Hypopigmented Iris Spot

A novel mouse model of anterior segment dysgenesis (ASD): conditional deletion of Tsc1 disrupts ciliary body and iris development

Development of the cornea, lens, ciliary body and iris within the anterior segment of the eye involves coordinated interaction between cells originating from the ciliary margin of the optic cup, the overlying periocular mesenchyme and the lens epithelium. Anterior segment dysgenesis (ASD) encompasses a spectrum of developmental syndromes that affect these anterior segment tissues. ASD conditions arise as a result of dominantly inherited genetic mutations and result in both ocular-specific and systemic forms of dysgenesis that are best exemplified by aniridia and Axenfeld-Rieger syndrome, respectively. Extensive clinical overlap in disease presentation amongst ASD syndromes creates challenges for correct diagnosis and classification. The use of animal models has therefore proved to be a robust approach for unravelling this complex genotypic and phenotypic heterogeneity. However, despite these successes, it is clear that additional genes that underlie several ASD syndromes remain unidentified. Here, we report the characterisation of a novel mouse model of ASD. Conditional deletion of Tsc1 during eye development leads to a premature upregulation of mTORC1 activity within the ciliary margin, periocular mesenchyme and lens epithelium. This aberrant mTORC1 signalling within the ciliary margin in particular leads to a reduction in the number of cells that express Pax6, Bmp4 and Msx1 Sustained mTORC1 signalling also induces a decrease in ciliary margin progenitor cell proliferation and a consequent failure of ciliary body and iris development in postnatal animals. Our study therefore identifies Tsc1 as a novel candidate ASD gene. Furthermore, the Tsc1-ablated mouse model also provides a valuable resource for future studies concerning the molecular mechanisms underlying ASD and acts as a platform for evaluating therapeutic approaches for the treatment of visual disorders.

Tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is a neurocutaneous syndrome that can affect the brain, skin, eyes, kidneys, heart, and lungs. TSC alters cellular proliferation and differentiation, resulting in hamartomas of various organs, tumor formation, and altered neuronal migration. The phenotype is highly variable. Most individuals have seizures, commonly including infantile spasms, and there is variable intellectual disability and autism. Neonates can present with cardiac failure due to intracardiac rhabdomyomas. The likelihood of renal angiomyolipomas increases with age, and renal disease is the most common cause of death in adults with TSC. Pulmonary involvement occurs predominantly in women and carries a high morbidity and mortality. TSC is inherited as an autosomal dominant trait, but spontaneous mutations are common. A mutation of either TSC1 on chromosome 9 or TSC2 on chromosome 16 leads to dysfunction of hamartin or tuberin, respectively. These two proteins form a functional complex that modulates the mammalian target of rapamycin (mTOR) pathway. Medications that inhibit mTOR are being used to treat TSC-related tumors, and current studies are investigating whether these agents could alleviate other TSC complications. Consensus statements guide identification and optimal management of many of the TSC-related complications at diagnosis and throughout the lifespan. A multidisciplinary approach is necessary for optimal management of individuals with TSC.

Molecular implications of skin lesions in tuberous sclerosis

Tuberous sclerosis (TS), neurocutaneous disorder resulting from the mutation of 1 of 2 genes, TSC1 or TSC2, is often associated with the formation of hamartomatous lesions in various organ systems, including the skin. TS patients may present with hypomelanic macules, confetti-like spots, facial angiofibromas, ungual fibromas, shagreen patches, forehead plaques, and other dermatological signs. Some of these manifestations are pathognomic for TS and thus should be carefully evaluated when TS diagnosis is suspected. Little is known however on molecular links connecting disease pathogenesis and formation of such hamartomas. In the current review, we describe molecular pathways thought to be responsible for the development of the disease and show how their upregulation may affect the skin. Special attention is paid to protein kinase B (PKB/Akt), extracellular signal-regulated kinase, and mammalian target of rapamycin, which have recently been found to participate in the control of melanin biosynthesis through microphthalmia-associated transcription factor and tyrosinase transcription.

Genetic disorders of pigmentation

More than 127 loci are actually known to affect pigmentation in mouse when they are mutated. From embryogenesis to transfer of melanin to the keratinocytes or melanocytes survival, any defect is able to alter the pigmentation process. Many gene mutations are now described, but the function of their product protein and their implication in melanogenesis are only partially understood. Each genetic pigmentation disorder brings new clues in the understanding of the pigmentation process. According to the main genodermatoses known to induce hypo- or hyperpigmentation, we emphasize in this review the last advances in the understanding of the physiopathology of these diseases and try to connect, when possible, the mutation to the clinical phenotype.

Abnormal sudomotor function in the hypomelanotic macules of tuberous sclerosis complex

To investigate the integrity of sympathetic innervation in the hypomelanotic macules of tuberous sclerosis complex, we studied sudomotor function in nine patients with tuberous sclerosis complex. Postganglionic sudomotor function was assessed using the Silastic imprint test in nine patients with tuberous sclerosis complex who have at least one hypomelanotic macule greater than 2 cm in diameter. Sweating was induced by iontophoresis with 0.5% pilocarpine nitrate and sweat droplets were counted under a microscope using a 1 x 1 cm grid. Silastic imprint testing of an analogous skin area contralateral to the hypomelanotic macule was measured as a control. Sweat volume quantitation using sweat collectors was performed in five of the subjects. The sweat volume collected from the hypomelanotic macule was reduced compared to the control skin in four of the five subjects. Sweat droplet counts from the hypomelanotic macule were significantly reduced in only one of nine subjects. These data suggest that, although there is no difference in the number of functioning sweat glands in most hypomelanotic macules, the sweat glands produce less sweat (ie, decreased sweat volume) than in normal skin. We hypothesize that focal abnormalities of sympathetic innervation might be responsible for the hypomelanotic macules of tuberous sclerosis complex.

Persistent hyperplastic primary vitreous with retinal tumor in tuberous sclerosis: report of a case including tumoral immunohistochemistry and cytogenetic analyses

OBJECTIVE

The authors describe an ocular lesion combining the characteristics of persistent hyperplastic primary vitreous (PHPV) and a retinal tumor in an infant with tuberous sclerosis complex (TSC).

STUDY DESIGN

Case report.

METHODS

Immunohistochemistry and cytogenetic studies were performed on TSC cells from an intraocular tumor in a 6-week-old infant.

RESULTS

Histopathologic examination showed a thick fibrovascular membrane between the aspect of the lens and the astrocytic component of the mass. Glial fibrillary acidic protein (GFAP) showed a variable intracytoplasmic reaction in the astrocytic proliferation, involving approximately 50% of the cells. Tissue culture studies showed a fairly rapid proliferation of fusiform cells, consistent with bipolar astrocytic cells. Cytogenetic studies showed one abnormal clone consisting of three hyperdiploid cells with a loss of chromosome 9 and a gain of chromosomes 6 and 12.

CONCLUSION

The atypical localization of the retinal tumor could be explained by the fact that it was trapped during its proliferation by the retinal detachment associated with the PHPV.

Neurocutaneous syndromes

Many of the neurocutaneous disorders are more common than once suspected, in part because patients with milder forms of the disorders are now more likely to be recognized. Improved diagnostic studies and increasingly specific medical and surgical therapy allow some previously untreatable complications to be successfully managed. Genetic linkage analysis has localized the abnormal gene for some of the hereditary neurocutaneous disorders onto specific chromosomes, and newly developed clinical diagnostic criteria have improved our ability to establish a definite diagnosis in less obvious patients. Thus, the outlook for these patients is no longer uniformly pessimistic.

Disorders of hypopigmentation in children

The most common disorders of hypopigmentation in children are pityriasis alba, vitiligo, nevus depigmentosus, and tinea versicolor. Pityriasis alba usually presents as ill defined, scaly patches of hypomelanosis on the cheeks of children with an atopic diathesis. The face is also a favored site for vitiligo, but the distribution is periorificial, and the pigment loss is complete because of a destruction of melanocytes. Vitiligo is an acquired, progressive disorder in contrast to nevus depigmentosus, which is a stable, congenital leukoderma. The localized form of nevus depigmentosus must be distinguished from an ash leaf spot, the earliest cutaneous manifestation of tuberous sclerosis, whereas the systematized form may be confused with hypomelanosis of Ito, another neurocutaneous disorder. The lesions of tinea versicolor favor the upper trunk of adolescents, and potassium hydroxide examination of the associated scale reveals hyphal and yeast forms of P. orbiculare. Any inflammatory process in the skin such as dermatitis or psoriasis can resolve with areas of hypopigmentation.

Ophthalmic manifestations of tuberous sclerosis

The ophthalmic manifestations of tuberous sclerosis include a variety of nonretinal ophthalmic findings which, other than adenoma sebaceum of the lids, are uncommon. Approximately half the patients with tuberous sclerosis have retinal or optic nerve hamartomas; in half of these patients the hamartomas occur bilaterally. Three basic morphologic types of retinal hamartomas are recognized: the most common type is a subtle, relatively flat, smooth-surfaced, salmon-colored, semitransparent, and circular or oval-shaped lesion located in the superficial retina, most commonly near or at the posterior pole. The second type is an easily recognized opaque, white, elevated, multinodular calcified lesion that is frequently described as resembling a mulberry. A third type of lesion contains features of the other two, being calcified and nodular centrally, whereas its perimeter is semitranslucent, smooth, and salmon-colored. The hamartomas may be richly vascularized. They generally do not grow, but over decades some of the lesions may become calcified. Visual loss from retinal and optic nerve hamartomas rarely occurs. Because growth and change of the fundus lesions are rare, treatment is not indicated. Retinoblastoma of the optic nerve and retina is the most important lesion that must be differentiated from the hamartomas seen with tuberous sclerosis.

Tuberous sclerosis. Diagnostic problems in a family

This paper describes a family in which the first child, a girl born in 1988, has tuberous sclerosis (TS). The 28-year-old mother had no symptoms of TS but at ocular examination she presented with progressively increasing prominence of the optic nerve heads with a few glistening spots due to deep and superficial drusen, and tilted disks. Visual fields showed enlarged blind spots. Fluorescein angiography showed lobular drusen of the optic nerve heads. The ERG showed an absent B-wave. The family history was negative for TS. The question arises whether these ocular findings have to be interpreted as part of the syndrome of TS. In that case the recurrence risk for the disease is 50% for each next child. Assuming a spontaneous mutation in the first child, however, the risk can be nearly neglected. Until the genetic defect in TS is found, these problems in genetic counselling of parents with a child with TS will remain unsolved. Prenatal diagnosis by DNA-analysis is not yet possible; however a few cases of TS have been diagnosed prenatally by fetal ultrasonography of the heart, as was the case in the affected child.

Tuberous sclerosis

Tuberous sclerosis is a multisystem disorder of autosomal dominant inheritance that has important eye signs which contribute substantially to the diagnosis. The disease has been recognized for over 100 years, classically by the occurrence of the triad of mental retardation, epilepsy and adenoma sebaceum of the face. Children with tuberous sclerosis often present with infantile spasms. Eye manifestations include retinal phakomata which may be granular, white and calcified, or flat, translucent and noncalcified. Additional findings include angiofibromas of the lids, poliosis, retinal and iris depigmentation, and atypical colobomata. Various systemic and ocular signs of tuberous sclerosis may develop over the years. For purposes of both patient management and genetic counselling, diagnosis based on early signs is important. The ophthalmologist may play an important role in this regard, as the characteristic phakomata can often be detected within the first two years of life.