Clinical and Molecular Genetic Aspects 10 of Leber’s Congenital Amaurosis Robert Henderson, Birgit Lorenz, Anthony T. Moore | Core Messages of about 2–3 per 100,000 live births [119, 50]. ∑ Leber’s congenital amaurosis (LCA) is a It occurs more frequently in communities severe generalized retinal dystrophy which where consanguineous marriages are common presents at birth or soon after with nystag- [128]. mus and poor vision and is accompanied by a nonrecordable or severely attenuated ERG 10.1.1 ∑ As some forms are associated with better Clinical Findings vision during childhood and nystagmus may be absent, a wider definition is early LCA is characterized clinically by severe visual onset severe retinal dystrophy (EOSRD) impairment and nystagmus from early infancy with LCA being the most severe form associated with a nonrecordable or substantial- ∑ It is nearly always a recessive condition but ly abnormal rod and cone electroretinogram there is considerable genetic heterogeneity (ERG) [32, 31, 118]. The pupils react sluggishly ∑ There are eight known causative genes and and, although the fundus appearance is often three further loci that have been implicated normal in the early stages,a variety of abnormal in LCA/EOSRD retinal changes may be seen. These include pe- ∑ The phenotype varies with the genes ripheral white dots at the level of the retinal pig- involved; not all are progressive. At present, ment epithelium, and the typical bone-spicule a distinct phenotype has been elaborated pigmentation seen in retinitis pigmentosa. for patients with mutations in RPE65 Other associated findings include the ocu- ∑ Although LCA is currently not amenable to lodigital sign, microphthalmos, enophthalmos, treatment, gene therapy appears to be a ptosis, strabismus, keratoconus [28], high re- promising therapeutic option, especially fractive error [143],cataract,macular coloboma, for those children with mutations in RPE65 optic disc swelling, and attenuated retinal vas- culature. 10.1.2 10.1 Differential Diagnosis Introduction It is now recognised that LCA represents the Leber’s congenital amaurosis, an infantile onset most severe end of the spectrum of infantile on- form of rod-cone dystrophy, is usually inherited set retinal dystrophies. Some genes originally as an autosomal recessive trait. First described described as causing LCA have since been by Theodor Leber in 1869 and 1871 [71], it ac- shown to have a less severe clinical pheno- counts for 3–5% of childhood blindness in type–with less nystagmus, normal pupil reac- the developed world [108] and has an incidence tions, and better vision despite an absent ERG 158 Chapter 10 Clinical and Molecular Genetic Aspects of Leber’s Congenital Amaurosis Table 10.1. Systemic disorders with associated LCA findings Disorder MIM No. Systemic features Senior-Loken syndrome MIM 266900 Kidney disease (nephronophthisis) Mainzer-Saldino syndrome MIM 266920 Skeletal anomalies including osteopetrosis giving cone shaped epiphyses of the hand bones and ataxia Lhermitte-Duclos syndrome MIM 158350 Cerebellar hyperplasia, macrocephaly and seizures Joubert syndrome MIM 213300 Cerebellar hypoplasia, oculomotor difficulties and respiratory problems Alstrom syndrome MIM 203800 Cardiomyopathy, deafness, obesity and diabetes Bardet-Biedl syndrome MIM 209900 Mental retardation, polydactyly, obesity and hypogonadism, abnormality in renal structure, function, or both Infantile Refsum disease MIM 266510 Abnormal accumulation of phytanic acid leading to peripheral neuropathy, ataxia impaired hearing, and bone and skin changes Zellweger disease MIM 214100 Cerebrohepatorenal syndrome Neonatal adrenoleucodystrophy MIM 202370 Similar in biochemical terms to Zellweger syndrome; it has characteristic facies, adrenal atrophy, and degenerative white matter changes Infantile Batten disease MIM 256730 Histopathologically: total derangement of cortical cytoarchitecture with severe degeneration of white matter. Clinically: rapid psychomotor deterioration, ataxia, and muscular hypotonia; microcephaly and myoclonic jerks are also features [99]. This has led some to use the term early on- set severe retinal dystrophy (EOSRD) to de- 10.2 scribe severe rod-cone dystrophies of infantile Molecular Genetics onset. The main differential diagnosis is from other LCA is usually inherited as an autosomal reces- causes of infantile nystagmus [14, 76] particu- sive disorder,although there are a few reports of larly early onset static retinal conditions such as autosomal dominant inheritance [33, 102, 135]. congenital stationary night blindness [5] and Although it had been recognised as early as 1963 the various forms of achromatopsia [90]. The that there was more than one causative gene clinical features, and in particular the ERG find- [142], it is only recently that the true extent of ings, allow these disorders to be distinguished the genetic heterogeneity has been identified. from LCA. Although most children with LCA To date,eight genes (GUCY2D [103],AIPL1 [123], have disease confined to the eye; there are a RPE65 [84, 44], RPGRIP1 [27, 40], CRX [35, 56], number of recessively inherited systemic dis- TULP1 [9, 43, 46], CRB1 [79], RDH12 [58, 101]) orders such as Joubert syndrome, and the and a three further loci (LCA3 [126], LCA5 [93] peroxisomal disorders in which an early onset and LCA9 [62]) have been reported in LCA/ retinal dystrophy similar to LCA may form part EOSRD. Furthermore, mutations in two other of the syndrome (Table 10.1). Usually the other genes (LRAT [131] and MERTK [87]) are as- systemic findings allow these disorders to be sociated with a very early onset form of RP distinguished from LCA but in some disorders, that could also be classified as EOSRD. All for example Alstrom syndrome [117] and juve- together these genes account for between 20% nile nephronothisis [7, 30, 132] the systemic fea- and 50% of cases of LCA cases and it is evident tures may not become apparent until later that more genes remain to be discovered [48,18] childhood. (Figs. 10.1, 10.2). 10.2 Molecular Genetics 159 Fig. 10.1. Median frequency of disease genes in LCA/EOSRD Fig. 10.2. Putative sites of gene expression for LCA/EOSRD 160 Chapter 10 Clinical and Molecular Genetic Aspects of Leber’s Congenital Amaurosis 10.2.1.1 10.2.1 Genotype–Phenotype Correlations GUCY-2D (LCA1 Locus) The findings of high hyperopia (>+7 DS),severe GUCY-2D (17p13.1) was the first gene identified photophobia, poor vision of count fingers (CF) as causative in LCA [103].The prevalence of mu- or light perception (LP) in addition to the pres- tations of GUCY-2D in LCA is much higher ence of early peripheral and macular degenera- (70%) in cases from Mediterranean countries tion with bone spicule pigments in the periph- [49]. GUCY-2D encodes a human photoreceptor ery, disc pallor and vessel attenuation has been specific guanylate cyclase which plays a key suggested by Perrault et al. to be pathognomon- role in phototransduction. Guanylyl cyclase cat- ic of the GUCY-2D mutation [103]. Other groups alyzes the conversion of guanosine triphos- have, however, reported LCA patients with phate (GTP) to cyclic guanosine monophos- GUCY-2D mutations who have no significant phate (cGMP). Guanylate cyclase function is photophobia, better visual acuity (20/200), and important in restoring levels of cGMP levels, mild to moderate hyperopia [25]. which keeps open the gated cation channels al- lowing recovery of the dark state after photo- transduction. Mutations in the gene would lead 10.2.2 to permanent closure of the c-GMP gated cation RPE65 (LCA2) channels with resultant hyperpolarisation of the plasma membrane. Mutations in RPE65 are associated with a num- Recessive mutations in GUCY-2D are associ- ber of different retinal degenerations, including ated with LCA and the causative mutations have LCA [44, 84, 95, 121], autosomal recessive RP been reported in the extracellular, kinase-like, [93], and an early onset severe rod-cone dystro- dimerisation, and catalytic domains [104, 116]. phy [77] (http://www.retina-international.org/ Heterozygous mutations in the same gene have sci-news/rpe65mut.htm). Mutations in RPE65 been reported to cause an autosomal dominant account for 3–16% of cases of LCA/EOSRD in cone rod dystrophy [26, 105, 134] (http://www. various series [25, 40, 48, 80, 95, 130]. The RPE65 retina-international.org/sci-news/gcmut.htm). gene localizes to 1p31.2 and consists of 14 exons. Histological examination of the eyes from an It encodes two distinct forms of a 65-kD protein 11-year-old LCA donor with a known GUCY-2D specific to the smooth endoplasmic reticulum mutation,who had light perception vision and a (ER) of the retinal pigment epithelium: a retinal nondetectable ERG at the time of death, showed pigment epithelium (RPE) membrane-associat- regional loss of photoreceptors. There were no ed form (mRPE65) and a lower molecular identifiable photoreceptors in the mid-periph- weight soluble form (sRPE65)[83]. RPE65 plays eral retina, but rods and cones lacking outer a key role in the metabolism of vitamin A with- segments were present in the macula and peri- in the retina. RPE65 serves as a binding protein pheral retina, although in reduced numbers. for all-trans-retinyl esters [41, 57] Analysis of The inner nuclear layer was found to be of nor- RPE65 knockout mice shows that it is involv- mal thickness but horizontal cell, amacrine and ed in the isomerisation of all-trans-retinol to ganglion cell numbers were decreased [91]. 11-cis-retinol within the RPE [107]. Xue et al. The fact that there were significant numbers of [148] demonstrated that the membrane associ- residual photoreceptors and inner retinal neu- ated form is triply palmitoylated and is a chap- rons despite a visual acuity of light perception erone for all-trans retinyl esters, allowing their gives rise to some optimism that any future entry into the visual cycle for processing into therapy may lead to improved retinal function. 11-cis retinal. The soluble form of RPE65 is not However, visual improvement following suc- palmitoylated and is a chaperone for vitamin A.