Nystagmus 1, Congenital, X-Linked

A number sign (#) is used with this entry because of evidence that X-linked congenital nystagmus-1 (NYS1) and infantile periodic alternating nystagmus (XIPAN) are caused by mutation in the FERM domain-containing-7 gene (FRMD7; 300628) on chromosome Xq26.

Description

Classic congenital or infantile nystagmus presents as conjugate, horizontal oscillations of the eyes, in primary or eccentric gaze, often with a preferred head turn or tilt. Other associated features may include mildly decreased visual acuity, strabismus, astigmatism, and occasionally head nodding. Eye movement recordings reveal that infantile nystagmus is predominantly a horizontal jerk waveform, with a diagnostic accelerating velocity slow phase. However, pendular and triangular waveforms may also be present. The nystagmus may rarely be vertical. As these patients often have normal visual acuity, it is presumed that the nystagmus represents a primary defect in the parts of the brain responsible for ocular motor control; thus the disorder has sometimes been termed 'congenital motor nystagmus' (Tarpey et al., 2006; Shiels et al., 2007).

Congenital nystagmus may also be a feature of other ocular diseases, such as albinism (see, e.g., OCA1A, 203100), achromatopsia (see, e.g., ACHM3, 262300), and Leber congenital amaurosis (see, e.g., LCA1, 204000). Congenital nystagmus is associated with at least 3 X-linked disorders: Nettleship-Falls ocular albinism (OA1; 300500), which maps to Xp22.3; complete congenital stationary night blindness (CSNB1; 310500), which maps to Xp11.4; and blue-cone monochromatism (CBBM; 303700), which maps to Xq28.

Genetic Heterogeneity of Congenital Nystagmus

Two other X-linked forms of congenital nystagmus have been reported: NYS5 (300589), which maps to Xp11.4-p11.3, and NYS6 (300814), which is caused by mutation in the GPR143 gene (300808) on Xp22.3. Autosomal dominant forms have been mapped to chromosomes 6p12 (NYS2; 164100), 7p11 (NYS3; 608345), 13q (NYS4; 193003), and 1q31-q32 (NYS7; 614826). Autosomal recessive inheritance may rarely occur (see 257400).

Clinical Features

Mellott et al. (1999) reported a large 4-generation pedigree segregating X-linked congenital nystagmus and deuteranomaly (green color vision defect; see 303800). Sixty-five family members were studied. Thirteen individuals had conjugate horizontal nystagmus with pendular and/or jerk waveforms. Several deceased family members were also determined to be affected by examination of medical records. Some had mildly decreased visual acuity, but none had significant ocular or neurologic abnormalities. Eighteen individuals were found to be deuteranomalous trichromats, including at least 5 women. One female carrier for the color vision abnormality and nystagmus had 1 son affected with deuteranomaly alone and a second who inherited both conditions. Linkage analysis was performed (see MAPPING). Mellott et al. (1999) noted that Rucker (1949) had reported a family with X-linked nystagmus in which a man with nystagmus and red-green colorblindness had 2 affected daughters and an affected grandson.

Oh et al. (2007) reported 5 patients from 3 Korean families with X-linked congenital nystagmus. All had a history of nystagmus from birth, bilateral conjugate ocular oscillations, and no abnormalities in the afferent visual pathways. Detailed report of the first family noted that a 6-year-old boy had horizontal, conjugate, and pendular nystagmus predominant in the primary position. The nystagmus changed into jerky nystagmus during lateral gaze. Left-beating, upbeating, and counterclockwise torsional nystagmus appeared during leftward gaze, while right-beating, upbeating, and clockwise torsional nystagmus appeared during rightward gaze. The direction of optokinetic nystagmus was reversed. These continuous eye movements did not cause oscillopsia, reading difficulty, or dizziness. His 39-year-old mother had abnormal eye movements and rightward head tilt since age 1 year. Ocular examination showed right exotropia and nystagmus with left-beating, downbeating, and counterclockwise torsional components in the primary position. Nystagmus was right-beating, upbeating, and clockwise torsional during rightward gaze, and left-beating, downbeating, and counterclockwise torsional during leftward gaze. The direction of optokinetic nystagmus was reversed in both patients. Neither complained of dizziness or oscillopsia. Several family members had abnormal ocular oscillations. The second family had affected brothers, an affected male cousin, and an affected maternal grandmother. The third family also had several affected individuals, both male and female. Overall, the eye-movement abnormalities were characterized by pendular or jerky oscillations, gaze-evoked nystagmus, poor or absent smooth pursuit, and poor or absent vestibuloocular reflexes. Other features included increased velocity waveforms, frequent foveation periods, direction change with gaze shift and reversed optokinetics. The patterns often differed, even in the same family. Most patients had mildly decreased visual acuity. Penetrance among female carriers was about 50%.

Thomas et al. (2008) compared the clinical features of 90 patients with nystagmus due to FRMD7 mutations with those of 48 patients with nystagmus without FRMD7 mutations. There were no differences in mean visual acuity or strabismus between the 2 groups; most had good acuity and stereopsis. Anomalous head posturing was significantly higher in the non-FRMD7 group. Pendular nystagmus was more common in the FRMD7 group. The amplitude of nystagmus was more strongly dependent on the direction of gaze in the FRMD7 group, being lower at primary position compared to the non-FRMD7 group. Fifty-three percent of obligate female carriers of FRMD7 mutations were affected.

X-linked Infantile Periodic Alternating Nystagmus

Periodic alternative nystagmus (PAN; nystagmus alternans) is spontaneous nystagmus with a sinusoidally modulated amplitude and spontaneous periodic changes in direction (Huygen et al., 1995). Huygen et al. (1995) reported a mother and daughter with congenital onset of periodic alternating nystagmus. Both had developed compensatory torticollis. Family history revealed many affected family members, and the pedigree pattern was consistent with X-linked dominant inheritance. The first generation contained 11 affected women. The authors proposed the designation XLPAN.

Ito et al. (2000) reported a woman with periodic alternating nystagmus whose mother had congenital fixed nystagmus. The daughter first noted intermittent oscillopsia at age 18 years. Examination showed spontaneous horizontal nystagmus that reversed its direction regularly on primary gaze. Smooth pursuit was also impaired. Her mother had pendular and jerky nystagmus on primary and lateral gaze. Smooth pursuit was also impaired. Ito et al. (2000) noted the similarities between the 2 patients and suggested that the daughter's nystagmus was most likely present from birth, even though it was not noticed until later. The authors suggested that the 2 disorders share a common underlying mechanism.

Hertle et al. (2005) defined infantile periodic alternating nystagmus as similar to infantile nystagmus, except that the null point shifts position in a cyclic pattern. This results in regular (periodic) or irregular (aperiodic) changes in the amplitude and direction of the nystagmus every few minutes or seconds. Hertle et al. (2005) described the clinical and electrophysiologic characteristics of 4 family members from 3 generations who had X-linked infantile periodic alternating nystagmus (symbolized XIPAN by the authors). Three males in 2 generations and 1 female were examined. Clinical examinations showed a jerk-pendular nystagmus with a latent component, strabismus, and a significant refractive error in the 3 affected males, and only myopic astigmatism in the female. All 4 family members showed eye movement recording (EMR) abnormalities with infantile jerk/dual jerk and pendular nystagmus waveforms. The female had nystagmus present on EMR only (clinically 'silent' periodic nystagmus that was probably a marker for the carrier state), and all patients showed a periodicity to their nystagmus. Mapping studies were not performed. In the family reported by Hertle et al. (2005), Thomas et al. (2011) identified a mutation in the FRMD7 gene (G24R; 300628.0005).

Khan et al. (2011) described a family with 2 brothers with X-linked infantile nystagmus and 3 asymptomatic females who had delayed corrective saccades (prolonged pursuit) during optokinetic nystagmus (OKN) drum testing. All of these individuals carried a mutation in FRMD7 (300628.0012). A maternal aunt had infantile nystagmus in addition to congenital fibrosis of the extraocular muscles (CFEOM; see 135700). She did not carry the FRMD7 mutation or a mutation in any known CFEOM genes, and Khan et al. (2011) concluded that her nystagmus represented a second disorder in this family, likely related to CFEOM.

Inheritance

Waardenburg (1962) stated that there was no reason to separate an X-linked recessive from an X-linked dominant form as some have attempted. In some families the disorder is recessive in one line and dominant in another (Hemmes, 1924; Waardenburg et al., 1961). The explanation could be that the mutation is identical but that a series of 'wildtype' isoalleles have different effects on penetrance of the mutation in the heterozygous female.

Mellott et al. (1999) reported a large 4-generation pedigree segregating X-linked congenital nystagmus and deuteranomaly (see 303800). At least 13 individuals had nystagmus and 18 had the green color vision defect. There were several affected females, including 5 with the color vision defect. The inheritance pattern was consistent with X-linked transmission of 2 closely linked genes. One female carrier for the color vision abnormality and nystagmus had 1 son affected with deuteranomaly alone and a second who inherited both conditions. Haplotype analysis revealed a recombination event that caused 1 brother to 'lose' the abnormal nystagmus gene (FRMD7; 300628). Similarly, one woman manifested only nystagmus because she had 1 normal copy of the red-green opsin cluster (see 300821) that she inherited from her father. In contrast, her brother manifested only nystagmus because a recombination event occurred between the nystagmus locus and the red-green opsin cluster, causing him to inherit only the nystagmus gene.

Mapping

Kerrison et al. (1999) studied 3 families with congenital motor nystagmus inherited in an X-linked, irregularly dominant pattern. The penetrance among obligate female carriers was 54%. Evaluation of markers in the region of the genes for OA1, CSNB1, and CBBM revealed no evidence of linkage, supporting the hypothesis that X-linked congenital motor nystagmus represents a distinct entity. Linkage analysis showed linkage to chromosome Xq26-q27. Assessment of haplotypes and multipoint linkage analysis, which gave a maximum lod score of 10.79 with the 1-lod-unit support interval spanning approximately 7 cM, placed the gene in a region between 2 specific markers. Evaluation of candidate genes SOX3 (313430), which maps to Xq26-q27, and CDR1 (302650), which maps to Xq27.1-q27.2, revealed no mutations in affected male subjects.

By linkage analysis of a large 4-generation family segregating both congenital nystagmus and deuten colorblindness (CBD; 303800), Mellott et al. (1999) found linkage to chromosome Xq26-q27 (maximum lod score of 4.84 at marker DXS8041). Recombination studies delineated a 5-cM candidate interval between ATA59C05 and DXS1192, which is approximately 11 cM centromeric to the red-green opsin gene cluster on Xq28 (see 300821), which explained the frequent coexistence of congenital nystagmus and deuteranomaly in this family. The identified locus was contained within the previously reported Xq26-q27 locus (Kerrison et al., 1999) and was 9.8 cM smaller.

Kerrison et al. (2001) refined the NYS1 locus to a 5-cM interval between DXS9909 and DXS1211 on Xq26-q27. Genetic analysis excluded mutations in the SLC25A14 gene (300242). Based on examination of an extended pedigree, the estimated penetrance among obligate female carriers was 29% (6 of 21).

By linkage analysis in 2 unrelated Chinese families segregating X-linked congenital motor nystagmus, Guo et al. (2006) identified a candidate region on Xq23-q27 (maximum lod score of 3.53 at DXS1047). Haplotype analysis identified a region between DXS8055 and DXS1205. Guo et al. (2006) noted that the region overlapped with that reported by Kerrison et al. (1999, 2001).

Cytogenetics

Gutmann et al. (1991) reported a 5-generation family in which a woman in the second generation with 46,XX/45,X Turner syndrome was affected. She had 2 normal children: a daughter who proved herself to be a carrier and an affected son. X-linked congenital nystagmus is associated with head oscillations. Gutmann et al. (1991) found that magnetic resonance images of the brain of affected individuals were normal. Berry and Docherty (1992) suggested that the mosaicism was likely to be simply that normally associated with increasing age. However, Gutmann et al. (1992) responded with the rebuttal that the percentage of 45,X cells was significantly different from that in females of comparable age.

Molecular Genetics

Tarpey et al. (2006) identified 22 novel mutations in the FRMD7 gene (300628) in 26 families with X-linked congenital nystagmus. Screening of 42 singleton cases of idiopathic congenital nystagmus (28 males, 14 females) yielded 3 mutations (7%). Tarpey et al. (2006) 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. All mutations identified in FRMD7 cosegregated with the disorder in the linked families and were absent from 300 male control chromosomes. The nonsense mutations leading to Q201X (300628.0001) and R335X (300628.0002) predicted truncated proteins containing 28% and 47% of the wildtype protein, respectively. Four of 5 splice site mutations occurred at conserved splice donor residues, position +1 and +2, and were predicted to be pathologic by classic exon skipping and nonsense-mediated decay. A silent variant (V84V; 300628.0004) created a new splice acceptor site within exon 4 of the FRMD7 gene that resulted in the loss of transcript containing the sequence of exons 1 through 5 and the rare presence of a transcript with exon 4 skipped in lymphocytes. Six missense mutations involved highly conserved residues that not only are invariant in rat, mouse, chicken, and Xenopus but are also located within invariant blocks of highly conserved residues, suggesting that mutations at these locations are critical to the normal function of the FRMD7 protein.

In affected members of 1 of the large Chinese families reported by Guo et al. (2006), Zhang et al. (2007) identified a mutation (300628.0006) in the FRMD7 gene. The authors noted that transmission in the Chinese family was consistent with X-linked recessive inheritance, but that this mutation had been identified by Tarpey et al. (2006) in an English family with X-linked dominant inheritance. Skewed X inactivation was offered as an explanation.

Shiels et al. (2007) identified the same FRMD7 mutation (300628.0007) in affected individuals of 2 unrelated families with NYS1.

In affected members of a large Turkish family with X-linked congenital nystagmus, Kaplan et al. (2008) identified a mutation (300628.0008) in the FRMD7 gene. There were at least 7 affected females, and molecular studies showed that some had markedly skewed X-chromosome inactivation.

In affected members of 6 unrelated Chinese families with X-linked congenital nystagmus, He et al. (2008) identified a truncating mutation in the FRMD7 gene (1274delTG; 300628.0009). Haplotype analysis indicated a founder effect. The phenotype was characterized by onset in infancy of horizontal pendular oscillations of both eyes and varying degrees of decreased visual acuity. Some patients had astigmatism. None had abnormal appearance of the fundus or color vision defects.

Thomas et al. (2011) identified FRMD7 mutations (see, e.g., 300628.0002, 300628.0005, 300628.0010-300628.0011) in 26 patients from 10 families in which at least 1 individual had X-linked infantile periodic alternating nystagmus, as well as in 1 singleton patient with the disorder. PAN was not diagnosed clinically in any of the individuals, but was apparent in some mutation carriers after eye movement recordings during prolonged fixation. Several families showed phenotypic heterogeneity, with only some having PAN on eye movement recordings; all had clinical nystagmus. Most patients had good visual acuity, but none with PAN had a horizontal optokinetic reflex. Based on immunohistochemical expression studies in human embryonic brain and phenotypic data, Thomas et al. (2011) hypothesized that periodic alternating nystagmus arises from instability of the optokinetic-vestibular systems.

Population Genetics

Stayte et al. (1993) found nystagmus in 1 per 1,000 children in a cohort in England followed from birth through the age of 5 years.

In a review of the literature, He et al. (2008) concluded that FRMD7 mutations account for about 47% of X-linked nystagmus in Chinese patients with the disorder.