GNPTAB-related disorders comprise the phenotypes mucolipidosis II (ML II) and mucolipidosis IIIa/ß (ML IIIa/ß), and phenotypes intermediate between ML II and ML IIIa/ß. ML II is evident at birth and slowly progressive; death most often occurs in early childhood. Orthopedic abnormalities present at birth may include thoracic deformity, kyphosis, clubfeet, deformed long bones, and/or dislocation of the hip(s). Growth often ceases in the second year of life; contractures develop in all large joints. The skin is thickened, facial features are coarse, and gingiva are hypertrophic. All children have cardiac involvement, most commonly thickening and insufficiency of the mitral valve and, less frequently, the aortic valve. Progressive mucosal thickening narrows the airways, and gradual stiffening of the thoracic cage contributes to respiratory insufficiency, the most common cause of death. ML IIIa/ß becomes evident at about age three years with slow growth rate and short stature; joint stiffness and pain initially in the shoulders, hips, and fingers; gradual mild coarsening of facial features; and normal to mildly impaired cognitive development. Pain from osteoporosis becomes more severe during adolescence. Cardiorespiratory complications (restrictive lung disease, thickening and insufficiency of the mitral and aortic valves, left and/or right ventricular hypertrophy) are common causes of death, typically in early to middle adulthood. Phenotypes intermediate between ML II and ML IIIa/ß are characterized by physical growth in infancy that resembles that of ML II and neuromotor and speech development that resemble that of ML IIIa/ß.

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). For a discussion of genetic heterogeneity of congenital nystagmus, see NYS1 (310700).

A rare genetic syndromic intellectual disability disorder with characteristics of mild to profound intellectual disability, delayed speech, obesity, ocular anomalies (blepharophimosis, blepharoptosis, hyperopic astigmatism, decreased visual acuity, strabismus, abducens nerve palsy, and/or accommodative esotropia), and dermal manifestations, such as chronic atopic dermatitis. Associated craniofacial dysmorphism includes macrocephaly, maxillary hypoplasia, mandibular prognathism and crowding of teeth.

Familial horizontal gaze palsy with progressive scoliosis-1 (HGPPS1) is an autosomal recessive neurologic disorder characterized by eye movement abnormalities apparent from birth and childhood-onset progressive scoliosis. These features are associated with a developmental malformation of the brainstem including hypoplasia of the pons and cerebellar peduncles and defective decussation of certain neuronal systems. Cognitive function is normal (summary by Bosley et al., 2005). Genetic Heterogeneity of Familial Horizontal Gaze Palsy with Progressive Scoliosis See also HGPPS2 (617542), caused by mutation in the DCC gene (120470) on chromosome 18q21.