X-linked ocular albinism: a family containing a manifesting heterozygote, and an affected male married to a female with autosomal recessive ocular albinism

New insights into ocular albinism type 1 (OA1): Mutations and polymorphisms of the OA1 gene

Albinism ocular type 1 (OA1) is an X-linked type of albinism that mainly effects pigment production in the eye, resulting in hypopigmentation of the retina, nystagmus, strabismus, foveal hypoplasia, abnormal crossing of the optic fibers, and reduced visual acuity. The OA1 gene is located on chromosome Xp22.32 and the coding sequence is divided into nine exons. The protein is an integral transmembrane protein that has weak similarities to G protein-coupled receptors. A total of 25 missense, two nonsense, nine frameshift, and five splicing mutations have been reported in the OA1 gene associated with OA1. There are also several deletions of some or all exons of the OA1 gene with deletions of exon 2 resulting from unequal crossing-over, due to flanking Alu repeats. Mutation and polymorphism data on this gene is available from the International Albinism Center - Albinism Database web site (http://www.cbc.umn.edu/tad).

Extracutaneous analogies of Blaschko lines

In females, early random X-chromosome inactivation in the late blastocyst and subsequent embryonic development cause a random distribution of cells with an active paternal or maternal X-chromosome. Carriers of X-linked disorders are mostly healthy but, when thoroughly examined, may display a characteristic pattern of partial involvement, which for the skin, is known to follow the lines of Blaschko. Comparable patterns of involvement have been seen in various other organs. The evaluation of carriers who are heterozygous for X-linked disorders, therefore, is an efficient method for the study of functional genetic mosaicism in humans. The same patterns can also be seen in case of early embryonic somatic mutations.

OA1 mutations and deletions in X-linked ocular albinism

X-linked ocular albinism (OA1), Nettleship-Falls type, is characterized by decreased ocular pigmentation, foveal hypoplasia, nystagmus, photodysphoria, and reduced visual acuity. Affected males usually demonstrate melanin macroglobules on skin biopsy. We now report results of deletion and mutation screening of the full-length OA1 gene in 29 unrelated North American and Australian X-linked ocular albinism (OA) probands, including five with additional, nonocular phenotypic abnormalities (Schnur et al. 1994). We detected 13 intragenic gene deletions, including 3 of exon 1, 2 of exon 2, 2 of exon 4, and 6 others, which span exons 2-8. Eight new missense mutations were identified, which cluster within exons 1, 2, 3, and 6 in conserved and/or putative transmembrane domains of the protein. There was also a splice acceptor-site mutation, a nonsense mutation, a single base deletion, and a previously reported 17-bp exon 1 deletion. All patients with nonocular phenotypic abnormalities had detectable mutations. In summary, 26 (approximately 90%) of 29 probands had detectable alterations of OA1, thus confirming that OA1 is the major locus for X-linked OA.

Ocular albinism with unilateral sectorial pigmentation in the fundus

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Genetic mapping of X linked ocular albinism: linkage analysis in British families

Genetic linkage studies were performed in 16 British families affected by X linked ocular albinism (XLOA) using RFLPs from the Xp22.3 region. Linkage was confirmed between the XLOA locus (OA1) and the loci DXS143 (dic56; Zmax = 15.90 at theta = 0.0, confidence interval (CI) 0-0.035), DXS85 (782; Zmax = 15.67 at theta = 0.04, CI = 0.007-0.11), and DXS237 (GMGX9; Zmax = 12.65 at theta = 0.08, CI = 0.03-0.17). Multipoint linkage analysis placed OA1 between DXS85 (782) and DXS237 (GMGX9) with odds exceeding 10(4):1 to give the map DXS85-(OA1,DXS143)-DXS237-XG-Xpter. OA1 lies close to DXS143 (dic56) but in the absence of recombinants the order of these loci could not be determined.

A practical approach to albino diagnosis. VEP misrouting across the age span

In addition to the genetic heterogeneity in albinism, widespread clinical heterogeneity frequently impedes albino detection and differential diagnosis. Further, several auxiliary ocular and/or cutaneous manifestations of this inherited error of pigmentary metabolism are neither pre-requisite nor specific to the albino condition. However, one feature that is specific to albinism regardless of genotype or phenotype is a unique pattern of abnormal visual pathway organization. With an appropriate test paradigm, the albino visual pathway can be revealed by the non-invasive recording of the visual evoked potential (VEP) distribution across the occiput which shows contralateral hemispheric asymmetry following full field monocular stimulation. As described in this report, the VEP albino misrouting detection test has been refined to yield extraordinarily high sensitivity and selectivity across the age span from the neonate to the elderly. As the VEP profile undergoes maturational changes, these changes have been taken into account in the development of an albino age-range VEP test recipe which includes the pattern onset paradigm for older albinos and a luminance flash paradigm for the albino infant. The age appropriate optic pathway misrouting test provides reliable albino detection and definitive differential diagnosis. Further, as the albino VEP signature of contralateral asymmetry is also age specific, the VEP misrouting test can be extended to the objective assessment of visual pathway maturation.

Genetic counselling in X-linked ocular albinism: clinical features of the carrier state

Forty-nine obligate heterozygotes and 64 at risk females were assessed from 17 families affected by X-linked ocular albinism to determine the relative values of clinical examination and skin biopsy in carrier detection. 92% of obligate heterozygotes had a mud-splattered appearance of the fundus with hyperpigmented streaks and in 74% this was associated with marked iris translucency. Skin histology showed macromelanosomes in 84%. 27 of 64 (42%) at risk females had definite features of the carrier state and 19 (30%) were unequivocally normal but a further 18 (28%) had mild RPE abnormalities, often with iris transillumination defects, of uncertain significance. Fundus examination will detect most carrier females but will not allow reliable genetic counselling for all at risk females, even when supplemented by skin biopsy.

Congenital nystagmus in a (46,XX/45,X) mosaic woman from a family with X-linked congenital nystagmus

X-linked congenital nystagmus is a rare disorder in which affected males manifest binocular uniplanar nystagmus with associated head oscillation. In the families previously reported, affected females have been described. We report on a multigeneration family with X-linked congenital nystagmus with an affected woman. She was a (46,XX/45,X) mosaic. Magnetic resonance images of the brain of affected individuals were normal.

Linkage analysis in X-linked ocular albinism

We studied the linkage of X-linked Nettleship-Falls ocular albinism (OA1) to Xp22.1-Xp22.3 RFLPs at 12 loci in five families, including one in which OA1 cosegregates with a deletion of steroid sulfatase (STS). We found evidence for tight linkage of OA1 to the Xp22.3 loci DXS143, STS, and DXS452. DXS452, a newly described polymorphism detected by the probe E25B1.8, is part of the sequence family "DXS278" (pCRI-S232), but represents a single genetic locus. Every female in this study was heterozygous for the DXS452 RFLP. Thus, this marker will be extremely useful for family studies and genetic counseling. Analysis of individual recombinations suggests that OA1 maps between DXS143 and DXS85. Multipoint linkage analysis was consistent with this localization but was not statistically significant. These data suggest that OA1 lies proximal to the deletion in a previously described family with OA1 and STS deletion, but maps within the Xp22.3-Xp22.2 region.

X-linked ocular albinism. Characteristic pattern of affection in female carriers

The authors report on a family with X-linked ocular albinism. Examined were one of three affected males and eight females. Six women were carriers and showed iris retroillumination and typical abnormalities of the fundus. The patchy or striated hypopigmentation of the retinal pigment epithelium is thought to be due to random inactivation of the paternal or maternal X-chromosome respectively. The pattern of affection however represents a nonrandom embryological developmental pattern of the retinal pigment epithelium.

Hypoplastic corpus callosum in ocular albinism: indication of a global disturbance of neuronal migration

Ocular albinism is distinguished from the more common oculocutaneous albinism by the presence of normal pigmentation of skin and hair in the former condition. Recent studies of ocular albinism have shown that the hypopigmentation of the optic fundus is associated with a number of anomalies of neuronal wiring involving the visual system. We present a patient with ocular albinism who also has a hypoplastic corpus callosum as determined by analysis of midsagittal magnetic resonance imaging scans. Previous studies of the hypoplastic corpus callosum indicate that this anomaly is a defect in neuronal migration as well. The finding of a hypoplastic corpus callosum in a patient with ocular albinism suggests a more generalized defect in neuronal migration not limited to the visual system.

Albinism in childhood: a flash VEP and ERG study

Flash visual evoked potentials (F. VEPs) and electroretinograms (ERGs) were recorded in a total of 20 young children with albinism (age range 5 months to 11 years, mean 4 years). All recordings were made without sedation. There were 13 oculocutaneous cases (one with Hermansky-Pudlak syndrome) and seven ocular albinos. Monocular flash stimulation commonly elicited an asymmetrical occipital VEP distribution with a well lateralised component at around 80 ms which was of opposite polarity in a comparison of VEPs from each eye. None of the normally pigmented matched controls or obligate female carriers showed this anomalous distribution. The albino electroretinogram, compared with controls, recorded under fully darkened conditions had a significantly larger a wave and significantly shorter latencies for both a and b waves. The accentuated ERG and asymmetrical VEP recorded in infants and young children with albinism permits distinction of these patients from those with congenital cone dysfunction and idiopathic nystagmus, with whom they may be confused by a clinical examination only.

The recognition and management of albinism

Albinism is not a single entity but represents a heterogenous group of inherited disorders of pigmentation. Despite a wide variety of manifestations, all forms of albinism are characterized by several ocular features, including: nystagmus, photophobia, reduced visual acuity and a lack of stereopsis. It is the intention of this review to describe and discuss the clinical implications of albinism with particular emphasis placed on these ocular features and their effects on visual performance. Various techniques that may be employed to aid the albino will also be described.

Retinal vascular and optic nerve abnormalities in albinism

Albinism is a condition in which the pigment melanin may not be synthesized from tyrosine. The fundus photographs of 24 eyes of 12 patients with the clinical diagnosis of either ocular or oculocutaneous albinism were reviewed. We observed two findings that to our knowledge have not been reported previously. Eighteen eyes of ten patients had prominent retinal vessels coursing through the putative macular area instead of arching around it. Large noncilioretinal vessels extended from the disc to the dysplastic "foveal zone" in eight eyes of five patients. Typical optic nerve hypoplasia (ONH) was seen in six eyes of four patients; features suggestive of ONH were present in an additional ten eyes of six patients. The cross-over (sharing) of clinical features between albinism and aniridia is striking and raises questions about pathophysiologic mechanisms.

Albinism

Albinism is the term applied to a heterogeneous group of genetically determined disorders characterized by hypopigmentation and affecting the eyes. After describing the clinical features of albinism in general, the authors discuss the various forms of oculocutaneous albinism, ocular albinism, and albinoidism that are of interest to the ophthalmologist. Emphasis is placed on the ocular features of each form. The visual pathway abnormalities and the clinical management of albinism are discussed.