Molecular Vision 2007; 13:1851-1855 <http://www.molvis.org/molvis/v13/a206/> Received 22 March 2007 | Accepted 27 September 2007 | Published 2 October 2007

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Mutation analysis in a family with oculocutaneous albinism manifesting in the same generation of three branches

Markus N. Preising,^1,6 Hedwig Forster,¹ H. Tan,² Birgit Lorenz,^1,6 Paulus T. V. M. de Jong,^2,3,4 Astrid S. Plomp^3,5
 
 

¹Regensburg University Medical Center, Department of Pediatric Ophthalmology, Strabismology and Ophthalmogenetics, Franz Josef Strauss, Regensburg, Germany; ²Department of Ophthalmology, Academic Medical Center, Amsterdam; ³Netherlands Institute for Neuroscience, KNAW, Department of Ophthalmogenetics, Meibergdreef, Amsterdam; ⁴Department of Epidemiology and Biostatistics, Erasmus Medical Center, Rotterdam; ⁵Department of Clinical Genetics, Academic Medical Center, Amsterdam, The Netherlands, ⁶Department of Ophthalmology, Universitaetsklinikum Giessen and Marburg GmbH, Giessen Campus, Giessen, Germany

Correspondence to: Markus Preising, Department of Ophthalmology, Universitaetsklinikum Giessen and Marburg GmbH Giessen Campus, Friedrichstrasse 18, 35385 Giessen, Germany, Phone: +49-641-99-43801; FAX: +49-641-43609; email: mpreising@arcor.de

Abstract

Purpose: To elucidate the molecular basis of oculocutaneous albinism with variable expressivity in a family from The Netherlands in which no consanguinity was reported.

Methods: Three affected family members were screened for mutations in tyrosinase (TYR) and the pink-eye-dilution gene (P) by using SSCP. The melanocortin receptor gene (MC1R) and amplimers of P showing an aberrant banding pattern in SSCP were analyzed by direct sequencing. All participants underwent ophthalmologic examination including funduscopy, and visually evoked potentials were recorded in two cases.

Results: The pedigree had three branches A, B, and C. We identified three mutations in P (V443I, N476S, C793F) that cause a compound heterozygous situation in cases from branch A (N476S/C793F) and B (V443I/C793F), who showed oculocutaneous albinism. Hair and skin color followed the light Nordic complexion that was also present in other affected and unaffected members of this family. Descendants of branches A and B showed light complexion with iris translucency and peripheral fundus hypopigmentation independent from the genotype identified. A single descendant had red hair, carrying a well known compound MC1R mutation combination for red hair color and a single heterozygous P mutation.

Conclusions: P mutations underlie oculocutaneous albinism in this family. Two known mutations in MC1R caused red hair color in one family member. No modifier effect of MC1R on P mutations could be deduced from the results of this study.

Introduction

Pigmentation of skin, hair, iris, and ocular fundus is a concerted activity of pigment production and distribution. Pigment is produced in specialized organelles-melanosomes-recruited from the lysosomal cycle [1]. Pigment occurs in two forms: black-brown eumelanin and yellow-red pheomelanin [2]. Both forms depend on the activity of tyrosinase (TYR) which controls the major steps in pigment production. TYR is guided through the cell from the endoplasmic reticulum to the late endosomal compartments, where it is essential for the production of melanin in the developing early-stage melanosomes.

The second important protein involved is encoded by the pink-eye dilution gene (P) and is thought to control the melanosomal pH value thus regulating TYR activity and melanosomal distribution [3,4].

The melanocortin-1 receptor (MC1R) determines the relative production of eumelanin and pheomelanin in the melanosome. Reduced activity of MC1R prevents the formation of eumelanin, thus causing red hair coloring and limiting the skin's ability to tan [5,6]. MC1R mutations have been characterized as strong (D84E, R142H, R151C, R160W, D294H) and weak alleles (V60L, V92M) in causing red hair as well as involvement in blond and light hair phenotypes [7-9].

Mutations acting in an autosomal recessive mode in any of these genes have been associated with phenotypes of reduced pigmentation. Oculocutaneous albinism (OCA) is a complete lack of pigmentation of any kind. This disorder presents with hypopigmented skin, white hair, and various ocular anomalies such as nystagmus, reduced visual acuity, macular hypoplasia, fundus hypopigmentation, and misrouting of the optic nerve fibers. This phenotype is usually classified as OCA1A1 [10]. A milder phenotype is present in patients carrying mutations in P. Patients may have red hair during the first weeks of life then develop a light blond hair color later on. This phenotype is usually classified as OCA2 [11]. Depending on the mutation type, the phenotype varies for both genes from OCA to autosomal recessive ocular albinism (AROA; OA3) characterized by macular hypoplasia and fundus hypopigmentation only. Recently, persistent red colored hair was reported in patients carrying mutations in P causing OCA modified by mutations in MC1R to cause red hair color [12]. MC1R is a highly polymorphous gene with variable influence of its various mutants on skin, hair, and even iris color [13].

In this study we report on the identification of P and MC1R mutations and exclusion of TYR mutations in two branches of a family with OCA, spanning three generations, in which no contribution of MC1R to the albinism phenotype was found.

Methods

Patients and controls

We investigated a Dutch family consisting of 21 descendants from a non-consangiuneous couple. The couple had seven children (three male, two female). The male children established three branches with three affected children (two female, one male) and an unaffected female in the first branch, an affected female and an unaffected male in the second branch, and an affected male and an unaffected female in the third branch. The affected male from the first branch had a pair of children and his affected sister two male children. The affected female in the third branch also had a pair of children. (Figure 1) Two of the five affected family members had already died (one female in branch A and a male in branch C) and the members of branch C did not cooperate. We investigated the living three affected family members from the third generation and further 11 unaffected members of the family as indicated in Figure 1. All participants in this study were informed about its objectives, and each gave informed consent according to the tenets of the declaration of Helsinki. The study was approved by the Ethics Committee of the University of Regensburg. DNA samples of 50 unaffected German individuals were included as controls for mutation analysis.

Clinical examination

Ophthalmic examination was performed on three OCA cases who presented with nystagmus, macular hypoplasia, hypopigmented fundus, reduced visual acuity, and iris translucency. We also conducted exams on seven unaffected members: six descendants from affected members from branches A and B and the father of case III:8 (Figure 1 indicated by arrows). Examinations included indirect ophthalmoscopy of the peripheral fundus, best-corrected visual acuity, fundus biomicroscopy with a 90D lens, and slit-lamp examination for checking abnormal iris translucency. In this latter exam, we looked for a generalized translucency with at least part of the lens equator visible. In two cases where we suspected OCA, we recorded visually evoked potentials according to the International Society for Clinical Electrophysiology of Vision (ISCEV) standard [14] to check if misrouting of the optic nerve fibers, pathognomonic in all forms of albinism, was present [15,16]. Medical records were available for case III:10, who was deceased.

Molecular genetic analysis

Peripheral blood was taken (Figure 1 indicated by DNA-IDs) from the three clinically affected participants as well as from 11 relatives who a regular macular reflex, normal visual acuity, and macular pigmentation. DNA was extracted by standard procedures using osmotic disruption of erythrocytes, protease digest of nucleated cells and erythrocyte membrane proteins followed by salting out of protein remnants and finally ethanol precipitation of DNA [17]. TYR, P, and MC1R were amplified in 34 amplimers using primers we designed (see Table 1) and those created by Lee et al. [18]. Amplimers were screened for mutations by single strand conformation polymorphism analysis in TYR and P. Amplimers of MC1R as well as amplimers of P that showed aberrant banding patterns were directly sequenced using the fluorescent dye termination method. Sequencing results were compared to standardized sequence listing of TYR, P, and MC1R (GenBank AY225228). V443I and C793F mutations within P^V443I, P^C793F were confirmed by PsyI, and BsgI restriction endonuclease assay, respectively.

Results

A summary of the available clinical data is given in Table 2. Case III:10 (Figure 1) was said to have had albinism. He had markedly reduced vision, but his hair and skin were less affected than those of his cousins. No further information was available.

Mutation analysis was first performed in III:3 and III:8, who were from the first and second branch of the family and later confirmed in other family members (Figure 1). Mutations in TYR were excluded. Case III:3 instead carried two novel compound heterozygous missense mutations in P (N476S, C793F) which was also found in his affected sister (III:6).

P^C793F could be followed by BsgI restriction endonuclease assay to segregate through the family to an affected paternal cousin III:8 (Figure 1). In addition III:8 carried another well known mutation in P (V443I) that she inherited from her mother II:7. All three mutations did not appear in 100 alleles from control samples. In the fourth generation, mutations of P could be shown in the single heterozygous state only.

Direct sequencing of MC1R in this family revealed four missense mutations (V60L, G104S, R151C, R160W). The distribution of the MC1R alleles is given in Table 2 and Figure 1. The mutations present in the family were investigated in the most recent generation. As would be assumed from his red hair color and his skin's limited ability to tan we were able to identify compound heterozygous MC1R mutations in case IV:5 (R151C, R160W).

Discussion

In this study we identified P mutations in a family with OCA and variable expressivity in five cases. The family had three branches with cases occurring in the same generation in all branches (Figure 1). Usually, such a situation can be observed in consanguineous families. Since the family was unaware of any consanguinity, monogenic inheritance seemed questionable; all spouses would have to be heterozygous carriers for a disease causing mutation, which is a rare condition. Family members from the fourth generation of the first branch (IV.1-4) consistently showed translucency of the irides, a history of easy burning when exposed to the sun, but no macular pathology.

These phenotypes made us consider the usual OCA genes, TYR, and P, as well as MC1R since King et al. reported on red hair, light tanning, and ocular signs of OCA due to mutations in P on various MC1R background in all eight cases investigated in his study [12]. No information was provided on the genotype of the carrier relatives in that publication.

We could assess the first and second branch of the family, and we conclusively showed P mutations (N476S/C793F/V443I) to cause OCA in these branches (see Figure 1). N476S and C793F mutations are reported here for the first time. The corresponding amino acid positions in P-protein are conserved throughout phylogeny from pig by mouse, rat, and rabbit to fish (Astyanax mexicanus). Cytosine loss is usually deleterious to tertiary and quartenary structures. The conversion of asparagine to serine removes an ion-binding site and introduces an aminoacid of lower hydrophilicity, which should affect the tertiary structure of the protein.

The mutations identified in MC1R were conclusive for the red hair phenotype in case IV:5 and have been reported previously by King et al. (MC1R^R151C, MC1R^R160W) [12]. Like his sister (IV:6) he did not show any signs of albinism. IV.6 and other unaffected relatives (II:7 and III:9) carried different other mutations in MC1R (V60L and G104S) in a compound heterozygous state but did not have red hair. The MC1R^G104S mutation is unlikely to contribute to the red hair phenotype since none of the persons carrying this mutation, together with a second MC1R mutation, could be shown to express the red hair phenotype. MC1R^G104S was reported in the population from The Netherlands at a frequency <0.5% without further associations with functional deficits [19]. All affected parents (III:3, III:6, and III:8) in this study showed P gene mutations that were sufficient to explain the phenotype of OCA. An influence of MC1R on the expression of OCA could not be deduced from the clinical data although these cases were at least heterozygous for MC1R^R160W. Our consideration of combinations of MC1R mutations (G104S/R160W) as a modifier of P mutations (P^C793F/P^V443I) in III:8 was inconclusive. In addition III:6, who did not have the MC1R^G104S allele, was similarly affected as III:8. Therefore, an influence of MC1R^G104S on the albino phenotype cannot be deduced.

Finally, a variation in MC1R (V60L) was present in the compound heterozygous state with other MC1R mutations in three unaffected individuals (II:7, III:9, IV:6; Table 2). MC1R^V60L has been reported to be a minor allele in red hair color [9]. Its influence cannot be assessed sufficiently in this family.

MC1R is a highly polymorphous gene with variable influence of its various mutations on skin, hair, and even iris color [13]. This may explain the iris translucency in the descendants of cases in branch 1 (IV:1, IV:2, and IV:4) who inherited a single heterozygous MC1R^R160W mutation from their parents III:3 and III:6 against a single heterozygous P background. However, since IV:3 did not carry any MC1R mutation, this reasoning seems invalid; the iris translucency has to be attributed to either mutations in other genes or the general Nordic phenotype of blond hair and sun sensitivity in these individuals, in combination with a mutation in the P gene.

In conclusion we examined an exceptional family with OCA in three branches. Although monogenic inheritance seemed unlikely in this family, we showed autosomal recessive inheritance in two branches by finding compound heterozygosity for mutations in the P gene that were sufficient to cause the affectation. We also identified several MC1R variations in the compound and single heterozygous state and on a compound and single heterozygous P background. We did not find a modifying effect on the OCA phenotype, but we independently identified some MC1R mutations as the cause of the red hair phenotype in one person.

Acknowledgements

We thank the family for participating in the study, and Ulrike Brauer and Renate Foeckler for excellent technical assistance.

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