A locus for autosomal dominant colobomatous microphthalmia maps to chromosome 15q12-q15

Novel eye genes systematically discovered through an integrated analysis of mouse transcriptomes and phenome

In the last few decades, reverse genetic and high throughput approaches have been frequently applied to the mouse (Mus musculus) to understand how genes function in tissues/organs and during development in a mammalian system. Despite these efforts, the associated phenotypes for the majority of mouse genes remained to be fully characterized. Here, we performed an integrated transcriptome-phenome analysis by identifying coexpressed gene modules based on tissue transcriptomes profiled with each of various platforms and functionally interpreting these modules using the mouse phenotypic data. Consequently, >15,000 mouse genes were linked with at least one of the 47 tissue functions that were examined. Specifically, our approach predicted >50 genes previously unknown to be involved in mice (Mus musculus) visual functions. Fifteen genes were selected for further analysis based on their potential biomedical relevance and compatibility with further experimental validation. Gene-specific morpholinos were introduced into zebrafish (Danio rerio) to target their corresponding orthologs. Quantitative assessments of phenotypes of developing eyes confirmed predicted eye-related functions of 13 out of the 15 genes examined. These novel eye genes include: Adal, Ankrd33, Car14, Ccdc126, Dhx32, Dkk3, Fam169a, Grifin, Kcnj14, Lrit2, Ppef2, Ppm1n, and Wdr17. The results highlighted the potential for this phenome-based approach to assist the experimental design of mutating and phenotyping mouse genes that aims to fully reveal the functional landscape of mammalian genomes.

Unilateral Autosomal Recessive Anophthalmia in a Patient with Cystic Craniopharyngioma

Anophthalmia is a rare ocular malformation. It is a genetically determined disorder and is typically associated with syndromes. However, sporadic nonsyndromic familial as well as non-familial cases of anophthalmia have also been reported. Non-syndromic familial cases are usually bilateral and have been attributed to autosomal recessive, autosomal dominant, and X-linked inheritance patterns. The authors hereby report a rare case of autosomal recessive unilateral anophthalmia in a patient with no other associated congenital anomaly. Patient was operated for craniopharyngioma. The clinical, radiological and intraoperative findings are discussed.

To investigate the necessity of STRA6 upregulation in T cells during T cell immune responses

Our earlier study revealed that STRA6 (stimulated by retinoic acid gene 6) was up-regulated within 3 h of TCR stimulation. STRA6 is the high-affinity receptor for plasma retinol-binding protein (RBP) and mediates cellular vitamin A uptake. We generated STRA6 knockout (KO) mice to assess whether such up-regulation was critical for T-cell activation, differentiation and function. STRA6 KO mice under vitamin A sufficient conditions were fertile without apparent anomalies upon visual inspection. The size, cellularity and lymphocyte subpopulations of STRA6 KO thymus and spleen were comparable to those of their wild type (WT) controls. KO and WT T cells were similar in terms of TCR-stimulated proliferation in vitro and homeostatic expansion in vivo. Naive KO CD4 cells differentiated in vitro into Th1, Th2, Th17 as well as regulatory T cells in an analogous manner as their WT counterparts. In vivo experiments revealed that anti-viral immune responses to lymphocytic choriomeningitis virus in KO mice were comparable to those of WT controls. We also demonstrated that STRA6 KO and WT mice had similar glucose tolerance. Total vitamin A levels are dramatically lower in the eyes of KO mice as compared to those of WT mice, but the levels in other organs were not significantly affected after STRA6 deletion under vitamin A sufficient conditions, indicating that the eye is the mouse organ most sensitive to the loss of STRA6.

Our results demonstrate that 1) in vitamin A sufficiency, the deletion of STRA6 in T cells does no affect the T-cell immune responses so-far tested, including those depend on STAT5 signaling; 2) STRA6-independent vitamin A uptake compensated the lack of STRA6 in lymphoid organs under vitamin A sufficient conditions in mice; 3) STRA6 is critical for vitamin A uptake in the eyes even in vitamin A sufficiency.

First implication of STRA6 mutations in isolated anophthalmia, microphthalmia, and coloboma: a new dimension to the STRA6 phenotype

Microphthalmia, anophthalmia, and coloboma (MAC) are structural congenital eye malformations that cause a significant proportion of childhood visual impairments. Several disease genes have been identified but do not account for all MAC cases, suggesting that additional risk loci exist. We used single nucleotide polymorphism (SNP) homozygosity mapping (HM) and targeted next-generation sequencing to identify the causative mutation for autosomal recessive isolated colobomatous microanophthalmia (MCOPCB) in a consanguineous Irish Traveller family. We identified a double-nucleotide polymorphism (g.1157G>A and g.1156G>A; p.G304K) in STRA6 that was homozygous in all of the MCOPCB patients. The STRA6 p.G304K mutation was subsequently detected in additional MCOPCB patients, including one individual with Matthew-Wood syndrome (MWS; MCOPS9). STRA6 encodes a transmembrane receptor involved in vitamin A uptake, a process essential to eye development and growth. We have shown that the G304K mutant STRA6 protein is mislocalized and has severely reduced vitamin A uptake activity. Furthermore, we reproduced the MCOPCB phenotype in a zebrafish disease model by inhibiting retinoic acid (RA) synthesis, suggesting that diminished RA levels account for the eye malformations in STRA6 p.G304K patients. The current study demonstrates that STRA6 mutations can cause isolated eye malformations in addition to the congenital anomalies observed in MWS.

Unilateral Isolated Microphthalmia Inherited as an Autosomal Recessive Trait

PURPOSE

To report a family with unilateral isolated microphthalmia showing an autosomal recessive pattern of inheritance.

CASE REPORT

We report a family in which three out of four children, one male and monozygotic female twins, were born with unilateral isolated microphthalmia to healthy consanguineous parents. One twin additionally had a horseshoe kidney. Rare cases of familial isolated microphthalmia/anophthalmia have been previously described. This is the first report of a family with autosomal recessive isolated microphthalmia occurring unilaterally in all affected individuals. It remains unknown how this inherited genetic disease results in unilateral manifestation.

CONCLUSION

Mirror imaging of this condition in the monozygotic twins may help elucidate the underlying mechanism. The constellation of features in this family may contribute to solve remaining questions of research into symmetry and asymmetry.

A genome-wide linkage scan in Tunisian families identifies a novel locus for non-syndromic posterior microphthalmia to chromosome 2q37.1

Posterior microphthalmia (PM) is a relatively rare autosomal recessive condition with normal anterior segment and small posterior segment resulting in high hyperopia and retinal folding. It is an uncommon subtype of microphthalmia that has been mostly reported to coexist with several other ophthalmic conditions and to occur in sporadic cases. The membrane-type frizzled-related protein (MFRP) is the only gene so far reported implicated in autosomal recessive, non-syndromic and syndromic forms of PM. Here, we performed a clinical and genetic analysis using six consanguineous families ascertained from different regions of Tunisia and affected with non-syndromic PM that segregates as an autosomal recessive trait. To identify the disease-causing defect in these families, we first analysed MFRP gene, then some candidate genes (CHX10, OPA1, MITF, SOX2, CRYBB1-3 and CRYBA4) and loci (MCOP1, NNO1 and NNO2) previously implicated in different forms of microphthalmia. After exclusion of these genes and loci, we performed a genome-wide scan using a high density single nucleotide polymorphism (SNP) array 50 K in a large consanguineous pedigree. SNP genotyping revealed eight homozygous candidate regions on chromosomes 1, 2, 3, 6, 15, 17 and 21. Linkage analysis with additional microsatellite markers only retained the 2q37.1 region with a maximum LOD score of 8.85 obtained for D2S2344 at theta = 0.00. Further investigations are compatible for linkage of four more families to this region with a refined critical interval of 2.35 Mb. The screening of five candidate genes SAG, PDE6D, CHRND, CHRNG and IRK13 did not reveal any disease-causing mutation.

Anophthalmia and microphthalmia

Anophthalmia and microphthalmia describe, respectively, the absence of an eye and the presence of a small eye within the orbit. The combined birth prevalence of these conditions is up to 30 per 100,000 population, with microphthalmia reported in up to 11% of blind children. High-resolution cranial imaging, post-mortem examination and genetic studies suggest that these conditions represent a phenotypic continuum. Both anophthalmia and microphthalmia may occur in isolation or as part of a syndrome, as in one-third of cases. Anophthalmia/microphthalmia have complex aetiology with chromosomal, monogenic and environmental causes identified. Chromosomal duplications, deletions and translocations are implicated. Of monogenic causes only SOX2 has been identified as a major causative gene. Other linked genes include PAX6, OTX2, CHX10 and RAX. SOX2 and PAX6 mutations may act through causing lens induction failure. FOXE3 mutations, associated with lens agenesis, have been observed in a few microphthalmic patients. OTX2, CHX10 and RAX have retinal expression and may result in anophthalmia/microphthalmia through failure of retinal differentiation. Environmental factors also play a contributory role. The strongest evidence appears to be with gestational-acquired infections, but may also include maternal vitamin A deficiency, exposure to X-rays, solvent misuse and thalidomide exposure. Diagnosis can be made pre- and post-natally using a combination of clinical features, imaging (ultrasonography and CT/MR scanning) and genetic analysis. Genetic counselling can be challenging due to the extensive range of genes responsible and wide variation in phenotypic expression. Appropriate counselling is indicated if the mode of inheritance can be identified. Differential diagnoses include cryptophthalmos, cyclopia and synophthalmia, and congenital cystic eye. Patients are often managed within multi-disciplinary teams consisting of ophthalmologists, paediatricians and/or clinical geneticists, especially for syndromic cases. Treatment is directed towards maximising existing vision and improving cosmesis through simultaneous stimulation of both soft tissue and bony orbital growth. Mild to moderate microphthalmia is managed conservatively with conformers. Severe microphthalmia and anophthalmia rely upon additional remodelling strategies of endo-orbital volume replacement (with implants, expanders and dermis-fat grafts) and soft tissue reconstruction. The potential for visual development in microphthalmic patients is dependent upon retinal development and other ocular characteristics.

Localization of a novel gene for congenital nonsyndromic simple microphthalmia to chromosome 2q11-14

Microphthalmia is a clinically and genetically heterogeneous disorder of eye development. The genetic basis of nonsyndromic microphthalmia is not yet fully understood. Previous studies indicated that disease pedigrees from different genetic backgrounds could be attributed to completely different gene loci. To investigate the etiology in a large autosomal-dominant inherited simple microphthalmia (nanophthalmia) pedigree, which is the first genetically analyzed Chinese microphthalmia pedigree, we performed a whole-genome scan using 382 micro-satellite DNA markers after the exclusion of reported candidates associated with microphthalmia. Strong evidence indicated that microphthalmia in this family was mapped to an unreported new locus on chromosome 2q. A significantly positive two-point LOD score was obtained with a maximum 3.290 at a recombination fraction of 0.00 for marker D2S2265. Subsequent haplotype analysis and recombination data further confined the disease-causing gene to a 15-cM interval between D2S1890 and D2S347 on 2q11-14. Our results further underlined the degree of heterogeneity in microphthalmia from Chinese background and localized a novel gene which regulates eye embryogenesis.

GDF6, a novel locus for a spectrum of ocular developmental anomalies

Colobomata represent visually impairing ocular closure defects that are associated with a diverse range of developmental anomalies. Characterization of a chromosome 8q21.2-q22.1 segmental deletion in a patient with chorioretinal coloboma revealed elements of nonallelic homologous recombination and nonhomologous end joining. This genomic architecture extends the range of chromosomal rearrangements associated with human disease and indicates that a broader spectrum of human chromosomal rearrangements may use coupled homologous and nonhomologous mechanisms. We also demonstrate that the segmental deletion encompasses GDF6, encoding a member of the bone-morphogenetic protein family, and that inhibition of gdf6a in a model organism accurately recapitulates the proband's phenotype. The spectrum of disorders generated by morpholino inhibition and the more severe defects (microphthalmia and anophthalmia) observed at higher doses illustrate the key role of GDF6 in ocular development. These results underscore the value of integrated clinical and molecular investigation of patients with chromosomal anomalies.

The PDAC syndrome (pulmonary hypoplasia/agenesis, diaphragmatic hernia/eventration, anophthalmia/microphthalmia, and cardiac defect) (Spear syndrome, Matthew-Wood syndrome): Report of eight cases including a living child and further evidence for autosomal recessive inheritance

The combination of pulmonary agenesis/dysgenesis/hypoplasia, microphthalmia/anophthalmia, and a diaphragmatic defect (agenesis or eventration) is a rare syndrome presumed to have an autosomal recessive mode of inheritance based on a report of affected siblings born to unaffected parents [Seller et al., 1996]. The condition is known as Spear syndrome and Matthew-Wood syndrome, although genetic heterogeneity cannot be ruled out. We report on eight patients with this condition including a living child, three sibs and three isolated cases. Most presented with fetal ultrasound findings of microphthalmia/anophthalmia, and diaphragmatic eventration/hernia and in five, cardiac abnormalities were also found. The earliest detection was at 20 weeks gestation. This is the second report of sibs affected with this condition, which supports an autosomal recessive mode of inheritance. We present the first and only reported living patient with this condition and expand the intrafamilial, interfamilial, and ethnic variability of this condition. We suggest changing the condition's name to PDAC to reflect the most important components of this condition.

Uveal coloboma: clinical and basic science update

PURPOSE OF REVIEW

To integrate knowledge on the embryologic and molecular basis of optic fissure closure with clinical observations in patients with uveal coloboma.

RECENT FINDINGS

Closure of the optic fissure has been well characterized and many genetic alterations have been associated with coloboma; however, molecular mechanisms leading to coloboma remain largely unknown. In the past decade, we have gained better understanding of genes critical to eye development; however, mutations in these genes have been found in few individuals with coloboma. CHD7 mutations have been identified in patients with CHARGE syndrome (coloboma, heart defects, choanal atresia, retarded growth, genital anomalies, and ear anomalies or deafness). Animal models are bringing us closer to a molecular understanding of optic fissure closure.

SUMMARY

Optic fissure closure requires precise orchestration in timing and apposition of two poles of the optic cup. The relative roles of genetics and environment on this process remain elusive. While most cases of coloboma are sporadic, autosomal dominant, autosomal recessive, and X-linked inheritance patterns have been described. Genetically, colobomata demonstrate pleiotropy, heterogeneity, variable expressivity, and reduced penetrance. Coloboma is a complex disorder with a variable prognosis and requires regular examination to optimize visual acuity and to monitor for potential complications.

Developmental eye disorders

In developed countries, malformations of the eye are among the most common causes of serious visual impairment in newborns. The identification of pathogenic mutations in autosomal and X-linked transcription factors has advanced our understanding of the critical stages in human eye development and has begun to explain some unusual inheritance characteristics of these disorders. The functional characterisation of these genes in model organisms has prompted reinvestigation of affected individuals to identify previously unrecognized but consistent extra-ocular malformations. This dialogue between clinical genetics and basic developmental biology provides a paradigm to enhance our understanding of many critical developmental processes in human embryogenesis.

Ocular coloboma: a reassessment in the age of molecular neuroscience

Congenital colobomata of the eye are important causes of childhood visual impairment and blindness. Ocular coloboma can be seen in isolation and in an impressive number of multisystem syndromes, where the eye phenotype is often seen in association with severe neurological or craniofacial anomalies or other systemic developmental defects. Several studies have shown that, in addition to inheritance, environmental influences may be causative factors. Through work to identify genes underlying inherited coloboma, significant inroads are being made into understanding the molecular events controlling closure of the optic fissure. In general, severity of disease can be linked to the temporal expression of the gene, but this is modified by factors such as tissue specificity of gene expression and genetic redundancy.

CHX10 mutations cause non-syndromic microphthalmia/ anophthalmia in Arab and Jewish kindreds

Microphthalmia/anophthalmia is a clinically heterogeneous disorder of eye formation, ranging from small size of a single eye to complete bilateral absence of ocular tissues. The genetic defect underlying isolated autosomal recessive microphthalmia/anophthalmia is yet unclear. We studied four families (two of Arab origin, one of Bedouin origin, and one of Persian-Jewish origin) with autosomal recessive microphthalmia/anophthalmia and no associated eye anomalies, and one Syrian-Jewish family with associated colobomas. Assuming a founder effect in each of the families, we performed homozygosity mapping using polymorphic markers adjacent to human homologues of genes known to be associated with eye absence in various species, namely EYA1, EYA2, EYA3, SIX4, SIX6, PAX6 and CHX10. No association was found with EYA1, EYA2, EYA3, SIX6 or PAX6. In two families, linkage analysis was consistent with possible association with SIX4, but no mutations were found in the coding region of the gene or its flanking intron sequences. In three of the five families, linkage analysis followed by sequencing demonstrated that affected individuals in each family were homozygous for a different CHX10 aberration: a mutation in the CVC domain and a deletion of the homeobox domain were found in two Arab families, and a mutation in the donor-acceptor site in the first intron in the Syrian-Jewish family. There was phenotypic variation between families having different mutations, but no significant phenotypic variation within each family. It has been previously shown that mutations in a particular nucleotide in CHX10 are associated with an autosomal recessive syndrome of microphthalmia/anophthalmia with iris colobomas and cataracts in two families. We now show that different mutations in other domains of the same gene underlie isolated microphthalmia/anophthalmia.

A novel GJA1 mutation causes oculodentodigital dysplasia without syndactyly

Oculodentodigital dysplasia (ODDD) is a rare autosomal dominant pleiotropic disorder, caused by mutations in the Connexin 43 gene (GJA1) [Paznekas et al. (2003): Am J Hum Genet 72:408-418], which is localized to human chromosome 6q22-q23. Here, we describe the identification of a novel heterozygous missense mutation in the GJA1 gene, (H194P) in an Italian family previously reported to be affected by isolated autosomal dominant microphthalmia [Vingolo et al. (1994): J Med Genet 31:721-725]. Careful clinical re-evaluation revealed that this family shows an atypical form of ODDD, characterized by the predominance of the ocular involvement and by the absence of hand and/or foot syndactyly. The mutation affects an amino acid residue localized in the second extracellular domain of the Cx43 protein and highly conserved across evolution. This finding confirms the highly variable phenotypic expression caused by GJA1 mutations.

Congenital Bilateral Severe Microphthalmia with Mental Retardation and Cerebral Palsy: Chromosome Aberration, 46, XY, t (2;6)(q31;q24)

Congenital bilateral anophthalmia and microphthalmia are rare conditions, with overall prevalence in one study set at 1.0 per 10,000 births. We report here a case of congenital bilateral severe microphthalmia with mental retardation and cerebral palsy. The patient was man aged 38 years with a chromosome aberration, namely a balanced translocation: 46, XY, t (2;6)(q31;q24). He had no other malformations apart from the severe microphthalmia. CT of the head showed no significant abnormal findings in the brain, but rudimentary eyeballs and external ocular muscles in the bilateral orbits. There was no family history of anophthalmia, microphthalmia, mental retardation or cerebral palsy. His mother had not used any medications or excessive alcohol during gestation. Putative genes of anophthalmia and microphthalmia reported to date include PAX6 (Glaser T et al 1994) and CHX10 (Ferda Percin E et al 2000). Further, some loci of these conditions have been reported (Graham CA et al 1991; Bessant DAR et al 1998; Morle L et al 2000: Forrester S et al 2001: Ng D et al 2002). To our knowledge, however, this is the first report of nonsyndromic microphthalmia or anophthalmia with chromosome 2q31 or 6q24 aberration. We consider that the putative gene may be located on the brake points of chromosome 2 and 6.

Recent advances in molecular genetics of glaucoma

Glaucoma represents a heterogeneous group of optic neuropathies, with different genetic bases. It can affect all ages generally with a rise in intra-ocular pressure. Three major types of glaucoma have been reported: primary open angle glaucoma (POAG), primary acute closed angle glaucoma (PACG) and primary congenital glaucoma (PCG), as well as a few others associated with developmental abnormalities. In recent years impressive progress has been made in the molecular genetic studies of POAG and PCG. These include the discovery of three genes--Myocilin, Optineurin and CYP1B1--defects in which results in Mendelian transmission of glaucoma. Identification of single nucleotide polymorphisms in multiple other genes that are associated with glaucoma and alteration of drug sensitivity are enriching our knowledge regarding the complex nature of the disease. This review attempts to present the recent progress made in the molecular genetics of glaucoma.

Physical and transcript map of the autosomal dominant colobomatous microphthalmia locus on chromosome 15q12-q15 and refinement to a 4.4 Mb region

Congenital microphthalmia is a developmental disorder characterized by shortened axial length of the eye. We have previously mapped the gene responsible for autosomal dominant colobomatous microphthalmia in a 5-generation family to chromosome 15q12-q15. Here, we set up a physical and transcript map of the 13.8 cM critical region, flanked by loci D15S1002 and D15S1040. Physical mapping and genetic linkage analysis using 20 novel polymorphic markers allowed the refinement of the disease locus to two intervals in close vicinity, namely a centromeric interval, bounded by microsatellite DNA markers m3-m17, and a telomeric interval, m76-m24, encompassing respectively 1.9 and 2.5 Mb. Moreover, we excluded three candidate genes, CKTSF1B1, KLF13 and CX36. Finally, although a phenomenon of anticipation was suggested by phenotypic and pedigree data, no abnormal expansion of three trinucleotide repeats mapping to the refine interval was found in affected individuals.

Heritable unilateral clinical anophthalmia

We examined a newborn child with unilateral right-sided clinical anophthalmos born to a mother with unilateral left-sided anophthalmos. Although rare, isolated nonsyndromic heritable unilateral anophthalmia and microphthalmia have been reported in the literature. We briefly review the genetics of such anomalies and discuss the importance of a full clinical genetics evaluation. Treatment of this patient's anophthalmic socket consists of progressive conformer expansion to be followed by placement of a self-inflating polymer expander.

Ocular genetics: current understanding

Over the past decade, there has been an exponential increase in our knowledge of heritable eye conditions. Coincidentally, our ability to provide accurate genetic diagnoses has allowed appropriate counseling to patients and families. A summary of our current understanding of ocular genetics will prove useful to clinicians, researchers, and students as an introduction to the subject.

The familial contribution to non-syndromic ocular coloboma in south India

AIMS

To identify the proportion of familial cases of isolated ocular colobomatous malformations in a case series from south India.

METHODS

Children with ocular coloboma without systemic features were recruited from multiple sources in Andhra Pradesh, India. Their families were traced, pedigrees drawn, and family members examined.

RESULTS

56 probands, 25 females (44.6%) and 31 males (57.4%) with a colobomatous malformation were identified. In 12 cases (21.4%) another family member was affected. The risk to siblings was 3.8%. The parents were consanguineous in 25 cases (44.6%).

CONCLUSIONS

21.4% of cases of isolated ocular coloboma in this highly consanguineous population of south India were familial, with both autosomal dominant and autosomal recessive mechanisms likely in different families.

A clinical and molecular genetic study of a rare dominantly inherited syndrome (MRCS) comprising of microcornea, rod-cone dystrophy, cataract, and posterior staphyloma

AIM

To phenotype and genetically map the disease locus in a family presenting with autosomal dominant microcornea, rod-cone dystrophy, cataract, and posterior staphyloma.

METHODS

Six affected and three unaffected members of the pedigree were examined. All individuals provided a history and underwent a full clinical examination with A-scan and B-scan ultrasonography and electrophysiological testing where appropriate. PCR based microsatellite marker genotyping using a positional candidate gene approach was then performed on DNA samples extracted from venous blood provided by each subject.

RESULTS

The disorder is inherited as an autosomal dominant trait with variable expressivity and has a complex phenotype. Affected individuals had bilateral microcornea, pulverulent-like lens opacities, a rod-cone dystrophy and posterior staphyloma (MRCS). Using a positional candidate gene approach, the authors have evidence suggestive of linkage of this disorder to a region on 11q13 within the nanophthalmos 1 (NNO1) genetic interval. The small family size militates against achieving a LOD score of 3, but the haplotype data and the position of the putative MRCS locus within a known nanophthalmos locus are suggestive of linkage. A candidate gene within this region (ROM1) was screened and no mutations were found in affected members of the family.

CONCLUSION

This rare developmental disorder has some phenotypic similarities to nanophthalmos and possibly maps to a locus within the genetic interval encompassing the NNO1 locus. Screening of candidate genes within this region continues.

Novel mutation in sonic hedgehog in non-syndromic colobomatous microphthalmia

Ocular (uveoretinal) colobomas occur in one in 10,000 individuals and present a substantive cause of congenital poor vision. The genetic bases of most forms of uveoretinal coloboma are elusive; mutations in PAX2 are found in only a few cases of coloboma of the retina and optic nerve that occur with renal anomalies as part of the renal-coloboma syndrome (MIM#120330; #167409). From experimental data that upstream expression of sonic hedgehog (SHH) controls Pax2 expression in mice and zebrafish, and from clinical experience that colobomas are observed frequently in patients with holoprosencephaly, we hypothesized that SHH could be a candidate for non-syndromic ocular colobomas (NSOC). We identified a three-generation family in which both a proband and his mother presented with iris and uveoretinal colobomas without optic nerve involvement. A novel 24 bp deletion in the gene SHH was identified in these affected family members, and cosegregated with the phenotype. This is the first report of the association of SHH mutations and uveoretinal coloboma.

Delineation of an estimated 6.7 MB candidate interval for an anophthalmia gene at 3q26.33-q28 and description of the syndrome associated with visible chromosome deletions of this region

Anophthalmia or microphthalmia occur in approximately one in 10 children who have severe visual impairment. These eye malformations are often of unknown aetiology, but can be inherited in autosomal dominant, recessive or X-linked forms, and can also occur in association with specific chromosome abnormalities. Four children are described in the medical literature with microphthalmia or anophthalmia in association with chromosome rearrangements involving distal 3q, suggesting the presence of a micro/anophthalmia gene in this region. We have identified two further patients with micro/anophthalmia and chromosome rearrangements involving 3q26-->3q27 and identified a 6.7 MB common deleted region. Patient 1 had multiple abnormalities including bilateral anophthalmia, abnormalities of the first and second cranial nerves and partial absence of the corpus callosum. His karyotype was 46,XY,del(3)(q26.33q28). Patient 2 had right anophthalmia and left extreme microphthalmia. Her karyotype was 46,XX,del(3)(q26.33q28)t(3;7)(q28;q21.1). Both patients had intrauterine growth retardation (IUGR) and strikingly similar dysmorphic facies consisting of bossed forehead, downward-slanting palpebral fissures, grooved bridge of the nose, prominent low-set ears, small down-turned mouth and small mandible. We identified BAC clones mapping to distal 3q from the ENSEMBL and NCBI Entrez databases. These BAC clones were used as fluorescence in situ hybridisation (FISH) probes to identify the minimum deleted region common to both patients. This interval, between clones RPC11-134F2 and RPC11-132N15, was estimated to be 6.7 MB. We conclude that there is an anophthalmia locus within this interval. Candidate genes mapping to this region include Chordin and DVL3, a homologue of the Drosophila Dishevelled gene.

Genetics of population isolates

Genetic isolates, as shown empirically by the Finnish, Old Order Amish, Hutterites, Sardinian and Jewish communities among others, represent a most important and powerful tool in genetically mapping inherited disorders. The main features associated with that genetic power are the existence of multigenerational pedigrees which are mostly descended from a small number of founders a short number of generations ago, environmental and phenotypic homogeneity, restricted geographical distribution, the presence of exhaustive and detailed records correlating individuals in very well ascertained pedigrees, and inbreeding as a norm. On the other hand, the presence of a multifounder effect or admixture among divergent populations in the founder time (e.g. the Finnish and the Paisa community from Colombia) will theoretically result in increased linkage disequilibrium among adjacent loci. The present review evaluates the historical context and features of some genetic isolates with emphasis on the basic population genetic concepts of inbreeding and genetic drift, and also the state-of-the-art in mapping traits, both Mendelian and complex, on genetic isolates.

National study of microphthalmia, anophthalmia, and coloboma (MAC) in Scotland: investigation of genetic aetiology

We report an epidemiological and genetic study attempting complete ascertainment of subjects with microphthalmia, anophthalmia, and coloboma (MAC) born in Scotland during a 16 year period beginning on 1 January 1981. A total of 198 cases were confirmed giving a minimum live birth prevalence of 19 per 100 000. One hundred and twenty-two MAC cases (61.6%) from 115 different families were clinically examined and detailed pregnancy, medical, and family histories obtained. A simple, rational, and apparently robust classification of the eye phenotype was developed based on the presence or absence of a defect in closure of the optic (choroidal) fissure. A total of 85/122 (69.7%) of cases had optic fissure closure defects (OFCD), 12/122 (9.8%) had non-OFCD, and 25/122 (20.5%) had defects that were unclassifiable owing to the severity of the corneal or anterior chamber abnormality. Segregation analysis assuming single and multiple incomplete ascertainment, respectively, returned a sib recurrence risk of 6% and 10% in the whole group and 8.1% and 13.3% in the OFCD subgroup. Significant recurrence risks were found in both unilateral and bilateral disease. In four families, one parent had an OFCD, two of which were new diagnoses in asymptomatic subjects. All recurrences in first degree relatives occurred in the OFCD group with a single first cousin recurrence seen in the non-OFCD group. A total of 84/122 of the MAC cases were screened for mutations in the coding regions of PAX6, CHX10, and SIX3. No pathogenic mutations were identified in the OFCD cases. A single PAX6 homeodomain missense mutation was identified in a subject with partial aniridia that had been initially misclassified as coloboma.