Phenotypic and molecular characterization of a novel DCX deletion and a review of the literature

To the Editor : Classical lissencephaly is a neuronal migration disorder characterized by agyria or pachygyria with a thickened, disorganized, four-layered cortex. The severity ranges from diffuse agyria to subcortical band heterotopia (SBH) (1, 2), consisting of bilateral symmetric ribbons of gray matter within the white matter between the cortex and ventricular surface. Various forms of lissencephaly have been associated with mutations in LIS1 (3), DCX (4), TUBA1A (5), YWHAE (6), ARX (7), RELN (8) and VLDLR (9) (Table S1, Supporting information online). Mutations of DCX, which resides on the X chromosome, cause a wide spectrum of lissencephaly phenotypes (4, 10, 11). Hemizygous males have the more severe forms, agyria or pachygyria (grade 1–4) (12), with global developmental delay, severe mental retardation and seizures. Heterozygous females have the less severe forms, mixed pachygyria and SBH (grade 5) or SBH alone (grade 6), presumably due to random X-inactivation (4, 11, 13, 14). We report a female patient who was born at term following an uncomplicated pregnancy. Her birth weight and head circumference were at the 75th and 50th percentile, respectively. She manifested globally delayed development from infancy. By 49 months, her developmental skills were appropriate for 11–20 months; she was able to cruise, speak several words and indicate her needs using a communication board. A psychoeducational assessment at 5.9 years confirmed severe to profound disability. At 6.0 years, she used a walker with assistance but was able to move independently on the floor. At 26 months, she developed generalized tonic– clonic seizures, and at 4 years, she developed daily clusters of head drops and episodes of staring and eye rolling. Her EEGs documented progressive worsening of her seizure disorder. Cranial magnetic resonance imaging (MRI) showed anterior prominent grade 3 lissencephaly, normal corpus callosum, moderately enlarged and dysplastic third and lateral ventricles (Fig. 1a, b). On physical examination at 7.3 years, her head circumference was 48.5 cm (<3rd percentile); her weight was 26 kg (50th–75th percentile); her height was 122.4 cm (25th–50th percentile). She had a sloping forehead with bitemporal narrowing, mild ptosis, a bulbous nasal tip, inverted nipples, valgus pronation of her feet, and distal contractures (Table 1). She had normal muscle bulk and strength, decreased central and peripheral tone, and appropriate deep tendon reflexes. Other tests showed normal blood cell counts and normal circulating levels of thyroid stimulating hormone, ammonia, lactate, and creatine phosphokinase. Her plasma amino acid profile, FMR1 triplet repeat length and peripheral lymphocyte karyotype (46, XX) were normal. Fluorescent in situ hybridization did not detect a microdeletion of LIS1. As detailed in the supplemental methods, multiplex ligation-dependent probe amplification (MLPA) and real-time quantitative polymerase chain reaction (RT-QPCR) analyses revealed an exon 5 deletion of DCX, predicted to cause splicing from exon 4–6 and a frameshift mutation that will either lead to nonsense-mediated RNA decay or a truncated protein with disrupted doublecortin domains (33). Also, consistent with the severe phenotype (Table 1), the proposita showed fully skewed X-inactivation (100:1) in peripheral lymphocytes suggesting the affected X chromosome was predominantly active and little functional DCX protein was produced. We were unable to determine if the deletion was inherited from a nondysmorphic mother or de novo because the family declined further testing. Array genomic hybridization placed the DCX deletion within introns 4 and 5. By long-range PCR and DNA sequencing, we delineated a 5359 bp