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Juvenile-Onset GM2-Gangliosidosis in an African-American Child With Nystagmus
Juvenile-Onset GM2-Gangliosidosis in an African-American Child With Nystagmus Alex R. Paciorkowski, MD*, Swati Sathe, MD†, Bei-Jin Zeng, MD†, Paola Torres, MS†, Sally S. Rosengren, MD*‡, and Edwin Kolodny, MD†
GM2-ganglioside by the lysosomal enzyme -hexosaminidase A, encoded by the HEXA gene [1]. The resulting accumulation of gangliosides in the central nervous system leads to neurodegeneration. In the infantile form (classic Tay-Sachs disease), rapid neurologic deterioration in the first year of life leads to death within 3-5 years [1]. A clinical subtype, late-onset Tay-Sachs disease, is marked by symptom onset in childhood (juvenile form) or later (adult-onset form) [2]. Patients with the juvenileonset subtype typically present by age 5 years with gait disturbance, incoordination, speech problems, and developmental delay [3]. Case Report
N-acetyl galactosamine gangliosidosis (GM2-gangliosidosis) is caused by a deficiency of hydrolysis of
The patient was a 2-year-old African-American girl with an unremarkable early medical history, although her father reported that she startled easily as an infant. She sat at age 5 months, and walked independently at age 11 months. Her fine motor skills were as expected. However, by age 23 months, she had spoken only two clear words. When she was 21 months of age, her parents noticed repetitive eye-blinking that worsened when the child was tired. The parents also reported that the child’s eyes “jiggled” when she looked to the left. On examination, her head circumference was just below the 50th percentile for age. She had pendular bilateral nystagmus, with titubation of the head when she examined objects closely. She exhibited a circumduction of her left leg when walking, but no dysmetria of reach or ataxia of gait. The remainder of her general and neurologic examinations was normal. Magnetic resonance imaging of her brain produced normal results. Electroencephalography revealed a normal background rhythm and no epileptiform discharges. A pediatric ophthalmologist observed rotary nystagmus and cherry-red spots of her maculae during a dilated funduscopic examination. Her visual-evoked potentials were abnormal. Her total hexosaminidase level was 94.8 nanomoles/mg protein/hour (normal range, 1890-2780 nanomoles/mg protein/hour). Further fractionation of her total hexosaminidase revealed the hexosaminidase A component to be only 3% (normal control range, ⬎59%). Sequence analysis of the HEXA gene in the patient, using standard bidirectional methods, disclosed the pathogenic mutations M1V and Y37N (Fig 1). The location of the mutations in the amino-acid sequence of the hexosaminidase A protein (sequence NM 000520) is shown in Figure 2. Further study of the family found the father to be a carrier of the M1V mutation, and the mother was a carrier of the Y37N mutation (Fig 3). The father’s total hexosaminidase activity was 2012 nanomoles/mg protein/hour, with a hexosaminidase A component of 39%. The mother’s total hexosaminidase activity was 1343 nanomoles/mg protein/hour, with a hexosaminidase A component of 43% (normal control range, ⬎59% hexosaminidase A; carrier range, ⬍53% hexosaminidase A). Hexosaminidase A and B enzymatic activity was assayed with the standard heat-inactivation method [4]. We used 4-MUB-N-acetylglucosaminide (Sigma-Aldrich, St. Louis, MO) as substrate. Fluorescence was determined at an alkaline pH of 10.3, using glycine-
From the *Division of Human Genetics, Department of Genetics and Developmental Biology, University of Connecticut Health Center, Farmington, Connecticut; †Department of Neurology, New York University Medical Center, New York, New York; and ‡Department of Pediatrics, University of Connecticut Health Center, Farmington, Connecticut.
Communications should be addressed to: Dr. Paciorkowski; Department of Neurology, Washington University School of Medicine; 660 South Euclid Ave., Campus Box 8111; St. Louis, MO 63110-1093. E-mail: [email protected] Received June 5, 2007; accepted December 3, 2007.
GM2-gangliosidosis is a neurodegenerative lysosomal disease with several clinical variants. We describe a 2-year-old black child with juvenile-onset disease, who presented with abnormal eye movements and cherryred spots of the maculae. Mutation analysis of the HEXA gene revealed the patient to be a compound heterozygote (M1V/Y37N). The M1V mutation was previously described in an African-American child with acute infantile GM2-gangliosidosis. The Y37N mutation is novel. This combination of mutations is consistent with juvenile-onset disease, and provides further evidence for the association of the M1V mutation with individuals of black ancestry. The presence of oculomotor abnormalities is an unusual finding in this form of GM2-gangliosidosis, and adds to the phenotypic spectrum. © 2008 by Elsevier Inc. All rights reserved. Paciorkowski AR, Sathe S, Zeng B-J, Torres P, Rosengren SS, Kolodny E. Juvenile-onset GM2-gangliosidosis in an African-American child with nystagmus. Pediatr Neurol 2008;38:284-286.
Introduction
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© 2008 by Elsevier Inc. All rights reserved. doi:10.1016/j.pediatrneurol.2007.12.004 ● 0887-8994/08/$—see front matter
Figure 3. Pedigree of family. Solid circle denotes patient. Dots denote carrier parents. Total Hex ⫽ total hexosaminidase level expressed in nanomoles/mg protein/hour; HexA ⫽ percentage of total hexosaminidase comprised of hexosaminidase A; yrs ⫽ years.
Figure 1. Arrows in chromatographs indicate M1V mutation (a) and Y37N mutation (b) in the gene HEXA. potassium hydroxide as a buffer. The enzymatic activity was proportional to the fluorescent standard curve. The possibility of substrate reduction therapy with oral miglustat was discussed with the family, although the data regarding its prevention of neurologic decline in GM2-gangliosidosis are not encouraging [5]. The family elected to pursue cord-blood stem-cell transplantation for their daughter. Unfortunately, the patient died of infectious complications 9 months after the transplant procedure.
Discussion This report describes a 2-year-old girl with juvenileonset GM2 gangliosidosis who presented with abnormal
eye movements and cherry-red spots of the maculae. Mutation analysis of the HEXA gene found her to be a compound heterozygote, with the molecular nature of the mutations consistent with her juvenile onset of disease. Patients with juvenile-onset GM2 gangliosidosis usually possess a missense mutation in one allele of HEXA, resulting in premature protein truncation and no enzyme activity [6]. The other allele is typically affected by a single amino-acid change, with less severe effects on enzyme structure and function [7-9]. In our patient, the substitution of valine for methionine at amino-acid position 1 was predicted to lead to protein truncation. The substitution of asparagine for tyrosine at amino-acid position 37, however, is the replacement of an aromatic and nonpolar amino acid for one that is neutral and polar. Such a change may lead to an error of protein-folding or transport, but may allow for an amount of residual enzyme activity, to explain her later onset of disease [7]. It is of phenotypic interest that our patient was found to have a macular cherry-red spot, as this feature is variable in juvenile-onset disease [1,3]. The developing retina may be more susceptible to storage products, because such material was detected at the fetal stage [10]. There is also documentation of the disappearance of the cherry-red spot by age 26 months in a child who presented with infantile disease, suggesting that this retinal lesion undergoes some evolution [11].
Figure 2. Arrows indicate locations of mutations in the amino-acid sequence of hexosaminidase A protein.
Paciorkowski et al: GM2-Gangliosidosis in a Child 285
Oculomotor abnormalities are not a typical feature of juvenile-onset disease, although optic atrophy and macular degeneration were described [3]. In adult-onset GM2gangliosidosis, however, saccadic dysmetria, abnormal optokinetic nystagmus, and supranuclear gaze palsies comprise more consistent features [12-14]. These abnormalities are thought to reflect the wider brainstem and cerebellar dysfunction seen in patients with late-onset disease [14]. It is therefore interesting that our patient shared characteristics of the infantile-onset disorder (the cherry-red spot) as well as the late-onset form (oculomotor abnormalities). Certain mutations in HEXA manifest distributions in specific ethnic groups [15,16]. Three mutant alleles account for 93% of the carriers in the Ashkenazi Jewish population [1]. The M1V mutation found in our patient was previously described in an African-American infant with classic Tay-Sachs disease [17]. The identification of the same mutation in our patient, also of African-American ancestry, confirms the ethnic predilection for that mutation. The novel mutation Y37N may also be more frequent in that population. Despite the suspected difference between protein expression and function between the M1V mutation (protein truncation) and the Y37N mutation (protein misfolding), the hexosaminidase A components in the carrier parents were similar. This may reflect a preferential expression of the normal HEXA allele in carrier parents. The phenotypic characteristics of juvenile-onset GM2gangliosidosis can vary. The cherry-red spot may or may not be present, but the disturbance of oculomotor function can be a notable sign. A high clinical suspicion is prudent because of the diverse forms of neurologic dysfunction this disease can take. The authors thank Philip Brunquell, MD, Division of Pediatric Neurology, Connecticut Children’s Medical Center, Hartford, Connecticut for referring the patient.
References [1] Gravel RA, Kaback MM, Proia RL, Sandhoff K, Suzuki K, Suzuki K. The GM2 gangliosidoses. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The metabolic and molecular bases of inherited diseases, Vol 3, 8th ed. New York: McGraw-Hill, 2001:3827-77.
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[2] Neudorfer O, Pastores GM, Zeng BJ, Gianutsos J, Zaroff CM, Kolodny EH. Late onset Tay-Sachs disease: Phenotypic characterization and genotypic correlations in 21 affected patients. Genet Med 2005; 7:119-23. [3] Maegawa GHB, Stockley T, Tropak M, et al. The natural history of juvenile or subacute GM2 gangliosidosis: 21 new cases and literature review of 134 previously reported. Pediatrics 2006;118:1550-62. [4] Kaback MM. Thermal fractionation of serum hexosaminidases: Applications to heterozygote detection and diagnosis of Tay-Sachs disease. Methods Enzymol 1972;28:862-7. [5] Bembi B, Marchetti F, Guerci VI, et al. Substrate reduction therapy in the infantile form of Tay-Sachs disease. Neurology 2006;66: 278-80. [6] Mahuran DJ. The biochemistry of HEXA and HEXB gene mutations causing GM2 gangliosidosis. Biochim Biophys Acta 1991; 1096:87-94. [7] Nakano T, Muscillo M, Ohno K, Hoffman AJ, Suzuki K. A point mutation in the coding sequence of the -hexosaminidase ␣ gene results in defective processing of the enzyme protein in an unusual GM2gangliosidosis variant. J Neurochem 1988;51:984-7. [8] Petroulakis E, Cao Z, Clarke JTR, Mahuran DJ, Lee G, Triggs-Raine B. W474C amino acid substitution affects early processing of the ␣-subunit of -hexosaminidase A and is associated with subacute GM2 gangliosidosis. Hum Mutat 1998;11:432-42. [9] Wicklow BA, Ivanovich JL, Plews MM, et al. Severe subacute GM2 gangliosidosis caused by an apparently silent HEXA mutation (V324V) that results in aberrant splicing and reduced HEXA mRNA. Am J Med Genet 2004;127A:158-66. [10] Cogan DG, Kuwabara T, Kolodny E, Driscoll S. Gangliosidoses and the fetal retina. Ophthalmology 1984;91:508-12. [11] Kivlin JD, Sanborn GE, Myers GG. The cherry-red spot in Tay-Sachs and other storage diseases. Ann Neurol 1985;17:356-60. [12] Musarella MA, Raab EL, Rudolph SH, Grabowski GA, Desnick RJ. Oculomotor abnormalities in chronic GM2 gangliosidosis. J Pediatr Ophthalmol Strabismus 1982;19:80-8. [13] Hund E, Grau A, Fogel W, et al. Progressive cerebellar ataxia, proximal neurogenic weakness and ocular motor disturbances: Hexosaminidase A deficiency with late clinical onset in four siblings. J Neurol Sci 1997;145:25-31. [14] Rucker JC, Shapiro BE, Han YH, et al. Neuro-ophthalmology of late-onset Tay-Sachs disease (LOTS). Neurology 2004;63:1918-26. [15] Karpati M, Gazit E, Goldman B, Frisch A, Colombo R, Peleg L. Specific mutations in the HEXA gene among Iraqi Jewish Tay-Sachs disease carriers: Dating of founding ancestor. Neurogenetics 2004;5:35-40. [16] Landels EC, Green PM, Ellis IH, Fensom AH, Bobrow M. -Hexosaminidase splice site mutation has a high frequency among non-Jewish Tay-Sachs disease carriers from the British Isles. J Med Genet 1992;29:563-7. [17] Mules EH, Hayflick S, Miller CS, Reynolds LW, Thomas GH. Six novel deleterious and three neutral mutations in the gene encoding the ␣-subunit of hexosaminidase A in non-Jewish individuals. Am J Hum Genet 1992;50:834-41.
GM2-ganglioside by the lysosomal enzyme -hexosaminidase A, encoded by the HEXA gene [1]. The resulting accumulation of gangliosides in the central nervous system leads to neurodegeneration. In the infantile form (classic Tay-Sachs disease), rapid neurologic deterioration in the first year of life leads to death within 3-5 years [1]. A clinical subtype, late-onset Tay-Sachs disease, is marked by symptom onset in childhood (juvenile form) or later (adult-onset form) [2]. Patients with the juvenileonset subtype typically present by age 5 years with gait disturbance, incoordination, speech problems, and developmental delay [3]. Case Report
N-acetyl galactosamine gangliosidosis (GM2-gangliosidosis) is caused by a deficiency of hydrolysis of
The patient was a 2-year-old African-American girl with an unremarkable early medical history, although her father reported that she startled easily as an infant. She sat at age 5 months, and walked independently at age 11 months. Her fine motor skills were as expected. However, by age 23 months, she had spoken only two clear words. When she was 21 months of age, her parents noticed repetitive eye-blinking that worsened when the child was tired. The parents also reported that the child’s eyes “jiggled” when she looked to the left. On examination, her head circumference was just below the 50th percentile for age. She had pendular bilateral nystagmus, with titubation of the head when she examined objects closely. She exhibited a circumduction of her left leg when walking, but no dysmetria of reach or ataxia of gait. The remainder of her general and neurologic examinations was normal. Magnetic resonance imaging of her brain produced normal results. Electroencephalography revealed a normal background rhythm and no epileptiform discharges. A pediatric ophthalmologist observed rotary nystagmus and cherry-red spots of her maculae during a dilated funduscopic examination. Her visual-evoked potentials were abnormal. Her total hexosaminidase level was 94.8 nanomoles/mg protein/hour (normal range, 1890-2780 nanomoles/mg protein/hour). Further fractionation of her total hexosaminidase revealed the hexosaminidase A component to be only 3% (normal control range, ⬎59%). Sequence analysis of the HEXA gene in the patient, using standard bidirectional methods, disclosed the pathogenic mutations M1V and Y37N (Fig 1). The location of the mutations in the amino-acid sequence of the hexosaminidase A protein (sequence NM 000520) is shown in Figure 2. Further study of the family found the father to be a carrier of the M1V mutation, and the mother was a carrier of the Y37N mutation (Fig 3). The father’s total hexosaminidase activity was 2012 nanomoles/mg protein/hour, with a hexosaminidase A component of 39%. The mother’s total hexosaminidase activity was 1343 nanomoles/mg protein/hour, with a hexosaminidase A component of 43% (normal control range, ⬎59% hexosaminidase A; carrier range, ⬍53% hexosaminidase A). Hexosaminidase A and B enzymatic activity was assayed with the standard heat-inactivation method [4]. We used 4-MUB-N-acetylglucosaminide (Sigma-Aldrich, St. Louis, MO) as substrate. Fluorescence was determined at an alkaline pH of 10.3, using glycine-
From the *Division of Human Genetics, Department of Genetics and Developmental Biology, University of Connecticut Health Center, Farmington, Connecticut; †Department of Neurology, New York University Medical Center, New York, New York; and ‡Department of Pediatrics, University of Connecticut Health Center, Farmington, Connecticut.
Communications should be addressed to: Dr. Paciorkowski; Department of Neurology, Washington University School of Medicine; 660 South Euclid Ave., Campus Box 8111; St. Louis, MO 63110-1093. E-mail: [email protected] Received June 5, 2007; accepted December 3, 2007.
GM2-gangliosidosis is a neurodegenerative lysosomal disease with several clinical variants. We describe a 2-year-old black child with juvenile-onset disease, who presented with abnormal eye movements and cherryred spots of the maculae. Mutation analysis of the HEXA gene revealed the patient to be a compound heterozygote (M1V/Y37N). The M1V mutation was previously described in an African-American child with acute infantile GM2-gangliosidosis. The Y37N mutation is novel. This combination of mutations is consistent with juvenile-onset disease, and provides further evidence for the association of the M1V mutation with individuals of black ancestry. The presence of oculomotor abnormalities is an unusual finding in this form of GM2-gangliosidosis, and adds to the phenotypic spectrum. © 2008 by Elsevier Inc. All rights reserved. Paciorkowski AR, Sathe S, Zeng B-J, Torres P, Rosengren SS, Kolodny E. Juvenile-onset GM2-gangliosidosis in an African-American child with nystagmus. Pediatr Neurol 2008;38:284-286.
Introduction
284
PEDIATRIC NEUROLOGY
Vol. 38 No. 4
© 2008 by Elsevier Inc. All rights reserved. doi:10.1016/j.pediatrneurol.2007.12.004 ● 0887-8994/08/$—see front matter
Figure 3. Pedigree of family. Solid circle denotes patient. Dots denote carrier parents. Total Hex ⫽ total hexosaminidase level expressed in nanomoles/mg protein/hour; HexA ⫽ percentage of total hexosaminidase comprised of hexosaminidase A; yrs ⫽ years.
Figure 1. Arrows in chromatographs indicate M1V mutation (a) and Y37N mutation (b) in the gene HEXA. potassium hydroxide as a buffer. The enzymatic activity was proportional to the fluorescent standard curve. The possibility of substrate reduction therapy with oral miglustat was discussed with the family, although the data regarding its prevention of neurologic decline in GM2-gangliosidosis are not encouraging [5]. The family elected to pursue cord-blood stem-cell transplantation for their daughter. Unfortunately, the patient died of infectious complications 9 months after the transplant procedure.
Discussion This report describes a 2-year-old girl with juvenileonset GM2 gangliosidosis who presented with abnormal
eye movements and cherry-red spots of the maculae. Mutation analysis of the HEXA gene found her to be a compound heterozygote, with the molecular nature of the mutations consistent with her juvenile onset of disease. Patients with juvenile-onset GM2 gangliosidosis usually possess a missense mutation in one allele of HEXA, resulting in premature protein truncation and no enzyme activity [6]. The other allele is typically affected by a single amino-acid change, with less severe effects on enzyme structure and function [7-9]. In our patient, the substitution of valine for methionine at amino-acid position 1 was predicted to lead to protein truncation. The substitution of asparagine for tyrosine at amino-acid position 37, however, is the replacement of an aromatic and nonpolar amino acid for one that is neutral and polar. Such a change may lead to an error of protein-folding or transport, but may allow for an amount of residual enzyme activity, to explain her later onset of disease [7]. It is of phenotypic interest that our patient was found to have a macular cherry-red spot, as this feature is variable in juvenile-onset disease [1,3]. The developing retina may be more susceptible to storage products, because such material was detected at the fetal stage [10]. There is also documentation of the disappearance of the cherry-red spot by age 26 months in a child who presented with infantile disease, suggesting that this retinal lesion undergoes some evolution [11].
Figure 2. Arrows indicate locations of mutations in the amino-acid sequence of hexosaminidase A protein.
Paciorkowski et al: GM2-Gangliosidosis in a Child 285
Oculomotor abnormalities are not a typical feature of juvenile-onset disease, although optic atrophy and macular degeneration were described [3]. In adult-onset GM2gangliosidosis, however, saccadic dysmetria, abnormal optokinetic nystagmus, and supranuclear gaze palsies comprise more consistent features [12-14]. These abnormalities are thought to reflect the wider brainstem and cerebellar dysfunction seen in patients with late-onset disease [14]. It is therefore interesting that our patient shared characteristics of the infantile-onset disorder (the cherry-red spot) as well as the late-onset form (oculomotor abnormalities). Certain mutations in HEXA manifest distributions in specific ethnic groups [15,16]. Three mutant alleles account for 93% of the carriers in the Ashkenazi Jewish population [1]. The M1V mutation found in our patient was previously described in an African-American infant with classic Tay-Sachs disease [17]. The identification of the same mutation in our patient, also of African-American ancestry, confirms the ethnic predilection for that mutation. The novel mutation Y37N may also be more frequent in that population. Despite the suspected difference between protein expression and function between the M1V mutation (protein truncation) and the Y37N mutation (protein misfolding), the hexosaminidase A components in the carrier parents were similar. This may reflect a preferential expression of the normal HEXA allele in carrier parents. The phenotypic characteristics of juvenile-onset GM2gangliosidosis can vary. The cherry-red spot may or may not be present, but the disturbance of oculomotor function can be a notable sign. A high clinical suspicion is prudent because of the diverse forms of neurologic dysfunction this disease can take. The authors thank Philip Brunquell, MD, Division of Pediatric Neurology, Connecticut Children’s Medical Center, Hartford, Connecticut for referring the patient.
References [1] Gravel RA, Kaback MM, Proia RL, Sandhoff K, Suzuki K, Suzuki K. The GM2 gangliosidoses. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The metabolic and molecular bases of inherited diseases, Vol 3, 8th ed. New York: McGraw-Hill, 2001:3827-77.
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[2] Neudorfer O, Pastores GM, Zeng BJ, Gianutsos J, Zaroff CM, Kolodny EH. Late onset Tay-Sachs disease: Phenotypic characterization and genotypic correlations in 21 affected patients. Genet Med 2005; 7:119-23. [3] Maegawa GHB, Stockley T, Tropak M, et al. The natural history of juvenile or subacute GM2 gangliosidosis: 21 new cases and literature review of 134 previously reported. Pediatrics 2006;118:1550-62. [4] Kaback MM. Thermal fractionation of serum hexosaminidases: Applications to heterozygote detection and diagnosis of Tay-Sachs disease. Methods Enzymol 1972;28:862-7. [5] Bembi B, Marchetti F, Guerci VI, et al. Substrate reduction therapy in the infantile form of Tay-Sachs disease. Neurology 2006;66: 278-80. [6] Mahuran DJ. The biochemistry of HEXA and HEXB gene mutations causing GM2 gangliosidosis. Biochim Biophys Acta 1991; 1096:87-94. [7] Nakano T, Muscillo M, Ohno K, Hoffman AJ, Suzuki K. A point mutation in the coding sequence of the -hexosaminidase ␣ gene results in defective processing of the enzyme protein in an unusual GM2gangliosidosis variant. J Neurochem 1988;51:984-7. [8] Petroulakis E, Cao Z, Clarke JTR, Mahuran DJ, Lee G, Triggs-Raine B. W474C amino acid substitution affects early processing of the ␣-subunit of -hexosaminidase A and is associated with subacute GM2 gangliosidosis. Hum Mutat 1998;11:432-42. [9] Wicklow BA, Ivanovich JL, Plews MM, et al. Severe subacute GM2 gangliosidosis caused by an apparently silent HEXA mutation (V324V) that results in aberrant splicing and reduced HEXA mRNA. Am J Med Genet 2004;127A:158-66. [10] Cogan DG, Kuwabara T, Kolodny E, Driscoll S. Gangliosidoses and the fetal retina. Ophthalmology 1984;91:508-12. [11] Kivlin JD, Sanborn GE, Myers GG. The cherry-red spot in Tay-Sachs and other storage diseases. Ann Neurol 1985;17:356-60. [12] Musarella MA, Raab EL, Rudolph SH, Grabowski GA, Desnick RJ. Oculomotor abnormalities in chronic GM2 gangliosidosis. J Pediatr Ophthalmol Strabismus 1982;19:80-8. [13] Hund E, Grau A, Fogel W, et al. Progressive cerebellar ataxia, proximal neurogenic weakness and ocular motor disturbances: Hexosaminidase A deficiency with late clinical onset in four siblings. J Neurol Sci 1997;145:25-31. [14] Rucker JC, Shapiro BE, Han YH, et al. Neuro-ophthalmology of late-onset Tay-Sachs disease (LOTS). Neurology 2004;63:1918-26. [15] Karpati M, Gazit E, Goldman B, Frisch A, Colombo R, Peleg L. Specific mutations in the HEXA gene among Iraqi Jewish Tay-Sachs disease carriers: Dating of founding ancestor. Neurogenetics 2004;5:35-40. [16] Landels EC, Green PM, Ellis IH, Fensom AH, Bobrow M. -Hexosaminidase splice site mutation has a high frequency among non-Jewish Tay-Sachs disease carriers from the British Isles. J Med Genet 1992;29:563-7. [17] Mules EH, Hayflick S, Miller CS, Reynolds LW, Thomas GH. Six novel deleterious and three neutral mutations in the gene encoding the ␣-subunit of hexosaminidase A in non-Jewish individuals. Am J Hum Genet 1992;50:834-41.