or intestinal malabsorption of cobalamin in Beagles
The vitamin B12 (cobalamin, Cbl) is a member of the B-group, water soluble vitamins. Higher organisms are unable to synthesize vitamin B12 and rely on either dietary cobalamin or symbiontic microorganisms. Humans obtain cobalamin exclusively with animal food. Vitamin B12 is important for cell multiplication (transfer of methyl residues during thymine synthesis and thus during DNA-synthesis). Cobalamin is important particularly for the activity of the nervous system and proper growth of haematopoietic cells, although its lack affects most organs with cell recovery. It is also involved in methionine and homocysteine metabolism.
It is assumed that other AMN and CUBN gene mutations will also be found in connection with this phenotype in other dog breeds.
The uptake of dietary cobalamin and its transport to the target cells is a complex multi-stage process and many inherited disorders in a number of these steps have been reported.
The intestinal malabsorption of cobalamin is a metabolic disorder that was described both in humans and animals. It is characterised by non-thriving, neutropenia, reduction of serum cobalamin (Cbl) and non-regenerative anaemia. The other symptoms include chronic loss of appetite, megalosblastic changes in bone marrow, methylmalonic aciduria and homocysteinemia. This phenotype can be caused either by AMN or CUBN gene mutation. CUBN-protein (cubilin) and AMN-protein (amnionless) create a transmembrane protein complex called "cubam". This receptor is essential for the uptake of cobalamin from the intestinal lumen. Defects in either the AMN or CUBN proteins lead to intestinal cobalamin malabsorption. The cobalamin malabsroprtion was described in several dog breeds. Two independent mutations in the AMN gene have been reported in Australian Shepherd Dogs and Giant Schnauzers (He et al 2003,2005).
Owczarek-Lipska et al. (2013) identified the mutation in the CUBN gene as the main cause of the intestinal malabsorption of cobalamin in Bordier Collies that was subsequently confirmed in the study by Fyfe et al. (2013). Mutation c.8392delC in CUBN gene causes shifting of the reading frame and creation of a premature stop codon (p.Q2798Rfs * 3). This mutation is predicted to result in a complete loss of the allele function (Owczarek-Lipska et al. 2013).
Drögemüller et al. (2013) have compared the sequences of both candidate genes (AMN and CUBN) in affected dogs of the beagle breed and identified 1 base pair deletion in CUBN gene as the main cause of the intestinal cobalamin malabsorption in this breed. The mutation c.786delC of CUBN gene causes shifting of reading frame and creation of premature stop codon (p.Asp262Glufs*47). This mutation is predicted to result in a complete loss of the allele function (Drögemüller et al. 2013). Fyte et al. (2014) and Kook et al. (2014) confirmed the c.786delC mutation in affected beagles and identified this mutation in one affected dog. The mutation c.786delC causes a more severe form of the disease than in Border Collies with IGS (Fyfe 2014). Beagles with cobalamin deficiency develop a degenerative liver disease similar to that of lamb disease with decreased cobalt intake known as ovine white liver disease (Drögemüller et al. 2013).
Dogs are born with a cobalamin reserve in the liver, however if it cannot be replaced with food, it is very soon exhausted during the postnatal growth (Fyfe et al., 1991). Therefore, the clinical signs of cobalamin malabsorption appear early in 6 to 12 week of dog´s age and include failure to thrive and chronic loss of appetite. The affected dogs can suffer from neutropia, non-regenerative anaemia, anisocytosis and poikilocytosis, megaloblastic changes in bone marrow, reduction of Cbl level, methylmalonic aciduria and homocysteinemia. The function of kidneys in the affected dogs is normal, but proteins with low molecular weight are eliminated in the urine (Fyfe et al., 1991).
In healthy dogs, the cobalamin (Cbl) taken up with food is bound to the internal factor, glycoprotein, produced by gastric mucosa and pancreatic duct epithelium. The intrinsic factor (IF) in complex with cobalamin (Cbl) is absorbed via enterocyte receptors in distal jejunum and ileum. The symptoms of intestinal cobalamin malabsroption are caused by the absence of the receptors for IF-Cbl complex on the brush border of the intestinal cells (Fyfe et al., 1991).
The intestinal malabsorption is an autosomal recessive disease. This means that a dog must inherit two copies of a mutated gene from its parents; these dogs are designated as P/P (positive/positive). The carriers of the mutated gene (N/P, i.e. negative/positive) are clinically healthy, but pass the gene on to their offsprings. If two heterozygous dogs (N/P) are mated, theoretically there will be 25 % healthy offsprings, 50 % offsprings will be the carriers and 25 % offsprings inherit the mutated gene from both parents and will suffer from the intestinal malabsorption (P/P).