DE NOVO MUTATION IN A CHOROIDEREMIA CARRIER Simge Bozbeyoglu, MD,* Gerald A. Fishman, MD,* Edwin M. Stone, MD, PHD,†‡ Ian M. MacDonald, MD,§ Luan M. Streb, BA†
Purpose: To describe a de novo gene mutation in a female patient with clinically characteristic findings of choroideremia (CHM). Methods: Determination of best-corrected visual acuity, fundus examination, visual field analysis, and electroretinography were performed on a female patient with an advanced CHM phenotype. Blood samples were obtained from the patient and both her parents, and direct genomic sequencing for DNA analysis was performed on the blood samples. Results: A single base-pair sequence change was identified in codon 293 in exon 7 (R293X) of the CHM gene in the proband. This mutation was not found to occur in her clinically unaffected parents. Conclusion: These findings document that a de novo point mutation should be considered when an isolated female family member is found to have CHM. RETINAL CASES & BRIEF REPORTS 1:182–184, 2007
From the *Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, Chicago, Illinois; the †Department of Ophthalmology and Visual Sciences, and ‡The Howard Hughes Medical Institute at the University of Iowa, Iowa City, Iowa; and the §Department of Ophthalmology, University of Alberta, Edmonton, Alberta, Canada.
Infrequently, carriers may manifest severe retinal pigmentary degenerative changes as well as choroidal atrophy and impairment of visual function.3 These differences in expression of disease severity are likely primarily the result of Lyonization with random inactivation in one X chromosome.4 The CHM gene has been identified as consisting of 15 exons, spanning at least 150 kilobases of chromosome Xq21.2 and encoding a protein of 653 amino acids.5 Mutations in the gene for the protein REP-1 (Rab escort protein) have been identified in patients with choroideremia.1 Rab proteins are geranylgeranyltransferases that control protein trafficking in secretory and endocytic pathways. REP is required for Rab geranylgeranylation and its subsequent association with the membrane of specific organelles.6 In the absence of REP, Rab proteins will not properly function, and cells cannot survive.6 REP-1 is expressed not only in rods and cones but also in retinal pigment epithelium cells.5 We describe a de novo mutation in the REP-1 protein sequence in a woman with the CHM phenotype who had extensive chorioretinal atrophy, severe nyctalopia, and restricted side vision.
C
horoideremia (CHM) is an X-linked recessive eye disorder characterized by progressive degeneration of the choroid, retinal pigment epithelium, and retina. Its prevalence is estimated as 1 case in 100,000 population.1 The severity of the ophthalmic manifestation of CHM may show considerable interfamilial as well as intrafamilial variability.2,3 Affected males have progressive loss of initially peripheral and subsequently central visual function.1,2 Female carriers generally do not have clinically significant visual impairment; most have some degree of pigmentary retinal degenerative change, which has been described as a “moth-eaten” fundus appearance.3 Supported by funds from the Foundation Fighting Blindness (Owings Mills, MD); the Karl Cless Family Foundation (Chicago, IL); the Grant Healthcare Foundation (Chicago, IL), and NEI (Core Grant EY01792; Bethesda, MD). The authors do not have any proprietary interest in this work. Reprint requests: Gerald A. Fishman, MD, UIC Eye Center, University of Illinois at Chicago (M/C 648), 1855 West Taylor Street, Chicago, IL 60612; e-mail: [email protected]
Case Report A 22-year-old white woman of German ancestry without a family history of CHM presented with poor night vision and
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Fig. 1. Fundus photographs of the right eye demonstrate extensive retinal pigment epithelium and choroidal atrophy in the posterior (A), temporal (B), and inferior (C) retina with a normal-appearing optic disk and retinal vessels.
peripheral field loss. These visual symptoms were first noted when she was 12 years of age and had been progressively worsening. Visual acuity was correctable to 20/20 in both eyes. Results of color vision screening with Ishihara plates were normal. Ocular pressure was 15 mmHg in each eye. Slit-lamp examination showed that the cornea, anterior chamber, and lens were clear in both eyes. The vitreous had 1⫹ cells in each eye. Fundus examination disclosed extensive retinal pigment epithelium and choroidal atrophy with a normal-appearing optic disk and retinal vessels. There were isolated areas of velvetlike pigment clumping as well as isolated areas of round black pigment clumping in the midperipheral retina. There was relative sparing of the macular region. The changes were essentially symmetric between the eyes (Fig. 1). Goldmann field testing showed ring scotomas with a normal peripheral boundary (Fig. 2). Electroretinogram testing showed markedly reduced rod and cone amplitudes with prolonged cone implicit times to both single-flash and 32-Hz flicker stimuli (Fig. 3). The patient’s phenotype was characteristic of CHM. Blood samples were obtained from the patient and her parents for gene mutation analysis and analyzed by previously described methods.2,7 (The patient and her parents provided informed consent, and the project was approved by an institutional review board at the University of Illinois.)
We sequenced each of the 15 exons of the CHM gene and observed a single heterozygous base-pair change in exon 7(R293X) of this gene. The variant characterized in this female patient was a C to T transition resulting in the conversion of an arginine residue (CGA) to a stop codon (TGA), which would be expected to cause a truncation of the REP-1 protein.2 A cytogenetic analysis for the patient showed a normal female karyotype. Specifically, there was no chromosomal translocation abnormality involving either of her X chromosomes. The parents were evaluated for the R293X mutation in the CHM gene; however, neither parent was found to harbor this change. Both parents also were screened for multiple informative genetic markers, and the results of this genotyping strongly supported the conclusion that they are the patient’s biologic parents. Entirely normal results of fundus examination were found for each parent. No other family members had a history of night blindness or peripheral field loss. Figure 4 shows the family pedigree.
Discussion Our findings are consistent with a de novo point mutation causing disease via one of two mechanisms. First, a heterozygous mutation in our patient has
Fig. 2. Goldmann field testing shows a ring scotoma with a normal peripheral boundary in the right eye.
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have no functional CHM allele and would be expected to have a phenotype similar to that of an affected male. Some cases of advanced CHM causing visual loss have been described in females.8,9 Partial or extensive inactivation of the normal X chromosome may cause either patchy or severe expression of disease in a CHM carrier.10 Other possible explanations for CHM expression in a female would include Turner syndrome or homozygous mutations,9 neither of which was found in our patient. Manifestation of disease in females can also result from disruption of the CHM gene by X chromosome translocations.1 In these cases, X chromosome inactivation is nonrandom; the normal X chromosome is preferentially inactivated, while both translocation fragments remain active.1 The normal karyotype determined in our patient does not support translocation or chromosomal rearrangement as the cause of her phenotype. Her de novo mutation has been previously identified and described in other patients with the clinical diagnosis of CHM.1,2,6 To our knowledge, this is the first case of a de novo mutation in the CHM gene described in either a carrier of or an affected individual with CHM. Key words: choroideremia, carrier, de novo mutation. References 1. Fig. 3. Electroretinogram (ERG) recording (left, normal findings; right, findings for the right eye of our patient) shows markedly reduced rod (top) and cone (bottom) amplitudes.
2.
3.
caused severe disease because of “skewed” inactivation leading to a relatively large fraction of retinal cells that have inactivated the normal allele. Second is the possibility that our patient is a carrier of a second CHM mutation (not detected by our mutation screening of the CHM coding sequences) that lies on the CHM allele opposite the one affected by the R293X mutation. In this latter scenario, the patient would
4.
5.
6.
7.
8. 9. Fig. 4. Pedigree of the family. Arrow and dark circle, the proband; white circles (females) and squares (males), unaffected family members; X, individuals examined by the authors.
10.
Van der Hurk JAJM, Schwartz M, Van Bokhoven H, et al. Molecular basis of choroideremia: mutations involving the Rab escort protein-1 gene. Hum Mutat 1997;9:110–117. McTaggart KE, Tran M, Mah DY, et al. Mutational analysis of patients with the diagnosis of choroideremia. Hum Mutat 2002;20:189–196. Francis PJ, Fishman GA, Trzupek KM, et al. Stop mutations in exon 6 of choroideremia gene, CHM, associated with preservation of electroretinogram. Arch Ophthalmol 2005; 123:1146–1149. Rudolph G, Preising M, Kalpadakis P. Phenotypic variability in three carriers from a family with choroideremia and a frameshift mutation 1388delCCinsG in the RPE-1 gene. Ophthalmic Genet 2003;24:203–214. Van Bokhoven H, Van der Hurk JAJM, Bogerd L, et al. Cloning and characterization of the human choroideremia gene. Hum Mol Genet 1994;3:1041–1046. MacDonald IM, Mah DY, Ho YK, et al. A practical diagnostic test for choroideremia. Ophthalmology 1998;105: 1637–1640. Fishman GA, Stone EM, Grover S, et al. Variation of clinical expression in patients with Stargardt dystrophy and sequence variations in the ABCR gene. Arch Ophthalmol 1999;117: 504–510. Fraser GR, Friedmann AI. Choroideremia in a female. BMJ 1968;2:732–734. Mijad MA, Horsborough B, Gray RH. Unusual macular findings in a known choroideremia carrier. Eye 1998;12:740–741. Cheung MC, Nune GC, Wang M, et al. Detection of localized retinal dysfunction in a choroideremia carrier. Am J Ophthalmol 2004;137:1:189–190.
Purpose: To describe a de novo gene mutation in a female patient with clinically characteristic findings of choroideremia (CHM). Methods: Determination of best-corrected visual acuity, fundus examination, visual field analysis, and electroretinography were performed on a female patient with an advanced CHM phenotype. Blood samples were obtained from the patient and both her parents, and direct genomic sequencing for DNA analysis was performed on the blood samples. Results: A single base-pair sequence change was identified in codon 293 in exon 7 (R293X) of the CHM gene in the proband. This mutation was not found to occur in her clinically unaffected parents. Conclusion: These findings document that a de novo point mutation should be considered when an isolated female family member is found to have CHM. RETINAL CASES & BRIEF REPORTS 1:182–184, 2007
From the *Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, Chicago, Illinois; the †Department of Ophthalmology and Visual Sciences, and ‡The Howard Hughes Medical Institute at the University of Iowa, Iowa City, Iowa; and the §Department of Ophthalmology, University of Alberta, Edmonton, Alberta, Canada.
Infrequently, carriers may manifest severe retinal pigmentary degenerative changes as well as choroidal atrophy and impairment of visual function.3 These differences in expression of disease severity are likely primarily the result of Lyonization with random inactivation in one X chromosome.4 The CHM gene has been identified as consisting of 15 exons, spanning at least 150 kilobases of chromosome Xq21.2 and encoding a protein of 653 amino acids.5 Mutations in the gene for the protein REP-1 (Rab escort protein) have been identified in patients with choroideremia.1 Rab proteins are geranylgeranyltransferases that control protein trafficking in secretory and endocytic pathways. REP is required for Rab geranylgeranylation and its subsequent association with the membrane of specific organelles.6 In the absence of REP, Rab proteins will not properly function, and cells cannot survive.6 REP-1 is expressed not only in rods and cones but also in retinal pigment epithelium cells.5 We describe a de novo mutation in the REP-1 protein sequence in a woman with the CHM phenotype who had extensive chorioretinal atrophy, severe nyctalopia, and restricted side vision.
C
horoideremia (CHM) is an X-linked recessive eye disorder characterized by progressive degeneration of the choroid, retinal pigment epithelium, and retina. Its prevalence is estimated as 1 case in 100,000 population.1 The severity of the ophthalmic manifestation of CHM may show considerable interfamilial as well as intrafamilial variability.2,3 Affected males have progressive loss of initially peripheral and subsequently central visual function.1,2 Female carriers generally do not have clinically significant visual impairment; most have some degree of pigmentary retinal degenerative change, which has been described as a “moth-eaten” fundus appearance.3 Supported by funds from the Foundation Fighting Blindness (Owings Mills, MD); the Karl Cless Family Foundation (Chicago, IL); the Grant Healthcare Foundation (Chicago, IL), and NEI (Core Grant EY01792; Bethesda, MD). The authors do not have any proprietary interest in this work. Reprint requests: Gerald A. Fishman, MD, UIC Eye Center, University of Illinois at Chicago (M/C 648), 1855 West Taylor Street, Chicago, IL 60612; e-mail: [email protected]
Case Report A 22-year-old white woman of German ancestry without a family history of CHM presented with poor night vision and
182
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DE NOVO MUTATION IN A CHM CARRIER
Fig. 1. Fundus photographs of the right eye demonstrate extensive retinal pigment epithelium and choroidal atrophy in the posterior (A), temporal (B), and inferior (C) retina with a normal-appearing optic disk and retinal vessels.
peripheral field loss. These visual symptoms were first noted when she was 12 years of age and had been progressively worsening. Visual acuity was correctable to 20/20 in both eyes. Results of color vision screening with Ishihara plates were normal. Ocular pressure was 15 mmHg in each eye. Slit-lamp examination showed that the cornea, anterior chamber, and lens were clear in both eyes. The vitreous had 1⫹ cells in each eye. Fundus examination disclosed extensive retinal pigment epithelium and choroidal atrophy with a normal-appearing optic disk and retinal vessels. There were isolated areas of velvetlike pigment clumping as well as isolated areas of round black pigment clumping in the midperipheral retina. There was relative sparing of the macular region. The changes were essentially symmetric between the eyes (Fig. 1). Goldmann field testing showed ring scotomas with a normal peripheral boundary (Fig. 2). Electroretinogram testing showed markedly reduced rod and cone amplitudes with prolonged cone implicit times to both single-flash and 32-Hz flicker stimuli (Fig. 3). The patient’s phenotype was characteristic of CHM. Blood samples were obtained from the patient and her parents for gene mutation analysis and analyzed by previously described methods.2,7 (The patient and her parents provided informed consent, and the project was approved by an institutional review board at the University of Illinois.)
We sequenced each of the 15 exons of the CHM gene and observed a single heterozygous base-pair change in exon 7(R293X) of this gene. The variant characterized in this female patient was a C to T transition resulting in the conversion of an arginine residue (CGA) to a stop codon (TGA), which would be expected to cause a truncation of the REP-1 protein.2 A cytogenetic analysis for the patient showed a normal female karyotype. Specifically, there was no chromosomal translocation abnormality involving either of her X chromosomes. The parents were evaluated for the R293X mutation in the CHM gene; however, neither parent was found to harbor this change. Both parents also were screened for multiple informative genetic markers, and the results of this genotyping strongly supported the conclusion that they are the patient’s biologic parents. Entirely normal results of fundus examination were found for each parent. No other family members had a history of night blindness or peripheral field loss. Figure 4 shows the family pedigree.
Discussion Our findings are consistent with a de novo point mutation causing disease via one of two mechanisms. First, a heterozygous mutation in our patient has
Fig. 2. Goldmann field testing shows a ring scotoma with a normal peripheral boundary in the right eye.
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have no functional CHM allele and would be expected to have a phenotype similar to that of an affected male. Some cases of advanced CHM causing visual loss have been described in females.8,9 Partial or extensive inactivation of the normal X chromosome may cause either patchy or severe expression of disease in a CHM carrier.10 Other possible explanations for CHM expression in a female would include Turner syndrome or homozygous mutations,9 neither of which was found in our patient. Manifestation of disease in females can also result from disruption of the CHM gene by X chromosome translocations.1 In these cases, X chromosome inactivation is nonrandom; the normal X chromosome is preferentially inactivated, while both translocation fragments remain active.1 The normal karyotype determined in our patient does not support translocation or chromosomal rearrangement as the cause of her phenotype. Her de novo mutation has been previously identified and described in other patients with the clinical diagnosis of CHM.1,2,6 To our knowledge, this is the first case of a de novo mutation in the CHM gene described in either a carrier of or an affected individual with CHM. Key words: choroideremia, carrier, de novo mutation. References 1. Fig. 3. Electroretinogram (ERG) recording (left, normal findings; right, findings for the right eye of our patient) shows markedly reduced rod (top) and cone (bottom) amplitudes.
2.
3.
caused severe disease because of “skewed” inactivation leading to a relatively large fraction of retinal cells that have inactivated the normal allele. Second is the possibility that our patient is a carrier of a second CHM mutation (not detected by our mutation screening of the CHM coding sequences) that lies on the CHM allele opposite the one affected by the R293X mutation. In this latter scenario, the patient would
4.
5.
6.
7.
8. 9. Fig. 4. Pedigree of the family. Arrow and dark circle, the proband; white circles (females) and squares (males), unaffected family members; X, individuals examined by the authors.
10.
Van der Hurk JAJM, Schwartz M, Van Bokhoven H, et al. Molecular basis of choroideremia: mutations involving the Rab escort protein-1 gene. Hum Mutat 1997;9:110–117. McTaggart KE, Tran M, Mah DY, et al. Mutational analysis of patients with the diagnosis of choroideremia. Hum Mutat 2002;20:189–196. Francis PJ, Fishman GA, Trzupek KM, et al. Stop mutations in exon 6 of choroideremia gene, CHM, associated with preservation of electroretinogram. Arch Ophthalmol 2005; 123:1146–1149. Rudolph G, Preising M, Kalpadakis P. Phenotypic variability in three carriers from a family with choroideremia and a frameshift mutation 1388delCCinsG in the RPE-1 gene. Ophthalmic Genet 2003;24:203–214. Van Bokhoven H, Van der Hurk JAJM, Bogerd L, et al. Cloning and characterization of the human choroideremia gene. Hum Mol Genet 1994;3:1041–1046. MacDonald IM, Mah DY, Ho YK, et al. A practical diagnostic test for choroideremia. Ophthalmology 1998;105: 1637–1640. Fishman GA, Stone EM, Grover S, et al. Variation of clinical expression in patients with Stargardt dystrophy and sequence variations in the ABCR gene. Arch Ophthalmol 1999;117: 504–510. Fraser GR, Friedmann AI. Choroideremia in a female. BMJ 1968;2:732–734. Mijad MA, Horsborough B, Gray RH. Unusual macular findings in a known choroideremia carrier. Eye 1998;12:740–741. Cheung MC, Nune GC, Wang M, et al. Detection of localized retinal dysfunction in a choroideremia carrier. Am J Ophthalmol 2004;137:1:189–190.
