[Analysis on the effect of secondary mutations on Leber's hereditary optic neuropathy]

OBJECTIVE

To investigate the effect of secondary mutations on Leber's hereditary optic neuropathy (LHON).

METHODS

Three primary and 24 secondary mutations were identified in 4 Chinese families which included male offspring.

RESULTS

All of the four pedigrees carried classic LHON mutations at nucleotide (nt) 11778, and did not carry any point of 24 secondary mutations. Nevertheless many polymorphic points were found in the nearby fragments of these pedigrees, such as 5178, 5108, 3705, 3721, 13734, etc.

CONCLUSION

Male offspring sequences should be analyzed in pedigrees with LHON to avoid the influence of familial inheritance characteristic which mitochondrial DNA polymorphism carried. Existence of the "repair genes" may affect the development of LHON.

Mutations in the AP1S2 gene encoding the sigma 2 subunit of the adaptor protein 1 complex are associated with syndromic X-linked mental retardation with hydrocephalus and calcifications in basal ganglia

Fried syndrome, first described in 1972, is a rare X-linked mental retardation that has been mapped by linkage to Xp22. Clinical characteristics include mental retardation, mild facial dysmorphism, calcifications of basal ganglia and hydrocephalus. A large four-generation family in which the affected males have striking clinical features of Fried syndrome were investigated for linkage to X-chromosome markers; the results showed that the gene for this condition lies within the interval DXS7109-DXS7593 in Xp22.2. In total, 60 candidate genes located in this region, including AP1S2, which was recently shown to be involved in mental retardation, were screened for mutations. A mutation in the third intron of AP1S2 was found in all affected male subjects in this large French family. The mutation resulted in skipping of exon 3, predicting a protein with three novel amino-acids and with termination at codon 64. In addition, the first known large Scottish family affected by Fried syndrome was reinvestigated, and a new nonsense mutation, p.Gln66X, was found in exon 3. Using CT, both affected patients from the French family who were analysed had marked calcifications of the basal ganglia, as previously observed in the first Scottish family, suggesting that the presence of distinctive basal ganglia calcification is an essential parameter to recognise this syndromic disorder. It may be possible to use this feature to identify families with X-linked mental retardation that should be screened for mutations in AP1S2.

[A novel mutation of the OPA1 gene responsible for isolated autosomal dominant optic atrophy in two brothers]

Autosomal dominant optic atrophy, or Kjer disease, is the most frequent form of autosomal dominant optic neuropathy. We report a novel mutation of the OPA1 gene in two brothers with autosomal dominant optic atrophy and describe their clinical features. The two patients, aged 41 and 37, presented with a bilateral visual impairment that had been detected at the age of 4 in both of them. Their ophthalmoscopic examinations disclosed a bilateral optic atrophy and their Goldmann visual fields showed cecocentral scotomas. The patients thought their disease might be a Leber's hereditary optic neuropathy; however, mutations had ever been sought. When first seen by us, they wished to know whether their disorder might be transmitted to their children. They had a family history of visual impairment. We carried out mtDNA sequencing but we did not identify any primary or rare Leber's hereditary optic neuropathy mutations. On the other hand, the 30 coding exons of the OPA1 gene and the intron-exon junctions were amplified by polymerase chain reaction and sequenced. A novel mutation of the OPA1 gene was found in both brothers: a deletion of four nucleotides in intron 19, associated with anomalous splicing, demonstrating the pathogenicity of the mutation. These molecular analyses contributed to identifying a novel mutation of the OPA1 gene with a clinical phenotype of isolated optic atrophy.

Ataxie cérébelleuse révélant un syndrome de Wolfram

INTRODUCTION

Wolfram syndrome is a genetic disease with recessive autosomic transmission, associating early-onset diabetes mellitus and bilateral optical atrophy.

CASE REPORT

We report the case of a 47-year-old patient for whom we diagnosed a Wolfram syndrome in view of a late neurological syndrome in association with ataxia and bilateral horizontal nystagmus. The brain resonance magnetic imaging revealed a major atrophy of the brainstem and cerebellum.

CONCLUSION

Wolfram syndrome is a rare pathology, with fatal consequences before the age of 50. The association of diabetes mellitus and optical atrophy, especially when there are other symptoms (ataxia, deafness, diabetes insipidus, neuropsychiatric manifestations or urinary tract disorders) should lead to this diagnosis and to carry out a genetic confirmation.

Early onset severe and late-onset mild Charcot-Marie-Tooth disease with mitofusin 2 (MFN2) mutations

Mutations in the mitofusin 2 (MFN2) gene, which encodes a mitochondrial GTPase mitofusin protein, have recently been reported to cause both Charcot-Marie-Tooth 2A (CMT2A) and hereditary motor and sensory neuropathy VI (HMSN VI). It is well known that HMSN VI is an axonal CMT neuropathy with optic atrophy. However, the differences between CMT2A and HMSN VI with MFN2 mutations remained to be clarified. Therefore, we studied the phenotypic characteristics of CMT patients with MFN2 mutations. Mutations in MFN2 were screened in 62 unrelated axonal CMT neuropathy families. We calculated CMT neuropathy scores (CMTNSs) and functional disability scales (FDSs) to quantify disease severity. Twenty-one patients with the MFN2 mutations were studied by brain MRI. Ten pathogenic mutations were identified in 26 patients from 15 families (24.2%). Six of these mutations had not been reported, and de novo mutations were observed in five families (33.3%). The electrophysiological patterns of affected individuals with the MFN2 mutations were typical of axonal CMT; however, the clinical and electrophysiological characteristics were markedly different in early (<10 years) and late disease-onset (> or =10 years) groups. All patients with an early onset had severe CMTNS (> or =21) and FDS (6 or 7), whereas most patients with late onset had mild CMTNS (< or =10) and FDS (< or =3). We identified two HMSN VI families with the R364W mutation in the early onset group; however, two other families with the same mutation did not have optic atrophy. In addition, two early onset families with R94W mutations, previously reported for HMSN VI, did not have visual impairment. Interestingly, eight patients had periventricular and subcortical hyperintense lesions by brain MRI. In the late-onset group, three patients had sensorineural hearing loss and two had bilateral extensor plantar responses. We found that MFN2 mutations are the major cause of axonal CMT neuropathy, and that they are associated with variable CNS involvements. Phenotypes were significantly different in the early and late disease-onset groups. Our findings suggest that HMSN VI might be a variant of the early onset severe CMT2A phenotype.

Novel OPA1 mutations identified in Japanese pedigrees with optic atrophy

PURPOSE

To determine whether mutations in the OPA1 gene were present in two Japanese families with optic atrophy.

METHODS

Thirty exons and their boundaries of the OPA1 gene were amplified by PCR with genomic DNA as templates and directly sequenced. The detected sequence changes were confirmed to be mutations by examining whether they were present in normal control individuals. A splicing mutation was characterized by RT-PCR of total RNA of leukocytes obtained from patients and one normal individual. The mutant transcripts resulting from the splicing mutation were further confirmed and quantified by sequencing and identifying the denatured RT-PCR products by polyacrylamide electrophoresis.

RESULTS

One novel splicing mutation of c.871-1G>T and one novel insertion mutation of c.579_580insTT (p.R194fsX228) were identified from two familial cases, respectively. Both mutations segregated within the family heterozygously and were not found in the 189 control individuals examined. Two mutant transcripts resulted from the splicing mutation were identified through amplified OPA1 cDNA prepared from the RNA of leukocytes of the patients. One had a 21 bp deletion at the beginning of the exon 9 leading to a 7 amino acid in-frame deletion of the protein. The expression level of this mutant transcript was similar to the transcript from the wild type allele of the patient. The other mutation was a 114 bp deletion, leading to a 38 amino acid in-frame deletion that skipped all of exon 9, and the expression of this mutant transcript was much lower than the 21 bp deletion.

CONCLUSIONS

The predicted consequence of both mutations is the loss of GTPase activity. Our findings further establish the involvement of OPA1 mutation in Japanese patients with optic atrophy and serve as supportive evidence that haploinsufficiency of the OPA1 gene is the cause of the optic atrophy.

The unique characteristics of Thai Leber hereditary optic neuropathy: analysis of 30 G11778A pedigrees

Leber hereditary optic neuropathy (LHON) is characterized by acute or subacute bilateral visual loss, and affects mostly young males. The most common mitochondrial DNA mutation responsible for LHON worldwide is G11778A. Despite different genetic backgrounds, which are believed to influence the disease expression, most features of LHON are quite common in different populations. However, there seem to be a few ethnic-specific differences. Analyses of our 30 G11778A LHON pedigrees in Thailand showed some characteristics different from those of Caucasians and Japanese. In particular, our pedigrees showed a lower male to female ratio of affected persons (2.6:1) and much higher prevalence of G11778A blood heteroplasmy (37% of the pedigrees contained at least one heteroplasmic G11778A individual). Heteroplasmicity seemed to influence disease manifestation in our patients but did not appear to alter the onset of the disease. The estimated overall penetrance of our G11778A LHON population was 37% for males and 13% for females. When each of our large pedigrees were considered separately, disease penetration varied from 9 to 45% between the pedigrees, and also varied between different branches of the same large pedigree. Survival analysis showed that the secondary LHON mutations G3316A and C3497T had a synergistic deleterious effect with the G11778A mutation, accelerating the onset of the disease in our patients.

[Electrophysiological tests in early and differential diagnosis of some hereditary retinal and optic nerve diseases]

PURPOSE

The goal of our study was to determine the value of electrophysiological tests in early and differential diagnosis of some hereditary retinal and optic nerve diseases.

MATERIAL AND METHODS

Review article on the basis of own experiences and results of another authors concerning to EOG, ERG, PERG, MfERG and VEP tests in evaluation of hereditary, stationary and progressive retinal diseases, as well as optic nerve diseases.

RESULTS

Electrophysiological tests can be abnormal even in patients without fundus changes seen in routine ophthalmological examination.

CONCLUSIONS

Electrophysiological tests are useful in early and differential diagnosis of some hereditary retinal and optic nerve diseases. General ophthalmologists should remember about its application, especially in difficult diagnostic cases.

[An OPA3 gene mutation is responsible for the disease associating optic atrophy and cataract with extrapyramidal signs]

INTRODUCTION

In 1961, Garcin et al. described a family with several members affected with optic atrophy associated with cataract, and neurological symptoms. The authors believed this condition to be distinct from other diseases known at that time, e.g. the Behr syndrome, Marinesco-Sjogren syndrome and Friedreich's ataxia.

METHOD

This family was followed over a period of 40 years and genes known to be responsible for optic atrophy were sequenced.

RESULTS

The G277A mutation of OPA3 gene was responsible for this familial disease.

DISCUSSION

A new clinical entity is identified: autosomal dominant optic atrophy and cataract, due to a heterozygous mutation of the OPA3 gene, a nuclear gene encoding a mitochondrial protein.

Spastic paraplegia, optic atrophy, and neuropathy is linked to chromosome 11q13

We report an autosomal recessive neurodegenerative disorder in 25 white members from a large inbred Brazilian family, 22 of whom were evaluated clinically. This condition is characterized by (1) subnormal vision secondary to apparently nonprogressive congenital optic atrophy; (2) onset of progressive spastic paraplegia in infancy; (3) onset of progressive motor and sensory axonal neuropathy in late childhood/early adolescence; (4) dysarthria starting in the third decade of life; (5) exacerbated acoustic startle response; and (6) progressive joint contractures and spine deformities. Motor handicap was severe, and all patients were wheelchair bound after 15 years old. We performed a genome-wide screen including 25 affected individuals and 49 of their unaffected relatives. Linkage was detected at 11q13 region with a maximum logarithm of odds score of +14.43, obtained with marker D11S1883. The candidate region, which lies between D11S1908 and D11S1889, encompasses approximately 4.8Mb and has more than 100 genes and expressed sequences. We propose the acronym SPOAN (spastic paraplegia, optic atrophy, and neuropathy) for this complex syndrome.

Hereditary optic neuropathies

AIMS

To provide a clinical update on the hereditary optic neuropathies.

METHODS

Review of the literature.

RESULTS

The hereditary optic neuropathies comprise a group of disorders in which the cause of optic nerve dysfunction appears to be hereditable, based on familial expression or genetic analysis. In some hereditary optic neuropathies, optic nerve dysfunction is typically the only manifestation of the disease. In others, various neurologic and systemic abnormalities are regularly observed.

CONCLUSION

The most common hereditary optic neuropathies are autosomal dominant optic atrophy (Kjer's disease) and maternally inherited Leber's hereditary optic neuropathy. We review the clinical phenotypes of these and other inherited disorders with optic nerve involvement.

CDG-Id caused by homozygosity for an ALG3 mutation due to segmental maternal isodisomy UPD3(q21.3-qter)

We report on a patient with a congenital disorder of glycosylation type Id (CDG-Id) caused by a homozygous mutation in the ALG3 gene, which results from a de novo mutation in combination with a segmental maternal uniparental isodisomy (UPD). The patient presented with severe psychomotor delay, primary microcephaly, and opticus atrophy, compatible with a severe form of CDG. Isoelectric focusing of transferrin showed a type I pattern and lipid-linked oligosaccharide analysis showed an accumulation of dol-PP-GlcNAc2Man5 in patient's fibroblasts suggesting a defect in the ALG3 gene. A homozygous ALG3 missense mutation p.R266C (c.796C > T) was identified. Further evaluation revealed that neither the mother nor the father were carrier of the p.R266C mutation. Marker analysis revealed a segmental maternal isodisomy for the chromosomal region 3q21.3-3qter. UPD for this region has not been described before. More important, the combination of UPD with a de novo mutation is an exceptional coincidence and an extraordinary observation.

Molecular genetic basis of primary inherited optic neuropathies

AIM

To review the molecular genetic basis of primary inherited optic neuropathies.

METHODS

Medline and Embase search.

RESULTS

Inherited optic neuropathies are a genetically diverse group of disorders that present with reduced visual acuity and the clinical appearance of optic atrophy. The inherited optic neuropathies may be sporadic or familial, in which case the mode of inheritance may be Mendelian (autosomal dominant, autosomal recessive, X-linked recessive) or non-Mendelian (mitochondrial). Two genes for dominantly inherited optic atrophy have been mapped (OPA1 and OPA4), of which the gene has been identified in one (OPA1). A gene for recessive optic atrophy (OPA3) has also been identified. X-linked optic atrophy (OPA2) has been mapped but to date no gene has been identified. Mutations in mitochondrial DNA have been identified in Leber's hereditary optic neuropathy.

CONCLUSIONS

Mutations in genes from both the nuclear and mitochondrial genomes appear to be responsible. Mitochondrial dysfunction, in the broadest sense, is emerging as central to the pathogenesis of this group of conditions.

First prenatal diagnosis for Wolfram syndrome by molecular analysis of theWFS1gene

Wolfram syndrome (WS) is an autosomal recessive neurodegenerative disorder characterized by early onset diabetes mellitus and progressive optic atrophy in the first decade of life. Other clinical features such as diabetes insipidus, deafness, renal tract abnormalities or psychiatric illnesses are often present. The sequence of the Wolfram syndrome gene (WFS1) was described in 1998, and mutations in the gene have been reported in many populations. To date, the function of the putative protein remains unknown. Here we report prenatal diagnosis by analysing the WFS1 gene, in a foetus belonging to a family with a child diagnosed for Wolfram syndrome. The parents are carriers of the c.2206G > C (G736R) mutation. To our knowledge this is the first description of prenatal diagnosis for Wolfram syndrome, based on the molecular analysis of the WFS1 gene.

Identification of GUCY2D gene mutations in CORD5 families and evidence of incomplete penetrance

Cone rod dystrophy 5 (CORD5) is an autosomal dominant retinal disease that primarily affects cone function. The locus has previously been mapped to human chromosome 17p12-p13 between the markers D17S926/D17S849 and D17S945/D17S804. One of our "unaffected" recombinant individual from family 1175 was subsequently found to cross through this interval. Reexamination revealed that he was in fact mildly affected. This expanded the minimum candidate region. Direct sequencing of the GUCY2D and other candidate genes within this interval was carried out on 2 American families affected with CORD5. There was an R838C missense mutation within the GUCY2D gene in one and a R838H missense mutation in another families. The previously reported mutations for CORD6 are clustered at the same position within the gene. These results indicate that both CORD5 (MIM# 600977) and CORD6 (MIM# 601777) are actually the same disease. We conclude that significant variability in expression and incomplete penetrance exists even within one family.

A review of primary hereditary optic neuropathies

The primary inherited optic neuropathies are a heterogeneous group of disorders that result in loss of retinal ganglion cells, leading to the clinical appearance of optic atrophy. They affect between 1:10,000 to 1:50,000 people. The main clinical features are a reduction in visual acuity, colour vision abnormalities, centro-caecal visual field defects and pallor of the optic nerve head. Electrophysiological testing shows a normal flash electroretinogram, absent or delayed pattern visually evoked potentials suggestive of a conduction deficit and N95 waveform reduction on the pattern electroretinogram, consistent with a primary ganglion cell pathology. The primary inherited optic neuropathies may be sporadic or familial. The mode of inheritance may be autosomal dominant, autosomal recessive, X-linked recessive or mitochondrial. Within each of these groups, the phenotypic characteristics vary in such features as the mode and age of onset, the severity of the visual loss, the colour deficit and the overall prognosis. A number of different genes (most as yet unidentified) in both nuclear and mitochondrial genomes, underlie these disorders. The elucidation of the role of the encoded proteins will improve our understanding of basic mechanisms of ganglion cell development, physiology and metabolism and further our understanding of the pathophysiology of optic nerve disease. It will also improve diagnosis, counselling and management of patients, and eventually lead to the development of new therapeutic modalities.

Evidence of a founder effect for the RETGC1 (GUCY2D) 2943DelG mutation in Leber congenital amaurosis pedigrees of Finnish origin

Leber congenital amaurosis (LCA) is the earliest and most severe form of all inherited retinal dystrophies. It is a genetically heterogeneous condition as six disease-causing genes have been hitherto identified. Among them, RETGC1 (GUCY2D), is more frequently implicated in our series of LCA patients. Interestingly, 70 % of the families with RETGC1 mutations are originating from Mediterranean countries, the remaining families (30%) being originating from various countries across the world. Here, we report, the identification of the same homozygous RETGC1 nonsense mutation in three unrelated and non-consanguineous LCA families of Finnish origin, suggesting a founder effect. Interestingly, no linkage desequilibrium was found using polymorphic markers flanking the RETGC1 gene, supporting the view that the mutation is very ancient. Haplotype studies and Bayesian calculation point the founder mutation to 150 generations (95% credible interval 80-240 generations), i.e., 3000 years ago.

No genetic differences between affected and unaffected members of a German family with Leber's hereditary optic neuropathy (LHON) with respect to ten mtDNA point mutations associated with LHON

In order to investigate possible synergistic influences of different mtDNA mutations on penetrance and severity of Leber's hereditary optic neuropathy (LHON), a large German LHON pedigree is characterized with respect to 10 different mutations associated with LHON. All members of the family carry three different mtDNA mutations (at nucleotide 4,216, 11,778 and 13,708) in a homoplasmic form, regardless of whether or not they are clinically affected. Testing for another 7 mutations reveals negative results in all family members. Hence, the variable disease expression of the family members cannot be explained by varying combinations of LHON-associated mtDNA mutations.

Type III 3-methylglutaconic aciduria (optic atrophy plus syndrome, or Costeff optic atrophy syndrome): identification of the OPA3 gene and its founder mutation in Iraqi Jews

Type III 3-methylglutaconic aciduria (MGA) (MIM 258501) is a neuro-ophthalmologic syndrome that consists of early-onset bilateral optic atrophy and later-onset spasticity, extrapyramidal dysfunction, and cognitive deficit. Urinary excretion of 3-methylglutaconic acid and of 3-methylglutaric acid is increased. The disorder has been reported in approximately 40 patients of Iraqi Jewish origin, allowing the mapping of the disease to chromosome 19q13.2-q13.3, by linkage analysis. To isolate the causative gene, OPA3, we sequenced four genes within the critical interval and identified, in the intronic sequence of a gene corresponding to cDNA clone FLJ22187, a point mutation that segregated with the type III MGA phenotype. The FLJ22187-cDNA clone, which we identified as the OPA3 gene, consists of two exons and encodes a peptide of 179 amino acid residues. Northern blot analysis revealed a primary transcript of approximately 5.0 kb that was ubiquitously expressed, most prominently in skeletal muscle and kidney. Within the brain, the cerebral cortex, the medulla, the cerebellum, and the frontal lobe, compared to other parts of the brain, had slightly increased expression. The intronic G-->C mutation abolished mRNA expression in fibroblasts from affected patients and was detected in 8 of 85 anonymous Israeli individuals of Iraqi Jewish origin. Milder mutations in OPA3 should be sought in patients with optic atrophy with later onset, even in the absence of additional neurological abnormalities.

The other human genome

In the past 13 years, a new chapter of human genetics, "mitochondrial genetics", has opened up and is becoming increasingly important in differential diagnosis. Although the clinical manifestations of disorders related to mitochondrial DNA (mtDNA) are extremely variable, recent advances in genetic testing aid in the identification of patients. Muscle morphology can give important clues for diagnosis, but histological features alone cannot define a specific disorder. Biochemical analysis may reveal a single enzyme defect, or when multiple activities are affected, suggest an mtDNA mutation. However, definitive diagnosis often requires DNA analysis and documentation of a specific mtDNA abnormality. Disorders associated with mtDNA mutations are associated with a wide variety of syndromes, and owing to the properties and characteristics of mtDNA, these are often transmitted by maternal inheritance. Although therapy for mitochondrial diseases is limited, identification of the molecular defect is important for genetic counseling.

Leber hereditary optic neuropathy, progressive visual loss, and multiple-sclerosis-like symptoms

PURPOSE

To report a case of Leber hereditary optic neuropathy with multiple-sclerosis-like symptoms.

METHODS

Observational case report. A 34-year-old man was found to have Leber hereditary optic neuropathy and a mutation at position 11778 of the mitochondrial genome. The progression of vision loss and onset of weakness in the right leg warranted neuroimaging.

RESULTS

Magnetic resonance imaging documented multiple lesions in the brain and spinal cord.

CONCLUSION

Although rarely reported, progression of optic neuropathy over months has been previously documented in Leber hereditary optic neuropathy. The emergence of multiple sclerosis-like symptoms and signs in our patient may be part of the spectrum of Leber hereditary optic neuropathy or a coincidental occurrence.

New views on RPE65 deficiency: the rod system is the source of vision in a mouse model of Leber congenital amaurosis

Leber congenital amaurosis (LCA) is the most serious form of the autosomal recessive childhood-onset retinal dystrophies. Mutations in the gene encoding RPE65, a protein vital for regeneration of the visual pigment rhodopsin in the retinal pigment epithelium, account for 10-15% of LCA cases. Whereas previous studies of RPE65 deficiency in both animal models and patients attributed remaining visual function to cones, we show here that light-evoked retinal responses in fact originate from rods. For this purpose, we selectively impaired either rod or cone function in Rpe65-/- mice by generating double- mutant mice with models of pure cone function (rhodopsin-deficient mice; Rho-/-) and pure rod function (cyclic nucleotide-gated channel alpha3-deficient mice; Cnga3-/-). The electroretinograms (ERGs) of Rpe65-/- and Rpe65-/-Cnga3-/- mice were almost identical, whereas there was no assessable response in Rpe65-/-Rho-/- mice. Thus, we conclude that the rod system is the source of vision in RPE65 deficiency. Furthermore, we found that lack of RPE65 enables rods to mimic cone function by responding under normally cone-isolating lighting conditions. We propose as a mechanism decreased rod sensitivity due to a reduction in rhodopsin content to less than 1%. In general, the dissection of pathophysiological processes in animal models through the introduction of additional, selective mutations is a promising concept in functional genetics.

Hereditary optic neuropathies

The hereditary optic neuropathies comprise a group of disorders in which the cause of optic nerve dysfunction seems to be heritable as demonstrated or suggested by familial expression or genetic analysis. Clinical variability, both within and among families with the same disease, often makes recognition and classification difficult. Genetic analysis now permits diagnosis of some of the hereditary optic neuropathies in the absence of family history or in the setting of unusual clinical presentations. The hereditary optic neuropathies are classified into three major groups: those that occur primarily without associated neurologic or systemic signs, those that frequently have associated neurologic or systemic signs, and those where the optic neuropathy is secondary in the overall disease process. This article reviewed the different hereditary optic neuropathies and emphasized the most common types.

Complete exon-intron structure of the RPGR-interacting protein (RPGRIP1) gene allows the identification of mutations underlying Leber congenital amaurosis

Leber congenital amaurosis (LCA) is a genetically heterogeneous autosomal recessive condition responsible for congenital blindness or greatly impaired vision since birth. So far, six LCA loci have been mapped but only 4 out of 6 genes have been identified. A genome-wide screen for homozygosity was conducted in seven consanguineous families unlinked to any of the six LCA loci. Evidence for homozygosity was found in two of these seven families at the 14q11 chromosomal region. Two retinal specific candidate genes were known to map to this region, namely the neural retina leucine zipper (NRL) and the retinitis pigmentosa GTPase regulator interacting protein (RPGRIP1). No mutation of the NRL gene was found in any of the two families. Thus, we determined the complete exon-intron structure of the RPGRIP1 gene. RPGRIP1 encompasses 24 coding exons, nine of which are first described here with their corresponding exon-intron boundaries. The screening of the gene in the two families consistent with linkage to chromosome 14q11 allowed the identification of a homozygous null mutation and a homozygous missense mutation, respectively. Further screening of LCA patients unlinked to any of the four already identified LCA genes (n=86) identified seven additional mutations in six of them. In total, eight distinct mutations (5 out of 8 truncating) in 8/93 patients were found. So far this gene accounts for eight out of 142 LCA cases in our series (5.6%).