FGF4L2 retrogene copy number is associated with intervertebral disc calcification and vertebral geometry in Nova Scotia Duck Tolling Retrievers

Abstract

OBJECTIVES To evaluate the effects of the chondrodystrophy-associated FGF4L2 retrogene on intervertebral disc (IVD) calcification and vertebral geometry. ANIMALS 22 Nova Scotia Duck Tolling Retrievers (NSDTR) with no FGF4L2 retrogene (n = 7, wild-type dogs), 1 retrogene copy (8, heterozygous dogs), or 2 retrogene copies (7, homozygous dogs). PROCEDURES Computed tomography (CT) scans of the vertebral column were analyzed using computer-aided design (CAD) software. IVD calcification, vertebral column length, and vertebral geometry of the third cervical (C3), 13th thoracic (T13), and first lumbar (L1) vertebrae were compared. RESULTS IVD calcification was not found in wild-type dogs. IVD calcification was more frequent in homozygous dogs than heterozygous (P = .008) or wild-type dogs (P < .001) and in heterozygous dogs compared to wild-type dogs (P < .001). Four IVDs were subclinically herniated in 3 dogs (2 homozygous, 1 heterozygous). Calcified IVD had a greater volume and surface area in heterozygous dogs than homozygous dogs. C3 vertebral canal height-to-width ratio was greater in homozygous dogs than heterozygous dogs (P = .044) and wild-type dogs (P = .010). CLINICAL RELEVANCE IVD calcification and vertebral geometry can be analyzed using CAD software. The presence of 1 or 2 FGF4L2 copies in the absence of the FGF4L1 retrogene has an additive effect on the number of calcified IVD and a minor effect on vertebral geometry in NSDTR dogs. Data support the use of FGF4L2 phenotyping to reduce clinical disease in segregating breeds and to monitor the introduction of wild-type alleles into fixed breed populations.

Association of the FGF4L2 retrogene with fibrocartilaginous embolic myelopathy in dogs

“…Two functional FGF4 retrogenes have been described in dogs located on chromosome 18 ( FGF4L1 ) and chromosome 12 ( FGF4L2 ), both associated with disproportionate dwarfism 16,50 . The FGF4L1 retrogene has been associated with marked limb shortening (termed chondrodysplasia) 50 whereas the FGF4L2 retrogene has been associated with more moderate limb shortening and premature degeneration of the intervertebral disc (termed chondrodystrophy) 14‐17,51,52 . No additional genetic loci have been identified associated with premature intervertebral disc degeneration, however additional risk loci might be present associated with premature disc degeneration in the absence of FGF4L2 or as modifiers of the phenotype 15,53 .…”

“… 16 , 50 The FGF4L1 retrogene has been associated with marked limb shortening (termed chondrodysplasia) 50 whereas the FGF4L2 retrogene has been associated with more moderate limb shortening and premature degeneration of the intervertebral disc (termed chondrodystrophy). 14 , 15 , 16 , 17 , 51 , 52 No additional genetic loci have been identified associated with premature intervertebral disc degeneration, however additional risk loci might be present associated with premature disc degeneration in the absence of FGF4L2 or as modifiers of the phenotype. 15 , 53 Other than anecdotally reported breed overrepresentations, there have been no genetically defined associations with FCE in dogs or in humans.…”

“…As postulated previously by several investigators, premature degeneration of nuclear material in chondrodystrophic breeds might make it non‐permissive to entry into the vascular system. Overexpression of FGF4 via the FGF4L2 retrogene is implicated in premature intervertebral disc degeneration in chondrodystrophic dogs 15 , 16 , 51 and degeneration of the NP is already apparent in chondrodystrophic, FGF4L2 retrogene carrying dogs as early as 10 weeks. 17 , 52 The exact mechanism relating FGF4 overexpression to premature IVDD is not defined, however recent single cell RNASeq studies defining the heterogenous cellular populations in the embryonic and adult intervertebral disc might provide a framework for potential mechanisms.…”

Duplications and retrogenes are numerous and widespread in modern canine genomic assemblies

“…Gene conversion and changes in pseudogene sequence can also give rise to actual and novel function, altering the name from pseudogenes into functional retrogenes. Retrocopies commonly occur in dogs [13,14] and confound the interpretation of genetic variation [15]. Novel developments in sequencing technology have allowed for the rapid and thorough analysis of thousands of samples.…”