Introduction

Protocadherins (PCDHs) are one of a group of calcium-dependent cell adhesion transmembrane protein belonging to the cadherin superfamily, and divided into clustered PCDH and non-clustered PCDH based on their genomic structures [1,2,3]. Both groups of the PCDHs are widely expressed in vertebrate nervous systems and play roles in neural development and neural circuit formation [1]. PCDH12 is a member of the non-clustered PCDH. However, its function in the central nervous systems (CNS) in human is yet to be clearly understood [2].

Aran et al. [4] first reported 10 Palestinian patients from four families who had loss of function variant of PCDH12. All four families shared the same homozygous variant in the PCDH12 gene. They showed congenital microcephaly, hyper-echogenic brain foci in utero, and intrauterine growth retardation. After birth, progressive microcephaly, intracranial calcification [5], and neonatal seizure with virtually no developmental milestones were evident. Brain imaging showed dysplastic, elongated midbrain with poor distinction between the crus cerebri of midbrain, hypothalamus, and optic tract. Here we report a Japanese patient with compound heterozygous truncating variants of PCDH12, which is thought to be highly pathogenic and may expand clinical spectrum of PCDH12-related disorders.

Case report

This 14-year-old Japanese boy was fourth child of healthy and un-consanguineous parents. He was born after 38 weeks of pregnancy without asphyxia. The birth weight, body length, and occipito-frontal head circumference (OFC) were 3114 g (+0.3 SD), 50 cm (+0.5 SD), and 34 cm (+0.5 SD), respectively. At 2 months of age, he showed epileptic spasms, which appeared two to three times a day. On brain computed tomography, he had spot calcification affecting right posterior limb of internal capsule and bilateral perithalamic regions as well as chronic subdural hematoma of the left hemisphere (Fig. 1). Brain magnetic resonance imaging (MRI) at the same period showed chronic subdural hematoma with normal myelination pattern. Along with increased subdural hematoma, his seizures changed to left hemiconvulsion with eye deviation to the right. The interictal electroencephalography (EEG) showed beta range rhythmic activity in bilateral parietal area independently. Anticonvulsants (valproic acid, later switched to zonisamide) were started on the diagnosis of focal epilepsy, which resulted in partial control. At 3 months old he received On Meyer reservoir placement to treat increased subdural effusion. After reservoir placement, his seizure was well controlled.

Fig. 1
figure 1

Brain CT at 2.5 months old. White arrowheads pointed chronic subdural hematoma. Spot calcification in posterior limb of internal capsule and bilateral perithalamic regions are indicated by black arrows

Full size image

Because of severe developmental delay, he was referred to our hospital for physiotherapy at the age of 9 months, when he showed no head control, eye contact, or purposeful hand use, characterized with asymmetrical tonic neck reflex and opisthotonic posture in some occasions. He was diagnosed as cerebral palsy (CP) with dyskinetic quadriplegia and severe psychomotor retardation. Brain MRI at 3 years old showed asymmetric basal ganglia and lateral ventricle and high intensity of central tegmental tract (CTT) on T2WI (Fig. 2). There were no abnormalities in infratentorial structures (Fig. 2). His EEG at 7 years old showed frequent spikes in the occipital and frontal areas independently during sleep. At the age of 5 and 7, he had status epilepticus, which started with vomiting and eye deviation to the right, followed by left hemiconvulsion. Despite of those episodes, his seizure was completely controlled by using topiramate and phenobarbital.

Fig. 2
figure 2

Brain MRI at 3 years old, axial T2WI (a), FLAIR (b), coronal T2WI (c), and sagittal T1WI (d). Asymmetric basal ganglia and lateral ventricle are indicated. High intensity of the central tegmental tract on T2WI was shown with white arrow. He had no midbrain-hypothalamus dysplasia. The infratentorial structures, optic nerve, and corpus callosum are considered as normal

Full size image

He had swallowing difficulty, which was evident on video fluorography and likely caused by dyskinetic quadriplegia, and nasogastric tube feeding was administered since 7 years old. He gained social smile and visual pursuit in late childhood.

At present, he had profound psychomotor retardation with no meaningful words, epilepsy with some sporadic seizures, dyskinetic quadriplegia characterized with axial hypotonia, variable tonus, occasional opisthotonic posture, and athetotic movement as well as growth failure; his body weight, height, and OFC were 16 kg (−3.3 SD), 116 cm (−5.7 SD), and 47.2 cm (−5.3 SD), respectively. He showed unstable head control, no achievement of sitting alone, self-standing, or purposeful hand use. He did not done any trachea surgery and his pharyngeal strider was lasting. He could step when seated in the posture control walker. He had some dysmorphic features (hypertelorism, broad nasal bridge, large mouth, and widely spaced teeth).

To clarify the etiology of the patient, we performed trio-based whole-exome sequencing and identified five possible candidate variants, including one de novo variant in the GPI gene and four compound heterozygous variants in the PCDH12 and MYH4 genes. Among them, compound heterozygous frameshift variants in PCDH12, c.448_449del:p.(Leu150Alafs*11) and c.522_525del:p.(Ser175Profs*22), were predicted to be deleterious (Table S). Segregation of PCDH12 variants was confirmed by Sanger sequencing. These two variants were absent in 575 in-house Japanese control exomes and public databases, including dbSNP137 data, ESP6500, 1KJPN, and The Exome Aggregation Consortium. We evaluated the pathogenicity of these two PCDH12 variants in accordance with the ACMG variant classification guideline. Both variants were classified as pathogenic.

Discussion

The present patient had new compound heterozygous truncating variants in the PCDH12 gene, associated with dyskinetic CP, severe intelligence disability, multifocal epilepsy, and severe growth failure. Brain imaging showed asymmetric basal ganglia and lateral ventricle, and high intensity of CTT on T2WI.

Several studies mentioned about the function of non-clustered PCDHs; axonal growth in striatal neurons (PCDH10) or amygdala (PCDH17), synapse function in hippocampal (PCDH8) or basal ganglia (PCDH17), cell migration in gastrulation (PCDH8) or ventral telencephalon (PCDH10), and cortical formation (PCDH19) [1, 2, 6]. These findings show that non-clustered PCDHs play roles as the wiring of neural circuit formation with region-specificity and spatiotemporal diversity, and this motility depends on the cell adhesion [1, 2, 6, 7].

PCDH12, previously called vascular endothelial cadherin-2, was first identified in mouse endothelial cells [8]. Studies in mice showed PCDH12 expression in angiogenic endothelium, placental glycogen cells, and messangial cells of renal glomeruli but barely detectable in brain [8, 9]. PCDH12-deficient mice showed reduced placental and embryonic size compared with wild-type mice, but no sign of intelligence disability or seizures. It did not affect survival or postnatal growth [9]. Human PCDH12 is located on chromosome 5q31 and encodes a protein of 1184 amino acid, with 81% sequence identity with the mouse [3]. In human, same as mice, PCDH12 showed high levels of expression in the placenta, but it has not been detected in the brain [10]. However, taken together with previous report [4], it is evident that PCDH12 has an important role on the development of CNS, which needs further investigation.

The present patient had some particular findings compared with previous report (Table 1). He had no congenital or postnatal microcephaly and intrauterine growth retardation. His severe growth failure may be explained as a result of hypothalamic pituitary hormonal deficiency, suggesting a continuum of the phenotypic spectrum.

Table 1 Detailed clinical summary and neuroimaging findings of the patient and previously reported patients
Full size table

At brain imaging, midbrain-hypothalamus dysplasia was not evident in this patient, while he had high intensity of CTT. CTT represents extrapyramidal tract connecting the red nucleus and inferior olivary nucleus. High intensity of CTT on T2WI has been detected in several disorders, e.g., CP, hypoxic ischemic encephalopathy, and congenital metabolic disorders [11,12,13]. Autopsy analysis of CTT demonstrated fiber loss, gliosis, and increased vacuoles in tegmental tracts [11]. Derinkuyu et al. reported high intensity of CTT on T2WI was most frequently detected in dyskinetic CP [12]. From these findings, high intensity of CTT may show the results of tegmental fiber injury and be caused by complex pathomechanism, including secondary degeneration [11,12,13].

His truncation mutation of both alleles suggests severe functional damage enough to be pathogenic and the variation of mutation site may partly explain clinical differences. As another explanation, epigenetic dysregulation of PCDH12 is possible, because several studies reported alterations in DNA methylation patterns of PCDHs, which was identified in postmortem brains of individuals with various brain disorders such as chromosome disorders, psychiatric or autism spectrum disorders, child maltreatment, and prenatal alcohol exposure [14].

Previous report highlighted congenital microcephaly and intracerebral calcification as a feature of PCDH12 gene variants. From our experience, however, we need to consider PCDH12 gene variants as a differential diagnosis for patients with dyskinetic CP associated with nonspecific MRI findings. Besides PCDH12 gene variants, several gene variants were previously identified as a candidate for dyskinetic CP, most of which were complicated with epilepsy and intellectual disability [15]. Further case presentations are required to elucidate clinical spectrum of this gene variants.