias. Although documented in humans and British Shorthair cats, ALPS has not been reported in dogs. Characterize the clinical presentation, laboratory findings and genetic basis of ALPS in Boerboel dogs. Four affected Boerboel pups from a prospectively studied litter, 2 additional retrospectively identified littermates from a different litter, and 17 unaffected relatives (including littermates, dam, sire). Prospective case series, with retrospective review of 2 additional affected dogs. Medical histories, clinicopathologic results and imaging findings were analyzed. Whole genome sequencing was conducted on 2 affected pups, with variant evaluation against 3023 canine whole genome sequences. Relatives were genotyped for the putative causal variant. Affected pups presented with lymphadenomegaly, splenomegaly, and variable cytopenias. Lymph node and splenic cytology showed reactive lymphoid hyperplasia and expanded large lymphocytes, with molecular clonality PCR indicating polyclonal lymphoproliferation. Lymph node flow cytometry and immunocytochemistry identified CD3+/CD4-/CD8- (double-negative) T cell proliferation. Whole genome sequencing of 2 affected pups identified a homozygous 14-base pair duplication in exon 2 of the FAS gene in both, predicted to result in a premature stop codon, absent from 3023 database dogs. All affected dogs were homozygous for the variant, whereas unaffected littermates, parents, and other relatives were heterozygous or clear of the variant. We describe the features of ALPS in dogs and emphasize the importance of considering non-neoplastic lymphoproliferative disorders in young dogs with lymphadenomegaly and splenomegaly.