1501
neuropathological features. Lack of haem synthesis or the toxic effects of pathway intermediates are two possibilities. 5-ALA itself may be directly neurotoxic. In-vitro studies have shown that -ALA inhibits y-aminbutyric acid release10 but at higher concentrations than those found in vivo.8 Hepatic haem deficiency may also have an indirect effect because the plasma concentration of haem determines brain levels of tryptophan and therefore the synthesis of 5-hydroxytryptamine.* Increased synthesis of 5-hydroxytryptamine may account for some of the symptoms and signs. Mitchell and colleagues suggest that, in the nervous system hereditary tyrosinaemia causes effects that are at least as severe as those of primary or secondary porphyrias; the onset is also earlier.7 Nevertheless, their patients may be especially predisposed to neurological crises, or the neonatal screening programme in Quebec, and therefore early treatment, may allow these extrahepatic manifestations to become apparent, because these features are not as common in patients seen elsewhere. However, neurological crises are clearly important and may be an additional reason to consider early liver transplantation in this condition. 1. Goldsmith LA, Laberge C. Tyrosinemia and related disorders. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The metabolic basis of inherited disease. 6th ed. New York: McGraw-Hill, 1989: 547-62. 2. Tuchman M, Freese DK, Sharp HL, et al. Persistent succinylacetone excretion after liver transplantation in a patient with hereditary tyrosinemia type 1. J Inherited Metab Dis 1985; 8: 21-24. 3. Kvittingen EA, Jellum E, Stoteke O, et al. Liver transplantation in a 23 year old tyrosinemia patient: effects on the renal tubular dysfunction. J Inherited Metab Dis 1986; 9: 216-24. 4. Lindblad B, Lindstedt S, Steen G. On the enzymic defects in hereditary tyrosinemia. Proc Natl Acad Sci USA 1977; 74: 4641-45. 5. Gentz J, Lindblad B, Lindstedt S, Levy L, Shasteen W, Zetterstrom R. Dietary treatment in tyrosinemia (tyrosinosis): with a note on the possible recognition of a carrier state. Am J Dis child 1967; 113: 31-37. 6. Strife CGF, Zuroseste EL, Emett EA, Finelli VN, Petering HG, Berry HK. Tyrosinemia with acute intermittent porphyria: &dgr;-aminolevulinic acid dehydratase deficiency related to elevated urinary aminolevulinic acid levels. J Pediatr 1977; 90: 400-04. 7. Mitchell G, Larochelle J, Lambert M, et al. Neurologic crises in hereditary tyrosinemia. N Engl J Med 1990; 322: 432-37. 8. Kappas A, Sassa S, Galbraith RA, Nordmann Y. The porphrias. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The metabolic basis of inherited disease. 6th ed. New York: McGraw-Hill, 1989: 1305-65. 9. Sassa S, Kappas A. Hereditary tyrosinemia and the heme biosynthetic pathway. Profound inhibition of &dgr;-aminolevulinic acid dehydratase activity by succinylacetone. J Clin Invest 1983; 71: 625-34. 10. Brennan MJW, Cantrill RC. &dgr;-aminolevulinic acid is a potent agonist for GABA autoreceptors. Nature 1979; 280: 514-15.
TREATMENT OF HERPES SIMPLEX LABIALIS After primary infection, herpes simplex virus (HSV) establishes latency, predominantly in the dorsal root ganglia. More than 90% of the UK population show serological evidence of previous infection. Periodic reactivation may lead to mucocutaneous lesions; such lesions usually resolve in immunocompetent individuals but occasionally cause severe disease with pain and general malaise. In immunocompromised subjects there may be a widespread prolonged active infection. Certain triggers or stimuli may stimulate reactivation, and because of the prodrome many
people can predict the onset of such an episode. Little is known about the detailed events, especially their timing, after HSV reactivation in human beings. However, it can be assumed that virus reactivates within one or more neural cell bodies and travels down the axon to the periphery where viral particles spread locally to infect cells in the skin adjacent to nerve terminals. Subsequently successive rounds of replication involve more cells, provoking an
immune response and in most cases leading to a characteristic herpes simplex labialis. In immunocompetent individuals disease severity ranges from symptomless shedding to a fullblown herpetic cold sore with crusting and eventual resolution. The role of the immune response in the pathogenesis of herpes simplex labialis is unclear and the size of the viral antigenic load at the periphery when prodromal symptoms first appear is unknown. There may be considerable interindividual variation in this respect. However, for a given individual there is often a remarkably consistent clinical pattern from one reactivation episode to the next. A detailed knowledge of this background is important if antiviral agents such as acyclovir are to be used to best effect. Acyclovir is available in various formats for topical, oral, or intravenous use; the first two are realistic options for herpes simplex labialis. Acyclovir becomes active after phosphorylation by the HSV-specific thymidine kinase and further phosphorylation by cellular kinases to the triphosphate derivative, which produces premature chain termination of DNA replication by the HSV-induced DNA polymerase. Although initial evidence indicated that treatment of herpes simplex labialis with topical acyclovir was successful,l more recent studies have not shown it to be of much value in immunocompetent individuals.2.3 The difficulty would appear to be how to achieve adequate drug delivery to the deeper cells of the epidermis. The next question is whether oral acyclovir is effective and if so when therapy should start to contain an individual episode. Spruance and colleagues showed that the oral preparation was effective if given prophylactically over a brief high-risk period.4 The same workers have now confirmed and extended their earlier studies, and have shown that some patients may benefit if oral acyclovir is started early in the development of herpes simplex labialis.5 In a double-blind randomised trial in adults, oral acyclovir reduced the mean duration of pain by 36 % and hastened the healing time to loss of crusts by 27%. Other features of the process were not affected significantly. However, therapy must begin early in the evolution of the lesion; in 97% of patients treatment started within an hour of the first sign or symptom of a recurrence and was initiated by the patient. Presumably the effect achieved by acyclovir is to reduce the viral antigenic load presented to the immune system, and since viral replication occurs exponentially the window of opportunity for achieving this goal is very small. Interestingly, acyclovir delayed the process of evolution to complete resolution, so prolonging the duration of residual
swelling. Other antiviral agents that are potentially effective topically have not been as extensively evaluated. In a similar study Spruance et al6 investigated the efficacy of topical 15% idoxuridine in 80% dimethylsulphoxide and 5% watery Duration of pain in the lesions and healing time to loss of hard crust were reduced by 49% and 38%, respectively, with little or no other effects on the evolution of the process. Essentially similar results had previously been reported for foscarnet sodium.7 Therapy with these agents should also begin as early as possible in the evolution of the disease. Another possibility is topical interferon beta. Glezerman et al8 recorded a reduction in symptoms and severity of herpes simplex labialis, although the number of patients in the trial was small; this approach needs further
investigation. Is the
cost
of
producing
what is
usually
a
minor and
1502
largely subjective alteration in the natural resolution of herpes simplex labialis worthwhile? The decision of those in charge of a health system with finite resources that has to meet the demands of a whole range of patients, some with life-threatening illnesses, may not be the same as that reached by a patient suffering from the pain and discomfort of a cold
sore.
Where does this leave clinicians?
1. Fiddian
AP, Yeo JM, Stubbings R, Dean D. Successful treatment of herpes labialis with topical acyclovir. Br Med J 1983; 286: 1699-701. 2. Raborn GW, McGaw WT, Grace M, Percy J, Samuels S. Herpes labialis with acyclovir 5% modified aqueous cream: a double blind randomised trial. Oral Surg Oral Med Oral Pathol 1989; 67: 676-79. 3. Raborn GW, McGaw WT, Grace M, Houle L. Herpes labialis treatment with acyclovir 5 per cent ointment. Can Dent Assoc J 1989; 55: 135-37. 4. Spruance SL, Hamill ML, Hoge WS, Davis LG, Mills J. Acyclovir prevents reactivation of herpes simplex labialis in skiers. JAMA 1988; 260: 1597-98. 5. Spruance SL, Stewart JCB, Rowe NH, McKeough MB, Wenerstrom G, Freeman DJ. Treatment of recurrent herpes simplex labialis with oral acyclovir. J Infect Dis 1990; 161: 185-90. 6. Spruance SL, Stewart JCB, Freeman DJ, et al. Early application of topical 15% idoxuridine in dimethyl sulfoxide shortens the course of herpes simplex labialis: a multicenter placebo-controlled trial. J Infect Dis 1990; 161: 191-97. 7. Lawee D, Rosenthal D, Akoi FY, Portnoy J. Efficicency and safety of foscarnet for recurrent orolabial herpes: a multi-centre randomized double-blind study. Can Med Assoc J 1988; 138: 329-33. 8. Glezerman M, Lunenfeld E, Cohen V, et al. Placebo-controlled trial of topical interferon in labial and genital herpes. Lancet 1988; i: 150-52. treatment
MOLECULAR BASIS OF ABO POLYMORPHISM The first steps in understanding the molecular basis of the ABO histo-blood group system have been reported by Yamamoto and colleagues in Seattle.1 The ABO antigens were first described by Landsteiner in 1900 as red cell antigens; ABO compatible blood is a basic requirement for safe transfusion. ABO determinants are carbohydrate with characteristic antigens, oligosaccharides immunodominant sugars whose structures and biosynthetic pathways were determined by Morgan and Watkins2 and by Kabat.3 Although carbohydrate antigens cannot be primary gene products, the ABO antigens are inherited in a simple mendelian manner. Morgan and Watkins suggested that A and B genes encode glycosyltransferases (alpha 1-3 N-acetyl-D-galactosaminyl and alpha 1-+3 D-galactosyl transferases, respectively) which transfer specific sugars from activated donor substrates to an acceptor precursor molecule. The 0 gene does not alter the precursor carbohydrate chain (H antigen) and, therefore, was thought either to be a silent gene or to produce an inactive protein. The molecular organisation of these genes was unknown. organisation of these genes was unknown. Yamamoto et al, using an A-transferase cDNA probe isolated after partial aminoacid sequencing with a human A transferase,4 cloned and sequenced cDNA from four cell lines of different ABO phenotypes to look for sequence differences between the various ABO genes. Several differences were observed but the clones broadly fitted into three types, representing A, B, or 0 cDNA. In comparisons of sequences of predicted A cDNA clones with those of B clones, four nucleotide substitutions were observed consistently. These substitutions led to alteration of four aminoacids. In all predicted 0 cDNA clones the Seattle workers found a single nucleotide deletion (position 258) in the coding region, which causes a shift of the reading frame and, hence, failure to encode a protein similar to the A and B transferases. Except for the deletion, the 0 cDNA sequence was identical to that of an A cDNA sequence.
Despite the initial failure to identify any restriction fragment length polymorphisms (RFLPs) correlating with ABO phenotypes,4 characterisation of the cDNA clones identified specific restriction enzyme cleavage sites at three of the four A:B cDNA substitution sites and at the deletion site found in 0 cDNA.1 Yamamoto et al exploited these findings by using specific RFLP analysis of genomic DNA extracted from buffy coats of fourteen blood samples of known ABO phenotype (four group A, four group B, four group 0, and two group AB). Their results support the molecular genetic basis suggested by the work with cell lines. All group 0 samples had cDNA with the same single deletion at nucleotide position 258. The AB samples had two alleles, called functional alleles because they were devoid of the deletion, and the A and B samples had at least one functional allele. Two critical experiments are required to confirm that the proposed molecular basis is responsible for the ABO polymorphism-chromosomal assignment (the ABO locus is mapped to 9q34’l-q34-2) and demonstration that functional transferases are encoded. Confirmation that the DNAs identified by these techniques carry the sequences necessary to encode functional transferases capable of producing A and B antigens is mentioned by the Seattle group in the discussion, and description of transfection experiments is promised in a future report. The exciting observations of Yamamoto and colleagues, although based on few samples, are convincing. Their hypothesis confirms that A and B genes are indeed alleles. Because of the different specificities of the A and B transferases, the allelic status of their genes was questioned until the transferases were shown to have overlapping specificities and also to be immunologically cross-reactive. At last the action of the 0 gene is explained: it is a non-functional allele because of a structural difference. Knowledge of the molecular basis of the ABO polymorphism opens a new approach to the study of ABO expression. The transferase responsible for At subgroup differs from that for Az subgroup in several kinetic characteristics; Yamamoto et al predict that a few aminoacid differences, additional to those observed for the A:B alleles, could be responsible for this polymorphism. The molecular approach will also help to distinguish and identify the genetic background of many rare ABO phenotypes. Determination of the molecular basis of ABO mosaics should allow easy distinction of people with ABO blood group mosaic phenotypes from those who are true chimaeras. Yamamoto and colleagues believe that DNA analyses will help to elucidate the mechanism causing variation of ABO expression during differentiation and oncogenesis. The ability to genotype A and B individuals will increase the usefulness of the ABO groups as a marker in linkage studies and in forensic invesigations. Description of the number and organisation of the exons and introns of the common and rare ABO alleles is eagerly awaited. F, Clausen H, White T, Marken J, Hakomori S. Molecular genetic basis of the histo-blood group ABO system. Nature 1990; 345:
1. Yamamoto
229-33. 2.
Morgan WTJ, Watkins WM. Genetic and biochemical aspects of human blood-group A-, B-, H-, Lea- and Leb-specificity. Br Med Bull 1969;
25: 30-34. 3. Kabat EA. In: Carbohydrates in solution. Adv Chemistry Series 1973; 117: 334-61. 4. Yamamoto F, Marken J, Tsuji T, White T, Clausen H, Hakomori S. Cloning and characterization of DNA complementary to human UDP-GalNAc: Fuc&agr;1 →2 Gal&agr;1→3 GalNAc transferase (histo-blood group A transferase) mRNA. J Biol Chem 1990; 265: 146-51.
neuropathological features. Lack of haem synthesis or the toxic effects of pathway intermediates are two possibilities. 5-ALA itself may be directly neurotoxic. In-vitro studies have shown that -ALA inhibits y-aminbutyric acid release10 but at higher concentrations than those found in vivo.8 Hepatic haem deficiency may also have an indirect effect because the plasma concentration of haem determines brain levels of tryptophan and therefore the synthesis of 5-hydroxytryptamine.* Increased synthesis of 5-hydroxytryptamine may account for some of the symptoms and signs. Mitchell and colleagues suggest that, in the nervous system hereditary tyrosinaemia causes effects that are at least as severe as those of primary or secondary porphyrias; the onset is also earlier.7 Nevertheless, their patients may be especially predisposed to neurological crises, or the neonatal screening programme in Quebec, and therefore early treatment, may allow these extrahepatic manifestations to become apparent, because these features are not as common in patients seen elsewhere. However, neurological crises are clearly important and may be an additional reason to consider early liver transplantation in this condition. 1. Goldsmith LA, Laberge C. Tyrosinemia and related disorders. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The metabolic basis of inherited disease. 6th ed. New York: McGraw-Hill, 1989: 547-62. 2. Tuchman M, Freese DK, Sharp HL, et al. Persistent succinylacetone excretion after liver transplantation in a patient with hereditary tyrosinemia type 1. J Inherited Metab Dis 1985; 8: 21-24. 3. Kvittingen EA, Jellum E, Stoteke O, et al. Liver transplantation in a 23 year old tyrosinemia patient: effects on the renal tubular dysfunction. J Inherited Metab Dis 1986; 9: 216-24. 4. Lindblad B, Lindstedt S, Steen G. On the enzymic defects in hereditary tyrosinemia. Proc Natl Acad Sci USA 1977; 74: 4641-45. 5. Gentz J, Lindblad B, Lindstedt S, Levy L, Shasteen W, Zetterstrom R. Dietary treatment in tyrosinemia (tyrosinosis): with a note on the possible recognition of a carrier state. Am J Dis child 1967; 113: 31-37. 6. Strife CGF, Zuroseste EL, Emett EA, Finelli VN, Petering HG, Berry HK. Tyrosinemia with acute intermittent porphyria: &dgr;-aminolevulinic acid dehydratase deficiency related to elevated urinary aminolevulinic acid levels. J Pediatr 1977; 90: 400-04. 7. Mitchell G, Larochelle J, Lambert M, et al. Neurologic crises in hereditary tyrosinemia. N Engl J Med 1990; 322: 432-37. 8. Kappas A, Sassa S, Galbraith RA, Nordmann Y. The porphrias. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The metabolic basis of inherited disease. 6th ed. New York: McGraw-Hill, 1989: 1305-65. 9. Sassa S, Kappas A. Hereditary tyrosinemia and the heme biosynthetic pathway. Profound inhibition of &dgr;-aminolevulinic acid dehydratase activity by succinylacetone. J Clin Invest 1983; 71: 625-34. 10. Brennan MJW, Cantrill RC. &dgr;-aminolevulinic acid is a potent agonist for GABA autoreceptors. Nature 1979; 280: 514-15.
TREATMENT OF HERPES SIMPLEX LABIALIS After primary infection, herpes simplex virus (HSV) establishes latency, predominantly in the dorsal root ganglia. More than 90% of the UK population show serological evidence of previous infection. Periodic reactivation may lead to mucocutaneous lesions; such lesions usually resolve in immunocompetent individuals but occasionally cause severe disease with pain and general malaise. In immunocompromised subjects there may be a widespread prolonged active infection. Certain triggers or stimuli may stimulate reactivation, and because of the prodrome many
people can predict the onset of such an episode. Little is known about the detailed events, especially their timing, after HSV reactivation in human beings. However, it can be assumed that virus reactivates within one or more neural cell bodies and travels down the axon to the periphery where viral particles spread locally to infect cells in the skin adjacent to nerve terminals. Subsequently successive rounds of replication involve more cells, provoking an
immune response and in most cases leading to a characteristic herpes simplex labialis. In immunocompetent individuals disease severity ranges from symptomless shedding to a fullblown herpetic cold sore with crusting and eventual resolution. The role of the immune response in the pathogenesis of herpes simplex labialis is unclear and the size of the viral antigenic load at the periphery when prodromal symptoms first appear is unknown. There may be considerable interindividual variation in this respect. However, for a given individual there is often a remarkably consistent clinical pattern from one reactivation episode to the next. A detailed knowledge of this background is important if antiviral agents such as acyclovir are to be used to best effect. Acyclovir is available in various formats for topical, oral, or intravenous use; the first two are realistic options for herpes simplex labialis. Acyclovir becomes active after phosphorylation by the HSV-specific thymidine kinase and further phosphorylation by cellular kinases to the triphosphate derivative, which produces premature chain termination of DNA replication by the HSV-induced DNA polymerase. Although initial evidence indicated that treatment of herpes simplex labialis with topical acyclovir was successful,l more recent studies have not shown it to be of much value in immunocompetent individuals.2.3 The difficulty would appear to be how to achieve adequate drug delivery to the deeper cells of the epidermis. The next question is whether oral acyclovir is effective and if so when therapy should start to contain an individual episode. Spruance and colleagues showed that the oral preparation was effective if given prophylactically over a brief high-risk period.4 The same workers have now confirmed and extended their earlier studies, and have shown that some patients may benefit if oral acyclovir is started early in the development of herpes simplex labialis.5 In a double-blind randomised trial in adults, oral acyclovir reduced the mean duration of pain by 36 % and hastened the healing time to loss of crusts by 27%. Other features of the process were not affected significantly. However, therapy must begin early in the evolution of the lesion; in 97% of patients treatment started within an hour of the first sign or symptom of a recurrence and was initiated by the patient. Presumably the effect achieved by acyclovir is to reduce the viral antigenic load presented to the immune system, and since viral replication occurs exponentially the window of opportunity for achieving this goal is very small. Interestingly, acyclovir delayed the process of evolution to complete resolution, so prolonging the duration of residual
swelling. Other antiviral agents that are potentially effective topically have not been as extensively evaluated. In a similar study Spruance et al6 investigated the efficacy of topical 15% idoxuridine in 80% dimethylsulphoxide and 5% watery Duration of pain in the lesions and healing time to loss of hard crust were reduced by 49% and 38%, respectively, with little or no other effects on the evolution of the process. Essentially similar results had previously been reported for foscarnet sodium.7 Therapy with these agents should also begin as early as possible in the evolution of the disease. Another possibility is topical interferon beta. Glezerman et al8 recorded a reduction in symptoms and severity of herpes simplex labialis, although the number of patients in the trial was small; this approach needs further
investigation. Is the
cost
of
producing
what is
usually
a
minor and
1502
largely subjective alteration in the natural resolution of herpes simplex labialis worthwhile? The decision of those in charge of a health system with finite resources that has to meet the demands of a whole range of patients, some with life-threatening illnesses, may not be the same as that reached by a patient suffering from the pain and discomfort of a cold
sore.
Where does this leave clinicians?
1. Fiddian
AP, Yeo JM, Stubbings R, Dean D. Successful treatment of herpes labialis with topical acyclovir. Br Med J 1983; 286: 1699-701. 2. Raborn GW, McGaw WT, Grace M, Percy J, Samuels S. Herpes labialis with acyclovir 5% modified aqueous cream: a double blind randomised trial. Oral Surg Oral Med Oral Pathol 1989; 67: 676-79. 3. Raborn GW, McGaw WT, Grace M, Houle L. Herpes labialis treatment with acyclovir 5 per cent ointment. Can Dent Assoc J 1989; 55: 135-37. 4. Spruance SL, Hamill ML, Hoge WS, Davis LG, Mills J. Acyclovir prevents reactivation of herpes simplex labialis in skiers. JAMA 1988; 260: 1597-98. 5. Spruance SL, Stewart JCB, Rowe NH, McKeough MB, Wenerstrom G, Freeman DJ. Treatment of recurrent herpes simplex labialis with oral acyclovir. J Infect Dis 1990; 161: 185-90. 6. Spruance SL, Stewart JCB, Freeman DJ, et al. Early application of topical 15% idoxuridine in dimethyl sulfoxide shortens the course of herpes simplex labialis: a multicenter placebo-controlled trial. J Infect Dis 1990; 161: 191-97. 7. Lawee D, Rosenthal D, Akoi FY, Portnoy J. Efficicency and safety of foscarnet for recurrent orolabial herpes: a multi-centre randomized double-blind study. Can Med Assoc J 1988; 138: 329-33. 8. Glezerman M, Lunenfeld E, Cohen V, et al. Placebo-controlled trial of topical interferon in labial and genital herpes. Lancet 1988; i: 150-52. treatment
MOLECULAR BASIS OF ABO POLYMORPHISM The first steps in understanding the molecular basis of the ABO histo-blood group system have been reported by Yamamoto and colleagues in Seattle.1 The ABO antigens were first described by Landsteiner in 1900 as red cell antigens; ABO compatible blood is a basic requirement for safe transfusion. ABO determinants are carbohydrate with characteristic antigens, oligosaccharides immunodominant sugars whose structures and biosynthetic pathways were determined by Morgan and Watkins2 and by Kabat.3 Although carbohydrate antigens cannot be primary gene products, the ABO antigens are inherited in a simple mendelian manner. Morgan and Watkins suggested that A and B genes encode glycosyltransferases (alpha 1-3 N-acetyl-D-galactosaminyl and alpha 1-+3 D-galactosyl transferases, respectively) which transfer specific sugars from activated donor substrates to an acceptor precursor molecule. The 0 gene does not alter the precursor carbohydrate chain (H antigen) and, therefore, was thought either to be a silent gene or to produce an inactive protein. The molecular organisation of these genes was unknown. organisation of these genes was unknown. Yamamoto et al, using an A-transferase cDNA probe isolated after partial aminoacid sequencing with a human A transferase,4 cloned and sequenced cDNA from four cell lines of different ABO phenotypes to look for sequence differences between the various ABO genes. Several differences were observed but the clones broadly fitted into three types, representing A, B, or 0 cDNA. In comparisons of sequences of predicted A cDNA clones with those of B clones, four nucleotide substitutions were observed consistently. These substitutions led to alteration of four aminoacids. In all predicted 0 cDNA clones the Seattle workers found a single nucleotide deletion (position 258) in the coding region, which causes a shift of the reading frame and, hence, failure to encode a protein similar to the A and B transferases. Except for the deletion, the 0 cDNA sequence was identical to that of an A cDNA sequence.
Despite the initial failure to identify any restriction fragment length polymorphisms (RFLPs) correlating with ABO phenotypes,4 characterisation of the cDNA clones identified specific restriction enzyme cleavage sites at three of the four A:B cDNA substitution sites and at the deletion site found in 0 cDNA.1 Yamamoto et al exploited these findings by using specific RFLP analysis of genomic DNA extracted from buffy coats of fourteen blood samples of known ABO phenotype (four group A, four group B, four group 0, and two group AB). Their results support the molecular genetic basis suggested by the work with cell lines. All group 0 samples had cDNA with the same single deletion at nucleotide position 258. The AB samples had two alleles, called functional alleles because they were devoid of the deletion, and the A and B samples had at least one functional allele. Two critical experiments are required to confirm that the proposed molecular basis is responsible for the ABO polymorphism-chromosomal assignment (the ABO locus is mapped to 9q34’l-q34-2) and demonstration that functional transferases are encoded. Confirmation that the DNAs identified by these techniques carry the sequences necessary to encode functional transferases capable of producing A and B antigens is mentioned by the Seattle group in the discussion, and description of transfection experiments is promised in a future report. The exciting observations of Yamamoto and colleagues, although based on few samples, are convincing. Their hypothesis confirms that A and B genes are indeed alleles. Because of the different specificities of the A and B transferases, the allelic status of their genes was questioned until the transferases were shown to have overlapping specificities and also to be immunologically cross-reactive. At last the action of the 0 gene is explained: it is a non-functional allele because of a structural difference. Knowledge of the molecular basis of the ABO polymorphism opens a new approach to the study of ABO expression. The transferase responsible for At subgroup differs from that for Az subgroup in several kinetic characteristics; Yamamoto et al predict that a few aminoacid differences, additional to those observed for the A:B alleles, could be responsible for this polymorphism. The molecular approach will also help to distinguish and identify the genetic background of many rare ABO phenotypes. Determination of the molecular basis of ABO mosaics should allow easy distinction of people with ABO blood group mosaic phenotypes from those who are true chimaeras. Yamamoto and colleagues believe that DNA analyses will help to elucidate the mechanism causing variation of ABO expression during differentiation and oncogenesis. The ability to genotype A and B individuals will increase the usefulness of the ABO groups as a marker in linkage studies and in forensic invesigations. Description of the number and organisation of the exons and introns of the common and rare ABO alleles is eagerly awaited. F, Clausen H, White T, Marken J, Hakomori S. Molecular genetic basis of the histo-blood group ABO system. Nature 1990; 345:
1. Yamamoto
229-33. 2.
Morgan WTJ, Watkins WM. Genetic and biochemical aspects of human blood-group A-, B-, H-, Lea- and Leb-specificity. Br Med Bull 1969;
25: 30-34. 3. Kabat EA. In: Carbohydrates in solution. Adv Chemistry Series 1973; 117: 334-61. 4. Yamamoto F, Marken J, Tsuji T, White T, Clausen H, Hakomori S. Cloning and characterization of DNA complementary to human UDP-GalNAc: Fuc&agr;1 →2 Gal&agr;1→3 GalNAc transferase (histo-blood group A transferase) mRNA. J Biol Chem 1990; 265: 146-51.
