Cobalamin (vitamin B12), a water-soluble vitamin required by all mammalian, is a vital cofactor in two enzymatic processes necessary for protein synthesis and cell metabolism.¹ It is involved in haematopoiesis, neuronal function, DNA and fatty acid synthesis, and energy production. The important active forms, (deoxy)adenosylcobalamin and methylcobalamin) are cofactors for two enzymatic processes.² Adenosylcobalamin is a cofactor for methylmalonyl-CoA mutase, located in the mitochondrion, which catalyzes the reaction of methylmalonyl-CoA isomerization to succinyl-CoA, which then enters the TCA cycle. Methylcobalamin predominates in the cytoplasm and is a cofactor for methionine synthase, which converts homocysteine to methionine, important for protein synthesis.

Cobalamin Deficiency Testing

Serum cobalamin concentration (usually assessed with serum folate) should be measured with a validated assay in all pets with signs of chronic gastrointestinal (GI) disease or other unexplained signs consistent with deficiency (e.g., immunodeficiencies, anaemias, neuropathies).³ Methylmalonic acid (MMA) is produced during amino acid metabolism. Cobalamin catalyses the conversion of methylmalonyl CoA to succinyl CoA. If cobalamin is insufficient, methylmalonyl CoA concentrations increase and is converted to MMA. Serum or urine MMA concentrations are indicators of cellular cobalamin status. Some dogs with low-normal serum cobalamin have increased serum MMA concentrations as decreased serum cobalamin concentration is not always sensitive for the diagnosis of cellular deficiency. While serum MMA concentration may be a better diagnostic test for cobalamin deficiency, it is difficult and expensive to run so not routinely used. If deficiency is likely to be present, supplementation should be considered as it is very safe, and toxicity has not been reported.⁴

Sources and Absorption

While cobalamin is produced by some gut bacteria, animal protein is the main source, especially liver. Cobalamin is ingested bound to protein and released from food in the stomach by pepsinogen and gastric acid. It is then complexed with intrinsic factor (IF), mainly produced by the exocrine pancreas in dogs and exclusively in the pancreas in cats.

Absorption involves a cubam receptor-mediated mechanism in the ileum; so, cobalamin deficiency can be a marker for ileal disease. Cubam receptors are comprised of the proteins cubilin and amnionless.⁵ Receptor recognition results in cobalamin absorption via ileal endocytosis followed by transportation in the circulation bound to transcobalamin I (haptocorrin) and transcobalamin II.

Cobalamin Supplementation

Cyanocobalamin is the most common form for supplementation, although hydroxocobalamin can be used. Oral as well as parenteral cyanocobalamin is effective in dogs and cats, even those with GI disease (250–1000 µg/day).^6-8 Six weeks of weekly subcutaneous doses (250–1500 µg) followed by a dose at one month and a 2 month recheck has been recommended.³ Parenteral doses of 1 mg hydroxycobalamin monthly or bimonthly appeared adequate in beagles with hereditary hypocobalaminaemia.⁹

Cobalamin Deficiency

Familial Cobalamin Deficiency

A familial cobalamin deficiency similar to human Imerslund-Grasbeck syndrome (IGS) has been reported in giant schnauzers, Australian shepherd, beagles, Komondors, and border collies. Clinical signs may begin in early adulthood and include anorexia, lethargy, cachexia, failure to thrive, poor body condition, dysphagia, vomiting, diarrhoea, glossitis, bradycardia, and oral ulcerations/erosions. Haematological abnormalities such as anaemia may be noted. Degenerative liver disease has been also reported in young beagle dogs with IGS.¹⁰

Australian shepherds and giant schnauzers have a genetic defect of chromosome 8, containing the amnionless gene.^11-13 In beagles, mutations of the cubilin genes have been found, similar to the mutation in border collies;^10,11,14,15 although the mutation location differs between the breeds. Familial cobalamin deficiency in shar-peis appears to be a different type of malabsorption, with a genetic defect on chromosome 13.^16,17 They have higher concentrations of serum MMA and homocysteine (HCY) than cobalamin-deficient dogs of other breeds. In humans, MMA levels are higher with genetic disorders that affect intracellular cobalamin processing compared those with disorders affecting gastrointestinal (GI) processing and extracellular transport. These defects result in deficient function of methylmalonyl-CoA mutase or methionine synthase, causing methylmalonic aciduria or homocysteinuria.¹⁸

Acquired Cobalamin Deficiency

Cobalamin deficiency can be caused by any disorder affecting IF production or intestinal absorption (e.g., chronic enteropathies [CE], exocrine pancreatic insufficiency [EPI] and intestinal lymphoma).¹⁹ In cats it is also reported to be associated with cholangitis/cholangiohepatitis and hyperthyroidism.²⁰ Cats with chronic kidney disease may have impaired function of cobalamin-dependent cellular metabolism.²¹

Cobalamin in GI Diseases

Low serum cobalamin deficiency occurs 6–73% of dogs with CE^22-25 and occurs more in cats with CE or lymphoma compared with other gastrointestinal cancers²⁶. In one study, only 25% of hypocobalaminemic dogs had increased serum MMA concentrations; so hypocobalaminemia was not always associated with a cellular cobalamin deficiency.²⁵ A suggested mechanism for cobalamin deficiency in CE is decreased expression of the cubam receptor causing impaired absorption.²⁷ In lymphoma, ileal infiltration may damage the receptors.¹⁹ Another suggested mechanism is GI dysbiosis²⁸ as some anaerobic bacteria (e.g., some Clostridia spp. and Bacteroides spp.) utilize cobalamin decreasing the amount available for absorption²⁹. The prevalence of cobalamin deficiency in canine and feline EPI is 74–83%.^30-32 Decreased production and secretion of IF occurs in EPI, reducing amounts of cobalamin-IF complexes available for absorption.^33-35 Hypocobalaminemia at the time of EPI diagnosis is a significant risk factor for decreased survival, while hyperfolatemia is associated with improved prognosis.³⁶

Hepatic Disease

Hypercobalaninaemia can be present with liver disease, solid tumours, haematological malignancies, and kidney disease. A functional cellular cobalamin deficiency can occur with high serum cobalamin. In liver disease, a functional deficiency may be caused by alterations in binding of cobalamin to haptocorrin proteins, causing altered delivery to cells and increased HCY and/or MMA levels.³⁷ In dogs with liver disorders 37.5% had hypercobalaminaemia and 26.7% had elevated serum MMA concentrations.³⁸ In cats with liver disease the odds ratio of increased serum cobalamin was 9.91 (95% CI 3.54–27.58).³⁹ Hypercobalaminaemia, elevated MMA and HCY are biomarkers for functional deficits in cobalamin and can indicate an underlying disease.

Relationship between Cobalamin (and Folate) and Anaemia

In humans, megaloblastic anaemia can occur with cobalamin (and folate) deficiency from defective DNA synthesis, ineffective haematopoiesis, and erythrocyte maturation arrest. Decreased white blood cell and platelet synthesis can occur due to bone marrow hypercellularity from impaired development and release of erythrocytes. Dogs with congenital hypocobalaminamia may show a nonregenerative anaemia, erythrocyte anisocytosis, megaloblastic bone marrow, and decreased white cells and platelets, although a study of anaemic dogs did not show a correlation between cobalamin and folate deficiency and macrocytic nonregenerative anaemia.⁴⁰ In some cats, low serum cobalamin has been linked to increased mean corpuscular volume and decreased serum phosphorus.^41,42 Hypocobalaminaemia in a cat has been reported to cause bone marrow failure and pancytopenia.⁴³

Neurological Disorders

Both folate and cobalamin deficiency may cause similar human neurological and psychiatric disturbances including depression, dementia, myelopathy and peripheral neuropathy.⁴⁴ In humans and rodents, subacute combined degeneration (SCD) is an adult onset neuropathy due to cobalamin deficiency.⁴⁵ This is related to interference with the methylation reactions in the CNS. These reactions are processed by S-adenosylmethionine (SAM), which is controlled by its product, S-adenosylhomocysteine (SAH). The relationship of these two compounds is the methylation ratio. If the ratio falls, it results in central nervous system (CNS) hypomethylation causing decreased function. Inhibition of the cobalamin-dependent methionine synthase causes a rapid fall in the ratio in the CNS as it does not have an alternative method of re-methylating homocysteine to maintain synthesis of SAM.^46,47

There are also non-coenzyme CNS functions of cobalamin. A rat CNS neuropathy due to cobalamin deficiency is associated with increases in CNS tissue and/or cerebrospinal fluid levels of some neurotoxic molecules (e.g., nerve growth factor, TNF-alpha), and decreases in levels of some neurotrophic molecules (epidermal growth factor, interleukin-6). Low cobalamin levels have also been observed Alzheimer’s patients, although the role is unclear. Cobalamin does delay the onset of signs of human dementia and improves cognitive functions in the elderly, if administered before the first symptoms.⁴⁸

In a rat sciatic nerve injury model high doses of methylcobalamin improved nerve regeneration and function.⁴⁹ Anecdotally, cobalamin has been used for treatment of feline diabetic neuropathy. Methylcobalamin has been suggested to be the form needed for neurological tissue health; however, in a meta-analysis of human diabetic neuropathy, vitamin B complex (including cyanocobalamin) and methylcobalamin both had beneficial effects on symptoms such as pain and paresthesia.⁵¹

Hypocobalaminaemia resulting in an encephalopathy and MRI lesions has been reported in a cat. The hypocobalaminaemia appeared to result in a urea cycle abnormality causing hyperammonaemia. Daily cobalamin injections resulted in a clinical improvement with resolution of MRI lesions at neurological signs after eight weeks.⁵²

Summary

Cobalamin supplementation is essential for many GI and pancreatic disorders and for individuals with congenital hypocobalaminaenia. Hypercobalaminaeamia may be a biomarker for hepatic and other disorders. Cobalammin supplementation could be considered for adjunct treatment of neuropathies and cognitive disorders, although research in dogs and cats is needed.

References

References are available upon request.