No abstract

Idiopathic congenital nystagmus is characterized by involuntary, periodic, predominantly horizontal oscillations of both eyes. We identified 22 mutations in FRMD7 in 26 families with X-linked idiopathic congenital nystagmus. Screening of 42 singleton cases of idiopathic congenital nystagmus (28 male, 14 females) yielded three mutations (7%). We found restricted expression of FRMD7 in human embryonic brain and developing neural retina, suggesting a specific role in the control of eye movement and gaze stability.

Idiopathic infantile nystagmus (IIN) consists of involuntary oscillations of the eyes. The familial form is most commonly X-linked. We recently found mutations in a novel gene FRMD7 (Xq26.2), which provided an opportunity to investigate a genetically defined and homogeneous group of patients with nystagmus. We compared clinical features and eye movement recordings of 90 subjects with mutation in the gene (FRMD7 group) to 48 subjects without mutations but with clinical IIN (non-FRMD7 group). Fifty-eight female obligate carriers of the mutation were also investigated. The median visual acuity (VA) was 0.2 logMAR (Snellen equivalent 6/9) in both groups and most patients had good stereopsis. The prevalence of strabismus was also similar (FRMD7: 7.8%, non-FRMD7: 10%). The presence of anomalous head posture (AHP) was significantly higher in the non-FRMD7 group (P < 0.0001). The amplitude of nystagmus was more strongly dependent on the direction of gaze in the FRMD7 group being lower at primary position (P < 0.0001), compared to non-FRMD7 group (P = 0.83). Pendular nystagmus waveforms were also more frequent in the FRMD7 group (P = 0.003). Fifty-three percent of the obligate female carriers of an FRMD7 mutation were clinically affected. The VA's in affected females were slightly better compared to affected males (P = 0.014). Subnormal optokinetic responses were found in a subgroup of obligate unaffected carriers, which may be interpreted as a sub-clinical manifestation. FRMD7 is a major cause of X-linked IIN. Most clinical and eye movement characteristics were similar in the FRMD7 group and non-FRMD7 group with most patients having good VA and stereopsis and low incidence of strabismus. Fewer patients in the FRMD7 group had AHPs, their amplitude of nystagmus being lower in primary position. Our findings are helpful in the clinical identification of IIN and genetic counselling of nystagmus patients.

Periodic alternating nystagmus consists of involuntary oscillations of the eyes with cyclical changes of nystagmus direction. It can occur during infancy (e.g. idiopathic infantile periodic alternating nystagmus) or later in life. Acquired forms are often associated with cerebellar dysfunction arising due to instability of the optokinetic-vestibular systems. Idiopathic infantile periodic alternating nystagmus can be familial or occur in isolation; however, very little is known about the clinical characteristics, genetic aetiology and neural substrates involved. Five loci (NYS1-5) have been identified for idiopathic infantile nystagmus; three are autosomal (NYS2, NYS3 and NYS4) and two are X-chromosomal (NYS1 and NYS5). We previously identified the FRMD7 gene on chromosome Xq26 (NYS1 locus); mutations of FRMD7 are causative of idiopathic infantile nystagmus influencing neuronal outgrowth and development. It is unclear whether the periodic alternating nystagmus phenotype is linked to NYS1, NYS5 (Xp11.4-p11.3) or a separate locus. From a cohort of 31 X-linked families and 14 singletons (70 patients) with idiopathic infantile nystagmus we identified 10 families and one singleton (21 patients) with periodic alternating nystagmus of which we describe clinical phenotype, genetic aetiology and neural substrates involved. Periodic alternating nystagmus was not detected clinically but only on eye movement recordings. The cycle duration varied from 90 to 280 s. Optokinetic reflex was not detectable horizontally. Mutations of the FRMD7 gene were found in all 10 families and the singleton (including three novel mutations). Periodic alternating nystagmus was predominantly associated with missense mutations within the FERM domain. There was significant sibship clustering of the phenotype although in some families not all affected members had periodic alternating nystagmus. In situ hybridization studies during mid-late human embryonic stages in normal tissue showed restricted FRMD7 expression in neuronal tissue with strong hybridization signals within the afferent arms of the vestibulo-ocular reflex consisting of the otic vesicle, cranial nerve VIII and vestibular ganglia. Similarly within the afferent arm of the optokinetic reflex we showed expression in the developing neural retina and ventricular zone of the optic stalk. Strong FRMD7 expression was seen in rhombomeres 1 to 4, which give rise to the cerebellum and the common integrator site for both these reflexes (vestibular nuclei). Based on the expression and phenotypic data, we hypothesize that periodic alternating nystagmus arises from instability of the optokinetic-vestibular systems. This study shows for the first time that mutations in FRMD7 can cause idiopathic infantile periodic alternating nystagmus and may affect neuronal circuits that have been implicated in acquired forms.

Background: Li-Fraumeni syndrome greatly increases the risk of developing several types of cancer and is usually caused by TP53 germline mutations. Predictive testing of at-risk family members is only offered after a complex genetic counselling process. Recently the clinical implementation of array comparative genomic hybridisation (CGH) has revolutionised the diagnosis of patients with syndromic or non-syndromic mental retardation and has evolved to a routinely performed high resolution whole genome scan. Methods and results: When using array CGH to identify the cause for mental retardation in a 7-year-old child we found a submicroscopic de novo deletion of chromosome 17p13.1, which includes several genes likely to be causative for her phenotype, and also of TP53. Conclusion: Thus, array CGH resulted in an unintended predictive diagnosis of an increased tumour susceptibility as observed in Li-Fraumeni syndrome.The list of well defined inherited cancer predisposition syndromes, which can be attributed to a hereditary susceptibility and have far reaching implications for all family members, is steadily growing. Many of these syndromes are caused by germline alterations in a tumour suppressor gene. Usually one functional copy of a tumour suppressor gene is sufficient to exert the function. However, inactivation of both alleles by mutation or deletion can result in uncontrolled proliferation and may therefore contribute to tumorigenesis. Hence, individuals who have already one dysfunctional tumour suppressor gene copy in all somatic cells due to a germline mutation may have such an increased tumour risk whenever the function of the second copy is compromised in a cell. This mechanism is usually referred to as Knudson's ''two-hit hypothesis''. In familial cancer syndromes the identification of at-risk family members by predictive testing is often recommended as enhanced surveillance for early diagnosis and prevention of disease is a critical part of primary care. Due to the extensive consequences of such predictive testing it is usually only offered after a complex genetic counselling process. Sophisticated surveillance guidelines with proven benefit were developed for several cancer prone syndromes such as Lynch syndrome (hereditary non-polyposis colon cancer (HNPCC)), 2 hereditary breast and ovarian cancer due to mutations in BRCA1 or BRCA2, 3 or for hereditary childhood tumours such as retinoblastoma. 5Array comparative genomic hybridisation (CGH) has evolved to a standard application in clinical genetics, especially in individuals with syndromic or non-syndromic mental retardation. As a consequence high resolution scans of entire genomes for small gains and losses anywhere in the genome are now conducted routinely. However, the commonly applied execution of array CGH bears the risk that deletions of genes involved in hereditary cancer syndromes, especially tumour suppressor genes, may also be identified, which would then result in an unintended predictive diagnosis of a cancer prone syndrome. Here we re...