Spectrum of pontocerebellar hypoplasia in 13 girls and boys with CASK mutations: confirmation of a recognizable phenotype and first description of a male mosaic patient

Abstract

Background: Pontocerebellar hypoplasia (PCH) is a heterogeneous group of diseases characterized by lack of development and/or early neurodegeneration of cerebellum and brainstem. According to clinical features, seven subtypes of PCH have been described, PCH type 2 related to TSEN54 mutations being the most frequent. PCH is most often autosomal recessive though de novo anomalies in the X-linked gene CASK have recently been identified in patients, mostly females, presenting with intellectual disability, microcephaly and PCH (MICPCH).

Methods: Fourteen patients (12 females and two males; aged 16 months-14 years) presenting with PCH at neuroimaging and with clinical characteristics unsuggestive of PCH1 or PCH2 were included. The CASK gene screening was performed using Array-CGH and sequencing. Clinical and neuroradiological features were collected.

Results: We observed a high frequency of patients with a CASK mutation (13/14). Ten patients (8 girls and 2 boys) had intragenic mutations and three female patients had a Xp11.4 submicroscopic deletion including the CASK gene. All were de novo mutations. Phenotype was variable in severity but highly similar among the 11 girls and was characterized by psychomotor retardation, severe intellectual disability, progressive microcephaly, dystonia, mild dysmorphism, and scoliosis. Other signs were frequently associated, such as growth retardation, ophthalmologic anomalies (glaucoma, megalocornea and optic atrophy), deafness and epilepsy. As expected in an X-linked disease manifesting mainly in females, the boy hemizygous for a splice mutation had a very severe phenotype with nearly no development and refractory epilepsy. We described a mild phenotype in a boy with a mosaic truncating mutation. We found some degree of correlation between severity of the vermis hypoplasia and clinical phenotype.

Conclusion: This study describes a new series of PCH female patients with CASK inactivating mutations and confirms that these patients have a recognizable although variable phenotype consisting of a specific form of pontocerebellar hypoplasia. In addition, we report the second male patient to present with a severe MICPCH phenotype and a de novo CASK mutation and describe for the first time a mildly affected male patient harboring a mosaic mutation. In our reference centre, CASK related PCH is the second most frequent cause of PCH. The identification of a de novo mutation in these patients enables accurate and reassuring genetic counselling.

Figure 1

RT-PCR analysis in patient 8 (mutation c.2302 + 5 G > A), showing exon skipping and aberrant transcript. a. RT-PCR products were amplified from cDNA of patient 8 and from one healthy individual using primers in exon 23 and in exon 25. The arrow indicates an aberrant sized product in patient 8 in addition to normal sized transcript observed in patient 8 and in healthy control. M marker, P8 patient 8, C control, N negative control. b. The PCR products were purified and sequenced. Sequence chromatograms from normal sized amplicon with normal sequence exon 23-24-25, and from abnormal amplicon, showing absence of exon 24.

Figure 2

Sequence analysis of exon 4 amplified from DNA obtained from lymphocytes and cheek swab of patient 12 and from lymphocytes of his parents. The upper two sequence profiles (patient) show low signals for the mutant variant superimposed on the wild-type sequence (arrow). The lower two sequence profiles (parents) show the wild type sequence.

Figure 3

Facial features of CASK-positive patients. A and B: patient 8 (1 year (A) and 4 years (B)). C: patient 7 (18 months). D: patient 12 (13 years). E: patient 9 (13 years). F: patient 4 (12 years). Note minor facial dysmorphism: round face, small chin, well-drawn eyebrows in the younger patients; longer face, high and large nasal bridge, long nose, protuding maxilla, in the older patients. Signed informed consent was obtained from the parents of the affected children for publication of the images.

Figure 4

MRIs in patients. A. Sagittal images showing spectrum of vermis and pons hypoplasia. Number represents the number of the patient. Figure 9 shows MRI of patient 9 at age 4 months and figure 9b patient 9 at age 11 years. Note that in all patients, the pons is very small but has a relative sparing of his buldging, mainly in its superior part. Hypoplasia predominates at the inferior part of the pons. Vermis hypoplasia is very variable, severe in patient 13, very slight in patient 10-11-12 and predominates at the inferior part. V4 is open in most cases. B. Coronal images showing spectrum of cerebellar hemispheric hypoplasia. Number represents the number of the patient. Hemispheres are frequently asymmetric. Note that the vermis does not protrude from the hemispheres indicating similar involvement of the vermis and the hemispheres. This pattern is different from that of PCH2 in which the vermis is relatively spared leading to the classic image of "dragonfly", the protruding vermis being the body of the dragonfly and the hemispheres, the wings. There is no progression of the lesions between successive MRI in patient 9.