Spectrum of RB1 Mutations in Argentine Patients: 20-years Experience in the Molecular Diagnosis of Retinoblastoma

ABSTRACT

Background: Retinoblastoma is a hereditary cancer of childhood caused by mutations in the RB1 tumor suppressor gene. An early diagnosis is critical for survival and eye preservation, thus identification of RB1 mutations is important for unequivocal diagnosis of hereditary retinoblastoma and risk assessment in relatives.

Methods: We studied 144 families for 20 years, performing methodological changes to improve detection of mutation. Segregation analysis of polymorphisms, MLPA, FISH and cytogenetic assays were used for detection of “at risk haplotypes” and large deletions. Small mutations were identified by heteroduplex/DNA sequencing.

Results: At risk haplotypes were identified in 11 familial and 26 sporadic cases, being useful for detection of asymptomatic carriers, risk exclusion from relatives and uncovering RB1 recombinations. Ten large deletions (eight whole gene deletions) were identified in six bilateral/familial and four unilateral retinoblastoma cases. Small mutations were identified in 29 cases (four unilateral retinoblastoma patients), being the majority nonsense/frameshift mutations. Genotype-phenotype correlations confirm that the retinoblastoma presentation is related to the type of mutation, but some exceptions may occur and it is crucial to be considered for genetic counseling. Three families included second cousins with retinoblastoma carrying different haplotypes, which suggest independent mutation events.

Conclusion: This study enabled us to obtain information about molecular and genetic features of patients with retinoblastoma in Argentina and correlate them to their phenotype.     

Sarcoma and familial retinoblastoma

Background: Retinoblastoma is the most common malignant ocular tumour of childhood. It results from mutations in the retinoblastoma gene, RB1, which may be sporadic or heritable. Only 10?25% of patients have a family history of retinoblastoma and can be assumed to have a heritable RB1 mutation. A small proportion of the remaining patients may have a heritable mutation despite the lack of relatives with retinoblastoma. Heritable RB1 mutations are associated with an increased risk of sarcoma. Described herein are three patients with a past history of retinoblastoma, no family history of retinoblastoma, and a first‐degree relative with sarcoma. Mutation analyses were performed on DNA samples from these patients to test for heritable RB1 mutations. Methods: A genomic DNA analysis of RB1 gene. Results: Heritable mutations in the RB1 gene were identified in two of the three cases. In these two cases, Bayesian risk calculation indicated that the chance of the affected relatives having the familial RB1 mutation was greater than 90%. In the case without an identified mutation, Bayesian risk analysis indicated that the chance of there being an unidentified familial RB1 mutation was low (16%) but could still be clinically significant in managing the family. Conclusion: The risk of non‐ocular malignancies and the availability of genetic testing for heritable RB1 mutations have important clinical implications for the management of children with retinoblastoma.     

Retinoblastoma in twins: Risk assessment of genotypic variants

Purpose:To describe methods of risk assessment in twins with retinoblastoma (RB).Methods:A case series of four RB probands with a twin sibling. Family status, clinical presentation, and RB1 germline status-based risk assessment were analyzed.Results:Two pairs had a positive family history (unilateral and bilateral RB in one of the parents (#1 and #2, respectively) and two pairs (#3 and #4) were sporadic. One of the familial twins (#1) had a high risk (90%) of manifesting RB in the twin. The other case (#2) with an absent RB1 germline mutation in the twin had a 0% risk of developing RB. Among sporadic cases of twins (#3), genetic testing did not identify a germline mutation (tumor sample unavailable) in the proband which downgraded the risk of germline mutation from 15% to <1%. The twin never developed RB (5 years of age at last follow-up). Pathogenic mosaicism for germline RB1 mutation (c.1723C>T) could be identified (tumor tissue available) in the proband (# 4). Identical germline mutation (and RB tumor) was also noted in the twin. In each case, there was concordance between the assessed risk and manifestation of RB.Conclusion:Assessment of risk of RB in a twin presents with a unique challenge. Depending upon the genotype variant, the risk of developing RB can vary from 0% to 90%. In addition to family history, clinical manifestation in the proband, zygosity status, and RB1 germline status are critical in formulating risk-appropriate surveillance guidelines.     

Myocilin mutations in a population-based sample of cases with open-angle glaucoma: the Rotterdam Study

PURPOSE: To investigate the prevalence of myocilin (MYOC) mutations in a population-based sample of open-angle glaucoma (OAG) cases and to describe a family with both juvenile and adult onset OAG caused by a mutation in MYOC. METHODS: MYOC was screened in cases derived from the Rotterdam Study in the Netherlands. Definite OAG was defined as a glaucomatous optic neuropathy together with a glaucomatous visual field defect. Upon the identification of the Asn480Lys mutation in one case, seven additional family members were studied. To test for a founder effect with earlier reported families with this mutation, the haplotypes of MYOC flanking markers D1S2851, D1S242, D1S218, and D1S1165 were compared. RESULTS: Seven sequence alterations in MYOC were found in 14 of 47 OAG cases; six of these were also found in controls. In one case, an Asn480Lys mutation was found. In relatives of the latter patient, the phenotype ranged from a glaucomatous optic neuropathy without visual field defect in a 70-year-old patient to severely affected optic discs and a remaining temporal remnant in a 34-year-old patient; those without the mutation had no signs of OAG. Haplotype analysis suggested a different origin of the mutation. CONCLUSIONS: The prevalence of MYOC mutations (2.2%) was similar to that found in hospital-based studies. Although mutations in MYOC are rare, relatives carrying this mutation run a high risk of developing the disease. Instead of submitting all members of a family with the Asn480Lys mutation to frequent follow-up, medical care can be restricted to those carrying the mutation.     

X-linked retinitis pigmentosa: re-evaluation of fundus findings and the use of haplotype analysis in clarification of carrier female status

The identification by fundus examination of those females carrying an X-linked retinitis pigmentosa (rp) gene can reportedly be as high as 87%. In genetic counselling sessions with young females with a 50% risk of being a carrier who wished to know their status, it has not been possible to achieve such a level of success. A review and reanalysis of previous reports indicated that if a tapetal-like reflex was not present in those age 35 years or less, the likelihood of identifying a carrier by fundus examination was small.

A family with 7 females with a 50% risk of being a carrier of X-linked rp was evaluated using haplotype analysis in an attempt to identify the X chromosome carrying the rp gene. In the family described, it was possible to establish that a mutation in the rp3 locus most likely causes the disease. This has permitted the determination of the carrier status in each of the females with a high degree of certainty.     

Prognostic DNA testing and counselling for dominant optic atrophy due to a novel OPA1 mutation

CASE REPORT: To report the case of a 26-year-old woman with a family history of dominant optic atrophy who requested DNA testing and counselling. Ophthalmologic examination showed her affected father had bilateral temporal papillary pallor. Direct genomic sequencing of the OPA1 gene revealed a novel heterozygous nonsense mutation (Arg879stop). Because no mutation in OPA1 was detected in the daughter, we could counsel her that the possibility was very low that she was a carrier or would pass the disease-causing gene to her children. COMMENTS: Our study provides evidence of the apparent value of molecular genetic analysis of OPA1 gene as predictive DNA testing, although the exact risk and benefit of this type of analysis awaits further study.     

Isolated sulfite oxidase deficiency: review of two cases in one family.

OBJECTIVE: The authors describe two cases of isolated sulfite oxidase deficiency found in one family. This is a rare autosomal-recessive disorder presenting at birth with seizures, severe neurologic disease, and ectopia lentis. It can be easily missed with metabolic screening; however, the finding of lens subluxation stresses the importance of ophthalmic assessment in making the diagnosis. DESIGN: Two observational case reports. INTERVENTION/METHODS: Ophthalmic assessment, biochemical assay for specific urinary and plasma metabolites, magnetic resonance imaging, and gene sequencing were used to make the diagnosis of the disease in the proband. The diagnosis was subsequently recognized in a previously affected sibling after the postmortem neuropathology was reviewed. Mutation analysis was performed on cultured fibroblasts from the proband to identify and categorize the specific mutation responsible for the disease in the family. From this, future prenatal detection of sulfite oxidase deficiency is possible. MAIN OUTCOME MEASURES: The diagnosis of sulfite oxidase deficiency was established in this family, enabling appropriate genetic counseling and recurrence risk estimation. RESULTS: Point mutations were found in both alleles of the sulfite oxidase gene in the proband. The first is a 623C-->A mutation, which predicts an A208D substitution, and the second is a 1109C-->A, which predicts an S370Y substitution. Both residues A208D and S370Y are critical for sulfite oxidase activity. CONCLUSIONS: Isolated sulfite oxidase deficiency is a rare heritable disease for which mutation analysis can allow accurate prenatal screening. It often is difficult to diagnose by clinical presentation alone, but the critical finding of lens subluxation accompanying seizures and diffuse neurologic disease in an infant should alert the physician to the diagnosis.     

108例视网膜母细胞瘤Rb基因突变的特征

目的；研究RB患者肿瘤及体细胞内Rb基因的存在状态、细微结构、Rb基因突变的特征、起源与传递。 方法；DNA分子杂交、SSCP分析、DNA序列测定。结果：108例RB肿瘤标本中80例(?4％)存在Rb基因点突变，其中44例为纯合型，20例有二个独立发生的杂合型点突变，16例只检出一个杂合型点突变。通过对比分析RB患者肿瘤与白细胞DNA、RB患者家庭成员白细胞DNARlo基因的结构，揭示了Rb基因点突变的起源与遗传特征。 结论：Rb基因是与RB肿瘤形成关系最为密切的基因。RB肿瘤发生需二次突变事件导致Rb基因的二个等位基因失活，第一次突变事件突出表现为点突变；第二次突变事件主要是LOH，其次是点突变。 （中华眼底病杂志，1997，13：12-16）     

Complete mitochondrial DNA sequence analysis in two southern Chinese pedigrees with Leber hereditary optic neuropathy revealed secondary mutations along with the primary mutation

AIM: To investigate mitochondrial factors associated with Leber hereditary optic neuropathy (LHON) through complete sequencing and analysis of the mitochondrial genome of Chinese patients with this disease. METHODS: Two unrelated southern Chinese families with LHON and 10 matched healthy controls were recruited, and their entire mitochondrial DNA (mtDNA) was amplified and sequenced with the universal M13 primer. Then DNA sequence analysis and variation identification were perfomed by DNAssist and Chromas 2 software and compared with authoritative databases such as Mitomap. RESULTS: Mutational analysis of mtDNA in these two Chinese pedigrees revealed one common LHON-associated mutation, G11778A (Arg→His), in the MT-ND4 gene. In addition, there were two secondary mutations in Pedigree 1: C3497T (Ala→Val), and C3571T (Leu→Phe) in the MT-ND1 gene, which have not been reported; and two secondary mutations occurred in Pedigree 2: A10398G (Thr→Ala) in the MT-ND3 gene, and T14502C (Ile→Val) in the MT-ND6 gene. Three polymorphisms, A73G, G94A and A263G in the mtDNA control region, were also found. CONCLUSION: Our study confirmed that the known MT-ND4*G11778A mutation is the most significant cause of LHON. The C3497T and C3571T mutations in Pedigree 1 were also both at hot-spots of MT-ND1; they may affect the respiratory chain in coordination with the primary mutation G11778A. In Pedigree 2, the two secondary mutations A10398G of MT-ND3 and T14502C of MT-ND6 may influence mitochondrial respiratory complex I, leading to the mitochondrial respiratory chain dysfunction which results in optic atrophy together with G11778A. Therefore, not only the common primary LHON mutation is responsible for the visual atrophy, but other secondary mtDNA mutations should also be considered when giving genetic counseling.     

原发性开角型青光眼分子遗传学研究进展

青光眼是继白内障之后的首要致盲眼病,其分子遗传学一直备受关注。MYOC(myocilin)、OPTN(opticneuropathy inducing gene)和WDR36(WD40-repeat36)是目前极少数已经确定的原发性开角型青光眼的致病基因。本文重点就上述3种基因的发现、定位、编码产物、突变研究及相关进展等作一综述。     

X-linked retinoschisis: novel mutation in the initiation codon of the XLRS1 gene in a large family

PURPOSE: To describe a novel point mutation in the initiation codon of the XLRS1 gene in a large family and the clinical features of males affected with X-linked juvenile retino-schisis. METHODS: Genealogic investigation and mutation screening of the XLRS1 gene were performed for a 4-generation family consisting of 72 members. Affected males were evaluated clinically between 1986 and 2004 with up to 18 years of follow-up. RESULTS: We identified a novel point mutation (1A>T transversion) in the initiation codon of the XLRS1 gene in affected males resulting in an amino acid substitution of methionine to leucine (Met1Leu), therefore abolishing the translation initiation Met codon. CONCLUSION: Identification of the disease-causing mutation in this family with long-term follow-up allows for earlier and more accurate identification of individuals at risk for this inherited progressive macular degeneration, provides for more accurate genetic counseling, and contributes to our understanding of the pathophysiology of this disorder.     

Mouse genetics: a tool to help unlock the mechanisms of glaucoma

Gene characterization holds great promise for understanding molecular mechanisms of disease. Although glaucoma gene identification is very valuable and allows assessment of an individual's genetic risk, it is not by itself sufficient to answer detailed questions about pathogenesis. Despite the recent identification of a number of glaucoma genes, there are still many questions regarding the ways in which mutations in these genes cause disease. The mouse system, including the ability to alter specific genes, provides a powerful experimental system for hypothesis testing and molecular dissection of pathogenesis subsequent to gene identification. The ability to control both genetic and environmental factors will allow the use of mice to identify modifier genes that alter complex glaucoma phenotypes and that are especially difficult to identify in human families. By providing a bridge between gene identification and tests of gene function, mouse studies will be an important complement to those in humans and other species. This article summarizes the recent use of mice and the future potential of applying approaches of mouse genetics to intraocular pressure and glaucoma research.     

Distribution of optineurin sequence variations in an ethnically diverse population of low-tension glaucoma patients from the United States

PURPOSE: Previous studies have suggested that Optineurin (OPTN) sequence variants contribute to low-tension glaucoma (LTG) in ethnically homogeneous populations. The purpose of this study is to evaluate the prevalence of OPTN sequence variants in an ethnically diverse population of LTG patients from the United States, and to describe the phenotype of patients with OPTN sequence variants preferentially found in LTG patients. METHODS: Genomic DNA purified from 67 LTG patients was screened for DNA sequence variants located in the exons and flanking introns of the OPTN gene using high-performance liquid chromatography analysis and direct genomic DNA sequencing. Eighty-six primary open-angle glaucoma probands and 100 control patients were also analyzed. RESULTS: Nine OPTN DNA sequence variants were identified in this patient population including the 2 previously identified heterozygous nonsynonymous single-nucleotide polymorphisms in exons 4 and 5. Four LTG patients with severe disease and positive family history of glaucoma, were found to have DNA sequence changes not found in primary open-angle glaucoma probands or control individuals including the previously reported E50K variation. CONCLUSIONS: The results of this study support the rare association of OPTN sequence variants with familial forms of LTG. The E50K mutation seems to be associated with a severe form of LTG, and although rare, the identification of this sequence variant in patients at risk may help direct appropriate therapy.     

A novel mutation in the short-wavelength-sensitive cone pigment gene associated with a tritan color vision defect

Inherited tritan color vision deficiency is caused by defects in the function of the short-wavelength-sensitive (S) cones. This heterozygous group of disorders has an autosomal dominant pattern of inheritance. Amino acid variations of the S cone opsin are rare and all that have been identified thus far are associated with inherited tritan color vision defects. Here we report the identification of a 30-year-old male who made errors on standard color vision tests consistent with the presence of a mild tritan color vision deficiency. We tested the hypothesis that his color vision impairment was due to a mutation in the S cone photopigment gene. He was found to be heterozygous for a mutation that caused the amino acid proline to be substituted in place of a highly conserved leucine at amino acid position 56 in the S cone opsin. This mutation was absent in 564 S cone photopigment genes from 282 subjects who did not make tritan errors. Thus, we conclude that this mutation disrupts the normal function of S cones.     

An audit of genetic testing in diagnosis of inherited retinal disorders: a prerequisite for gene-specific intervention

Background:  There has been an exponential increase in the number of genes implicated in inherited retinal disease over the last decade, but the genetic and phenotypic heterogeneity limited mutation detection. The high cost of sequencing and long turn around times meant that gene testing was not a viable option, particularly in New Zealand. Recently, advancements including development of micro-array-based mutation analysis and non-for-profit laboratories have resulted in affordable and time-efficient testing. This has enabled genetic diagnostics to become an integral component of the work-up for inherited retinal disease.
Methods:  Genetic testing for inherited retinal disorders was initiated via the Ocular Genetic Clinic in Auckland 2 years ago. A retrospective audit of genetic testing over this period was carried out. The results of these tests and outcomes are discussed.
Results:  Thirty-five probands have undergone genetic testing for retinal disorders. This has included X-Linked Retinoschisis, Leber Congenital Amaurosis, Retinitis Pigmentosa, Albinism, Achromatopsia, Usher syndrome, Stargardt disease and Mitochondrial disease. Of these, 54% of tests (19/35) showed a rare variant or pathogenic mutation. Three couples have proceeded to investigate the options of prenatal diagnosis and/or pre-implantation genetic diagnosis.
Conclusion:  The introduction of genetic testing, largely via disease arrays, has been highly successful at clarifying disease genotype in our cohort. It is now a timely and cost-effective investigation that should be elemental to the assessment of inherited retinal disease. Genetic testing in an opportune fashion permits genetic counselling, enables families to make reproductive choices and might allow the possibility of gene therapy interventions.     

The hunt for the secrets of uveal melanoma

The many secrets of uveal melanoma are being uncovered. Information on host and environmental factors that predispose to uveal melanoma has been published. The most important factors include light eye colour, fair skin, inability to tan and chronic sun exposure. Clinical clues that are visible on ophthalmoscopy have been shown to be significant factors in predicting growth of small borderline tumours and allow for early detection of melanoma. These factors include thickness over 2 mm, subretinal fluid, symptoms, orange pigment overlying the tumour and tumour margin within 3 mm of the disc. Refined methods of cytogenetic analysis have identified several chromosomal mutations associated with uveal melanoma. Currently, the most important mutation proves to be chromosome 3 monosomy, an abnormality associated with greater risk for metastatic disease.     

Evolving global risk assessment of ocular hypertension to glaucoma

PURPOSE OF REVIEW: To discuss current knowledge of global risk assessment in ocular hypertension. RECENT FINDINGS: The ophthalmologist treating patients with ocular hypertension is frequently faced with the clinical dilemma of which patients to treat and how vigorous treatment should be. The goal of risk assessment for glaucoma is to identify patients at greatest risk for symptomatic vision loss. Risk factors can be identified by history such as age, race, and family history or can be clinically observed by examination such as elevated intraocular pressure, optic nerve head appearance, central corneal thickness, and visual field abnormalities. Risk assessment is a well accepted tool in other fields of medicine. Parallels can be drawn between the evolution of risk assessment for coronary artery disease and glaucoma. Validated risk calculators for ocular hypertension are currently available mostly derived from the Ocular Hypertension Treatment Study. SUMMARY: The aim of assessing global risk for conversion from ocular hypertension to glaucoma is to identify patients who are most likely to benefit from early treatment. Calculation of risk should be accompanied by thorough analysis of risks, benefits, and alternatives for the individual patient.     

Familial non-syndromic macular pseudocoloboma secondary to homozygous CLDN19 mutation

Purpose: The purpose of this study is to uncover the genetic cause for non-syndromic macular “coloboma” (pseudocoloboma) in three brothers from a consanguineous family.

Methods: Homozygosity mapping for the three affected brothers and whole-exome sequencing in one affected brother, followed by confirmatory Sanger sequencing and segregation analysis of the candidate gene for all immediate family members; molecular modeling of the candidate mutation; and review of clinical, imaging, and laboratory findings.

Results: Three otherwise-healthy brothers (age 10, 10, and 6 years) had macular pseudocoloboma. Both parents and the fourth brother were not affected. Parents were first cousins. A novel homozygous missense variant in claudin 19 (CLND19: NM_148960.2:c. 263T>A; p.Val88Glu) segregated with the phenotype, and molecular modeling predicts an unfavorable effect to protein function. All prior reported biallelic CLND19 mutations cause symptomatic hypomagnesemia with hypercalciuria and nephrocalcinosis, often with concurrent macular pseudocoloboma. However, general physical assessment, metabolic profile, and renal imaging for the three affected brothers were normal.

Conclusions: A homozygous CLDN19 mutation can cause macular pseudocoloboma without evidence for systemic disease in children. This is the first reported family with CLDN19 mutations to have an ocular phenotype only; however, those identified to harbor biallelic CLDN19 mutations should be considered at risk for the extraocular manifestations that have previously been associated with mutations in the gene.     

常染色体隐性遗传视网膜色素变性家系视紫红质基因突变的检测分析

目的 观察一个近亲婚配常染色体隐性遗传视网膜色素变性（ARRP）家系中视紫红质基因（RHO）的突变特征，并探讨其视网膜色素变性(RP)发病机制。 方法 抽取8例该ARRP家系成员及10例正常对照者的外周静脉血5~8 ml；提取基因组DNA；采用聚合酶链反应（PCR）方法扩增RHO基因第1~5外显子和第1内含子基因 片段 ，用直接DNA测序法筛查RHO基因突变。 结果 来自同一家系3例患者RHO 基因的第5外显子第344密码子发生了A→G碱基的错义突变，导致了谷氨酰胺变成了精氨酸(G ln344Arg)，3例患者为该突变的纯合子。患者近亲婚配父母及1例未患病家庭成员为该突变 的杂合子。2例未患病家庭成员及10例正常对照者均未发现RHO基因突变。 结论 Gln344Arg突变可能是该ARRP家系的致病原因；在近亲婚配ARRP家系中RHO基因突变频率可能增加。 （中华眼底病杂志，2004，20：145-148）