Prenatal diagnosis of oculocutaneous albinism type I: review and personal experience

Oculocutaneous albinism

Oculocutaneous albinism (OCA) is a group of inherited disorders of melanin biosynthesis characterized by a generalized reduction in pigmentation of hair, skin and eyes. The prevalence of all forms of albinism varies considerably worldwide and has been estimated at approximately 1/17,000, suggesting that about 1 in 70 people carry a gene for OCA. The clinical spectrum of OCA ranges, with OCA1A being the most severe type with a complete lack of melanin production throughout life, while the milder forms OCA1B, OCA2, OCA3 and OCA4 show some pigment accumulation over time. Clinical manifestations include various degrees of congenital nystagmus, iris hypopigmentation and translucency, reduced pigmentation of the retinal pigment epithelium, foveal hypoplasia, reduced visual acuity usually (20/60 to 20/400) and refractive errors, color vision impairment and prominent photophobia. Misrouting of the optic nerves is a characteristic finding, resulting in strabismus and reduced stereoscopic vision. The degree of skin and hair hypopigmentation varies with the type of OCA. The incidence of skin cancer may be increased. All four types of OCA are inherited as autosomal recessive disorders. At least four genes are responsible for the different types of the disease (TYR, OCA2, TYRP1 and MATP). Diagnosis is based on clinical findings of hypopigmentation of the skin and hair, in addition to the characteristic ocular symptoms. Due to the clinical overlap between the OCA forms, molecular diagnosis is necessary to establish the gene defect and OCA subtype. Molecular genetic testing of TYR and OCA2 is available on a clinical basis, while, at present, analysis of TYRP1 and MATP is on research basis only. Differential diagnosis includes ocular albinism, Hermansky-Pudlak syndrome, Chediak-Higashi syndrome, Griscelli syndrome, and Waardenburg syndrome type II. Carrier detection and prenatal diagnosis are possible when the disease causing mutations have been identified in the family. Glasses (possibly bifocals) and dark glasses or photocromic lenses may offer sufficient help for reduced visual activity and photophobia. Correction of strabismus and nystagmus is necessary and sunscreens are recommended. Regular skin checks for early detection of skin cancer should be offered. Persons with OCA have normal lifespan, development, intelligence and fertility.

Rapid genetic analysis of oculocutaneous albinism (OCA1) using denaturing high performance liquid chromatography (DHPLC) system

OBJECTIVES

To present the prenatal genetic diagnoses and counseling for two cases of oculocutaneous albinism (OCA) type I family by detection of mutations in the OCA1 gene by denaturing high performance liquid chromatography (DHPLC) system and a review of the literature.

METHODS

All DNA samples were extracted from peripheral whole blood and amniocentesis-derived cells. Mutation analysis was performed for all five coding exons of the TYR gene, which were amplified by PCR. DHPLC was used for heteroduplex detection and sequence analysis was performed to demonstrate the mutation loci.

RESULTS

Case 1: After sampling of blood from the family members and performing amniocentesis of the fetus, it was demonstrated that the affected boy and the female fetus were shown to be compound heterozygotes for mutations in the TYR gene. In addition, it was shown that the parents were carriers of the two mutations. However, the couple chose to keep the baby. Case 2: Mutation analysis of the DNA of the siblings revealed two heterozygous mutations in the TYR gene. Her husband is free of the disease. According to the principles of autosomal recessive inheritance, the incidence of affected offspring is very low.

CONCLUSIONS

Herein we introduce a novel application for molecular diagnostic of DHPLC coupled with direct sequencing, which can provide an effective and exact diagnosis in patients with albinism. Clinicians should be cognizant of the risk of OCA inheritance by the offspring through careful identification of genetic mutations and the inheritance mode, both important to ensure comprehensive genetic counseling.

Establishment and characterization of pheochromocytoma tumor models expressing different levels of trkA receptors

To date experimental in vivo pheochromocytoma (PC) models have not been available. A major in vitro PC model consists of PC12 cells that respond to nerve growth factor (NGF) by differentiation, mediated by the trkA receptor. We report the establishment of PC12 tumor models expressing low and high levels of trkA receptor in CD1 nude mice. The tumors are characterized by their responsiveness to NGF, karyotype, presence of enolase, and chromaffin granules, as well as dopamine release. These novel PC models facilitate research on the role of the trkA receptor in cancer and the development of trkA-selective anti-cancer agents.

Behavioral visual responses of wild-type and hypopigmented zebrafish

Zebrafish possess three classes of chromatophores that include iridophores, melanophores, and xanthophores. Mutations that lack one or two classes of chromatophores have been isolated or genetically constructed. Using a behavioral assay based on visually mediated escape responses, we measured the visual response of fully and partially pigmented zebrafish. In zebrafish that lack iridophores (roy mutants), the behavioral visual responses were similar to those of wild-type animals except at low contrast stimulation. In the absence of melanophores (albino mutants) or both melanophores and iridophores (ruby mutants), the behavioral visual responses were normal under moderate illumination but reduced when tested under dim or bright conditions or under low contrast stimulation. Together, the data suggest that screening pigments in the retina play a role in the regulation of behavioral visual responses and are necessary for avoiding "scatter" under bright light conditions.