MDR1 gene defect

The MDR1 gene (Multi-Drug Resistance Gene) (ABCB1) encodes P-glycoprotein, a drug transporter that plays a key role in drug disposition. P-glycoprotein is a very important component of hematoencephalic barrier (blood-brain barrier) that prevents entry of many potentially toxic compounds into the CNS (central nervous system). P-glycoprotein also promotes excretion of drugs into bile and urine.

Defective function of P-glycoprotein can lead to severe, potentially fatal, adverse drug reactions because of enhanced CNS exposure to drugs (lack of functional P-glycoprotein at the hematoencephalic (blood-brain) barrier) or enhanced systemic exposure of drugs (lack of P-glycoprotein-mediated biliary and/or renal excretion).

What are the causes of P-glycoprotein dysfunction?

Distinct mutations of the MDR1 gene have been described in both dogs and cats as the cause of P-glycoprotein dysfunction. In dogs, the mutation (a 4-base pair deletion) is known as the MDR1 mutation or ABCB1-1Δ and creates a stop codon that prematurely terminates P-glycoprotein synthesis. Dogs that inherited this mutation from both parents (homozygous for this mutation) have no P-glycoprotein function, whereas heterozygotes have partial P-glycoprotein function. Some heterozygotes have adverse reactions after anaesthetic without knowing the specific reason. It is supposed that these reaction can be influenced not only by genetic factors (So far, not all MDR1 mutations have been detected), but also by the general health condition and the amount of the anaesthetic administered.

In Border collies, another two mutations of ABCB1 gene and that c.73insAAT and c.-6-180T>G have been found. The existence of other mutations of ABCB1 gene in various breeds cannot be excluded. The occurrence of compound heterozygotes that carry two different mutations of ABCB1 gene, where each mutation was inherited from one of the parents, is possible. The compound heterozygotes also have defective P-glycoprotein function.

In cats, this is caused by 2-base pair MDR1 deletion known as ABCB11930_1931del TC that creates a premature stop codon that can lead to abnormally truncated P-glycoprotein with similar impairment of P-glycoprotein function as those described in dogs.

Defective P-glycoprotein function can occur in dogs and cats with MDR1 mutations, but can also result from drug-drug interactions. Some commonly used drugs can inhibit P-glycoprotein function, even in animals with normal MDR1 gene structure. Consequently, veterinarians may encounter dogs and cats with intrinsic (genetically mediated) P-glycoprotein dysfunction, as well as with extrinsic, or acquired, P-glycoprotein dysfunction (animals receiving a drug that inhibits P-glycoprotein function). To avoid causing P-glycoprotein-associated adverse drug reactions, veterinarians should consider all potential causes of P-glycoprotein dysfunction - both intrinsic and acquired.

Which drugs should be avoided or used at reduced doses?

Not all drugs must be avoided in animals with altered P-glycoprotein function; many drugs can be used safely with no need to alter the dose. However, animals with altered P-glycoprotein function have been documented to experience enhanced toxicity to the drugs listed in the table:

In dogs without MDR1 mutation, ketoconazole and spinosad are most often associated with severe adverse effects because of their ability to inhibit P-glycoprotein function. Ivermectin toxicosis has been documented in MDR1 wild-type dogs (without MDR1 mutation) receiving ivermectin in combination with either ketoconazole or spinosad. In addition, severe vinblastine toxicosis was documented in a MDR1 wild-type dog concurrently receiving ketoconazole.

Which dog breeds can be affected?

The MDR1 mutation has been identified primarily in dogs of herding breed ancestry. In fact, all dogs affected by MDR1 mutation are thought to be descendants of a single dog that lived in Great Britain before the genetic isolation of breeds.

Can mixed breeds be affected?

Mixed breeds can be affected because dogs only need to receive 1 copy of the mutant MDR1 gene from a parent with at-risk breed lineage. Based on clinical findings and observations, many mixed-breed dogs have experienced serious adverse effects because they were treated with drugs inhibiting P-glycoprotein function before being tested for the MDR1 mutation.

Table 2 shows breeds and approximate frequency of MDR1 mutation occurrence.

Worries about drugs

Based on studies conducted in rodents or humans, but not in dogs and cats, some drugs have been designated as being unsafe to use and therefore the information can be distorted. Most of the nearly 100 drugs that could fit criteria cited in these studies do not cause toxicity in animals with P-glycoprotein dysfunction. Many (e.g. cephalosporins, penicillins, tetracyclines, antihistamines, beta-adrenergic antagonists) have one or more characteristics that allow for safe use in animals with P-glycoprotein dysfunction, for example, wide therapeutic margin, minimum neurologic toxicity, alternate drug clearance mechanisms or ability to be pumped by a different drug transporter.

Metronidazole is inappropriately listed by many websites as a drug that should not be used in animals with P-glycoprotein dysfunction. Although metronidazole can cause neurologic toxicity, it is not transported by P-glycoprotein. The risk for toxicity increases with dose and duration of therapy and it is not related to P-glycoprotein function.

Ivermectin administered at low dose up to 300-600 μg/kg is safe to use in dogs with heterozygous or homozygous MDR1 mutation.

Is the use of pesticides a problem in affected animals?

Abamectin (similiar to ivermectin) is the active component in many pesticides licensed for both indoor and outdoor use. These products can cause severe neurologic toxicity in dogs with MDR1 mutation. Animals with MDR1 mutation should avoid contact with these products. Clinical signs in dogs are similar to those observed with invermecin toxicosis. So far, nothing is known about the specific toxicity in cats, but clinical signs are expected to be similar to those seen with ivermectin.

It is also necessary to pay attention to excrements of cattle that was treated with anti-parasitic drugs that are not safe to use in dogs with P-glycoprotein dysfunction. After ingestion of such excrements, neurotoxic reaction may occur in dogs; however it depends on the amount of the substance in the excrement.

Pharmacological treatment of dogs or cats with P-glycoprotein dysfunction involves either decreasing drug doses or considering alternate drugs.

Further information is available on following website

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Stating of results of MDR1 gene mutations

Original identification: c.227_230delATAG MDR1, c.295_298delAGAT genu ABCB1

Current identification: c.228_231del genu ABCB1

These are identical gene variants – for references see OMIA

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Breeders are often confused with stating of MDR1 test results. Generally, each laboratory can use its system of result codes. Always is necessary to connect a legend explaining the result codes. Genomia lab uses for testing the same code system for all providing tests:

N = negative - searched character (such as mutation, deletion) is not present
P = positive - searched character (such as mutation, deletion) is present

In the case of MDR1 gene testing:

• N/N genotype means that the absence of 4 base pair deletion in both alleles MDR1 gene is present. Individual with this genotype (N/N) is not threatened by the emergence of neurotoxic reaction during administration of certain drugs or other substances that are substrates of P-glycoprotein. Mutation which is responsible for hypersensitivity of the drugs above, is not present in individuals with N/N genotype.
  
• P/P genotype means that an individual has two copies of the deleted MDR1 gene (two copies of mutation in general). Such individual is threatened by the emergence of neurotoxic reaction after administration of those drugs. Always, one copy of the gene with the 4 base deletion is inherited from one parent (mother) and the second copy of the gene with the deletion is inherited from the other parent (father).
  

• N/P genotype menas that an animal has one copy of mutated MDR1 gene and is likely to have neurotoxic reaction upon administration of the drugs listed.

There are some laboratories in the world that use different result codes, symbols + and -. In these cases, pay increased attention to the legend of the results. Some laboratories use the + symbol in meaning of our P (+ = mutation present), and symbol - in meaning of our N (mutation is not present).

Anyway, you may encounter with the opposite designation:

• -/- Mutated homozygote (an individual who has a mutation (deletion) in both alleles) 
  
• +/+ For healthy individual (an individual without any mutation (deletion))
  

In this system, a deletion (mutation) is generally represented as a minus. The designation is based on the fact that a part of a gene (e.g. MDR1 gene) is erased (deleted). In case of MDR1 gene, 4 bases are erased. (reference: Geyer et. al.: Development of a PCR-based diagnostic test detecting a nt230 (del4) MDR1 mutation in dogs: verification in a moxidectin-sensitive Shepherd Australia)

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References:

Campbell, W. C., Fisher, M. H., Stapley, E. O., Albers-Schonberg, G. & Jacob, T. A. (1983) Science 221, 823-828.

Preston, J. M. (1983) Vet. Rec. 112, 286 (lett.).

Shan, Q., Haddrill, J. L. & Lynch, J. W. (2001) J. Biol. Chem. 276, 12556-12564.
Fisher, M. H. & Mrozik, H. (1992) Annu. Rev. Pharmacol. Toxicol. 32, 537-553.
Mealey, K.L., Bentjen, S.A., Gay, J.M. & Cantor, G.H. (2001) Ivermectin sensitivity in collies is associated with a deletion mutation of the mdr1 gene. Pharmacogenetics, 11, 727-733.

Roulet, A., Puel, O., Gesta, S., Lepage, J.F., Drag, M., Soll, M., Alvinerie, M. & Pineau, T. (2003) MDR1-deficient genotype in Collie dogs hypersensitive to the P-glycoprotein substrate ivermectin. European Journal of Pharmacology, 460, 85-91.

Mealey, K.L., Northrup, N.C. & Bentjen, S.A. (2003) Increased toxicity of P-glycoprotein-substrate chemotherapeutic agents in a dog with the MDR1 deletion mutation associated with ivermectin sensitivity. Journal of the American Veterinary Medical Association, 223, 1453-1455,

Yas-Natan, E., Shamir, M., Kleinbart, S. & Aroch, I. (2003) Doramectin toxicity in a collie. The Veterinary Record, 153, 718-720.

Sartor, L.L., Bentjen, S.A., Trepanier, L. & Mealey, K.L. (2004) Loperamide toxicity in a collie with the MDR1 mutation associated with ivermectin sensitivity. Journal of Veterinary Internal Medicine, 18, 117-118.

Neff MW, Robertson KR, Wong AK, Safra N, et al. Breed distribution and history of canine mdr1-1∆, a pharmacogenetic station that marks the emergence of breed from the collie lineage 2004. PNAS 101:11725-11730.

Geyer J, Döring B, Godoy JR, Moritz A, Petzinger E. 2005. Development of a PCR-based diagnostic test detecting a nt230(del4) MDR1 mutation in dogs: verification in a moxidectin-sensitive Australian Shepherd. J Vet Pharmacol Therap 28:95-99.

Katrina Mealey: MDR1 GENE MUTATIONS & DRUG THERAPY, Washington State University, May 2016