The depolarizing muscle relaxant succinylcholine entered clinical practice in 1951. Paralysis from succinylcholine peaks in approximately 45 seconds and usually lasts for 2 to 6 minutes, convenient for facilitating tracheal intubation. In 1952, anesthesiologists reported rare cases of prolonged paralysis after its use. Biochemical studies revealed why. Plasma butyrylcholinesterase (BChE, also called pseudocholinesterase) hydrolyzes succinylcholine to inactive components. Succinylcholine acts quickly and briefly in individuals with normal BChE activity, but abnormal BChE activity in 1:2500 (0.04%) of Caucasians [8,9] prolongs succinylcholine activity’and paralysis’by 5 to 10-fold. In 1952, Bourneet al. [10] reported that “those patients who recover slowly from succinylcholine had significantly lower pseudocholinesterase levels than did patients recovering at the normal speed.” Lehmann and Evans, and Kalow independently established the mechanism. Lehmann’s group detected decreased levels of BChE in patients with prolonged paralysis [11], leading to their suggestion that a genetic cause, rather than chronic disease or chemical poisoning, altered BChE activity. In their 1956 landmark publication, Lehmann and Evans investigated families of 5 unrelated patients having prolonged responses to succinylcholine [12], finding otherwise healthy family members with low BChE activity without any reason for such levels. These findings provided a genetic link to prolonged paralysis after succinylcholine.

In 1951, Werner Kalow (Fig. 44.2) proved that procaine-esterase was actually BChE. He provided a more efficient method to measure esterase activity. Serendipitously, he contacted Donald Gunn, a physician using low-dose succinylcholine to dampen muscular contractions during electric shock therapy. Gunn noted that a few patients remained relaxed for almost an hour rather than 2 to 6 minutes. Kalow discovered that BChE enzymes from 2 of these patients behaved differently from normal BChE, and inferred that a genetic change caused a structural difference in the enzyme. In the families of these two patients, Kalow found differential inhibition of BChE by a local anesthetic called dibucaine (also known as cinchocaine or nupercaine). Dibucaine inhibited 79% of normal BChE activity. In both parents of the two patients, inhibition was only 62%, and minimal inhibition (16%) occurred in patients having a prolonged effect of succinylcholine. Such patients had two abnormal genes for BChE (homozygous trait); the parents were heterozygous with one normal and one abnormal gene, and the average person had two normal genes. Succinylcholine was one of the first drugs whose metabolism was shown to be affected by genes displaying simple Mendelian patterns of inheritance [13].

Kalow and his co-workers then demonstrated that individuals having a prolonged response to succinylcholine had abnormally functioning BChE rather than a low quantity of enzyme. They had a genetically determined defect in BChE’a so-called dibucaine resistant enzyme, or atypical enzyme’causing the abnormal reaction to succinylcholine. Kalow’s use of dibucaine inhibition percentage remains the standard for classifying heterozygous, homozygous, and wild-type phenotypes of BChE. His findings and his study of other examples of varied responses to drugs, motivated him to write his 1962 book “Pharmacogenetics: Heredity and the Response to Drugs” [7] establishing pharmacogenetics as a new scientific discipline. Subsequently, over 40 polymorphisms associated with BChE deficiency were described, the most common being the A and the K enzyme variants. The A (for “atypical”) variant has an altered activity while the K variant (after Kalow) results in a lesser production of enzyme. Although prolonged paralysis following succinylcholine is now a rare but easily managed clinical problem, the finding of prolonged muscle relaxation with succinylcholine in specific individuals provided the first evidence for serious clinical effects from genetic variation, and highlighted anesthesiology as a discipline wherein genetics had immediate clinical implications.

Succinylcholine contributed to another anesthesia-related pharmacogenetic discovery. Malignant hyperthermia (MH) is an uncommon adverse pharmacogenetic syndrome associated with administration of succinylcholine and/or inhaled volatile anesthetics [14]. The mechanism triggering this reaction remains unknown (severe stress alone can trigger MH). When triggered, MH increases cytosolic Ca²⁺ in skeletal muscle cells, and this increase can produce hyperthermia, hypermetabolism, muscle rigidity, breakdown of muscle tissue, and death [15].

Here is the story of the discovery of MH. On 8 April 1960, a car hit a 21 year old student, fracturing his right tibia and fibula [16]. Upon arrival at the casualty department of the Royal Melbourne Hospital, in Victoria, Australia, he and his mother expressed more concern about the anesthesia than his leg. Ten family members (3 cousins and 7 aunts and uncles), out of 38, had died during or shortly after receiving ether anesthesia. This family history caused the student’s mother to request a local anesthetic when the boy had an appendectomy at age 12. The consultant anesthesiologist on 8 April was James Villiers. Having heard the story, he and the surgeon learned that the patient’s cousin had recently been safely anesthetized with a “modern anesthetic” at the nearby children’s hospital. After excluding porphyria, the only condition known at the time to affect anesthesia, Villiers believed that the deaths must have been due to “ether convulsions” and recommended using a new, possibly safer anesthetic, halothane’plus vigilant monitoring. Ten minutes into anesthesia, hypotension, tachycardia, cyanosis, and hyperventilation occurred, and the soda lime canister became very hot. The fractures were quickly set, and anesthesia administration discontinued. The patient remained cyanotic and unresponsive and became hot and sweaty. He was immediately packed in ice from a nearby cardiac theater, and within an hour, he was awake and alert. His subsequent course was uneventful. He was the first recorded patient to have had’and survive’MH [17].

Villiers referred the patient to Richard Lovell, who, in turn asked his research assistant, Michael Denborough (Fig. 44.3), to see the patient. Denborough had an interest in genetics, and the familial history led him to suspect a previously unrecognized inborn error of metabolism. When the student again required anesthesia for an impacted renal calculus in 1961, John Forster of the Royal Melbourne Hospital gave him an uneventful spinal anesthetic [16].

In 1964, deaths were noted to have occurred during anesthesia in a large Wisconsin (USA) family. One child had died in Toronto, Canada, and Beverly Britt, the anesthesiologist, along with Kalow and RA Gordon, a professor of anesthesia, organized inquiries throughout Canada. This led to presentation of 13 cases of MH at a 1966 symposium in Toronto [18]. These patients had evinced two common features: muscle rigidity and high temperature. The first description in the American literature of a patient with MH, with reported blood gas data, was similar to that of Villiers in that the patient survived after being packed in ice [19]. When this case was presented at the annual American Medical Association meeting, several in the audience reported caring for similar patients who had received halothane and succinylcholine.

In November 1969, a patient at the Royal Melbourne Hospital received halothane and succinylcholine and died from MH [20]. This patient’s right arm had became rigid, prompting a test of plasma creatine kinase activity, an enzyme involved in energy production in muscle fibers. Creatine kinase activity was increased, indicating muscle breakdown. The original student who had broken his leg in 1960, and some of his relatives also were found to have increased serum creatine kinase activity’evidence of a generalized muscular disease [21]. In 1970, in Johannesburg South Africa, H Isaacs and M Barlow independently observed increased serum creatine kinase levels in relatives of MH patients [22]. Impressed by the muscle rigidity, Kalow and Britt tested their muscle samples for contractile forcein vitro. Muscle from patients with MH exhibited abnormally strong contractions in the presence of caffeine. Caffeine was used because it inducesin vitro skeletal muscle contraction. This resulted in the first clinically useful test for detection of a predisposition to MH in 1970 [23]. In 1971, F Ellis and colleagues showed that halothane augments muscle contraction in patients who are susceptible to MH [24].

In the early 2000s, molecular genetic testing for MH susceptibility was introduced in Europe [25] and North America [14]. The genetic assessments were based on DNA sequence variations in the skeletal muscle type ryanodine receptor (RYR1) gene, which encodes a protein involved in the excitation-contraction process of muscle cells. In 1990, MacLennanet al. had identified theRYR1 gene as a candidate gene that might mediate MH [26]. More than 180RYR1 variants have since been documented, with 29 considered mutations predisposing to MH [27,28]. In 2001, the European Malignant Hyperthermia Group published criteria for proof of causation, and agreed on guidelines for molecular genetic detection for susceptibility to MH. These guidelines are the first to describe genetic screening for testing in anesthesiology [25,29,30].

The next pharmacogenetic discovery relevant to anesthesia came in the late 1960s, in association with a description of an enzyme, now called CYP2D6. CYP2D6 affects metabolism of 25% of the drugs used in medicine, including anesthesiology [31]. CYP2D6 is part of a family of cytochrome P450 (CYP) drug metabolizing enzymes, enzymes involved in metabolism of morphine derivatives, β-blockers, antiarrhythmics, antipsychotics, and antiemetic drugs [31]. Polymorphisms (specific variations in protein structure and function) inCYP2D6 can affect drug efficacy, side effects, and toxicity. Beginning in 1967, 3 laboratories independently discovered polymorphic CYP2D6.

Folk Sjöqvists’ group in Sweden found that metabolism of two CYP2D6 substrates showed considerable inter-individual variation in metabolite plasma levels. Two phenotypes were identified in 1967 [32]. Studies in 19 identical (monozygotic) and 20 fraternal (dizygotic) sets of twins in 1969, suggested the difference was gene-based [33].

At St. Mary’s Medical School in London in 1975, Robert Smith studied debrisoquine, a new agent for treating hypertension. Several healthy volunteers, including Smith, received 40 mg of debrisoquine. Everyone but Smith seemed unfazed by the injection, but Smith became severely hypotensive and collapsed. He recovered fully, but his startling reaction prompted further investigation, revealing that his debrisoquine hydroxylase, the enzyme that metabolizes debrisoquine, eliminated only 10% of the drug. The normal enzyme eliminates 80%. Smith then identified 2 medical students with deficient debrisoquine hydroxylase, traced the family histories, and found that the trait was inherited. His team then tested hundreds of volunteers, discovering that 1 in 10 developed hypotension after debrisoquine. Although 10% of the population were homozygous carriers, 45% were heterozygote carriers of the deficient gene and could transmit it to their children. Smith’s observations had uncovered a mechanism explaining numerous adverse drug reactions [34].

Finally, Michel Eichelbaum’s group in Bonn, Germany, studied the antiarrhythmic effects of sparteine [35]. Two subjects complained of double vision, blurred vision, dizziness, and headache after receiving the drug. Both had 4–5 times higher sparteine plasma levels than other individuals, demonstrating a defect in the capacity to oxidize some drugs, a defect inherited as an autosomal recessive trait. In 1980, Bertilssonet al. in Sweden showed that subjects having an impaired metabolism of debrisoquine, also had an impaired metabolism of sparteine [36]. Also in 1980, Kalow found that students of Europeanversus Chinese descent differed in their capacity to metabolize debrisoquine and sparteine [37], the first observation of inter-ethnic differences in drug metabolism. Debrisoquine and sparteine metabolism were suspected to be genetically linked [38]. A search then began for the gene underlying their variable pharmacokinetics.

In 1985, Distlerathet al. purified the protein’CYP2D6’responsible for the altered metabolism from human liver microsomes [39]. In 1987, Eichelbaumet al. localized the gene encoding CYP2D6 to chromosome 22 [40]. CYP2D6 was the first polymorphic P450 to be characterized at a molecular level, initially identified as a gene responsible for altered metabolism of debrisoquine, sparteine, and other drugs [41]. Further study revealed that CYP2D6 is the only non-inducible enzyme of the P450 family of enzymes. Non-inducible means that it is constantly expressed rather than being turned on or off. As a result, a mutation of the enzyme may cause sustained inter-individual variation in enzyme activity [31]. Genotyping for CYP2D6 activity is drug specific, and the need for typing an individual is limited. If genotyping is needed, patients are assigned to one of four categories based on their ability to metabolize CYP2D6 substrates: ultrarapid metabolizers (UM), extensive metabolizers (EM), intermediate metabolizers (IM), and poor metabolizers (PM) [42]. PMs (two non-functional gene alleles) metabolize CYP2D6 substrates less well than their counterparts (IMs, EMs, UMs). UMs possess multiple functional copies of the same normal CYP2D6 gene, and an increase in the number of functional copies in an individual increases the rate of metabolism of CYP2D6 substrates (Fig. 44.4).

CYP2D6 genetic variations have other clinical implications. In 2006, Korenet al. [43] described a neonatal death leading to the suggestion that breastfeeding mothers taking codeine for postpartum pain should be genotyped for CYP2D6 activity. CYP2D6 O-demethylates codeine to morphine. Mothers and neonates that are CYP2D6 UMs or EMs may produce toxic blood levels of morphine or its active metabolite morphine-6-glucuronide (M6G). The infant in this case report was a CYP2D6 EM, and had a blood concentration of 70 ng/ml morphine; neonates breastfed by mothers receiving codeine normally have concentrations of 0–2.2 ng/ml [43]. The mother was a CYP2D6 UM, and her breast milk had 87 ng/ml morphine’the normal range being 1.9–20.5 ng/ml at doses of 60 mg codeine every 6 hours [43]. Thus the infant had 2 reasons for increased morphine levels. These findings and those from case-control studies [44] indicate that codeine should be avoided in breastfeeding mothers with CYP2D6 EM and UM genotypes.

Pharmacokinetics (what the body does to a drug) and pharmacodynamics (what a drug does to the body) can influence the effectiveness and metabolism of medicines given to produce anesthesia. The previous section concentrated on pharmacokinetics, identifying genetic alterations in drug metabolizing enzymes that change circulating drug levels. The next section explores pharmacodynamic effects, specifically genetic variations in receptor proteins that may alter activation of signaling pathways, despite the presence of constant drug concentration. We present 3 clinically relevant examples.

Variation in response to disease onset and severity, or to drug treatments, may result from augmenting/dampening of cascades of genes. Animal models have facilitated examination of the effect of genetic variability in the context of a whole organism. We now review some of the most common of these non-human models to see how they have advanced the field.