What’s new in pontocerebellar hypoplasia? An update on genes and subtypes

PCH1 is characterized by PCH with in addition bulbar and spinal motor neurodegeneration identical to spinal muscular atrophy (SMA) as seen in routinely stained spinal cord sections. Early reports describe PCH1 as a neonatally lethal disorder with polyhydramnios, congenital contractures, respiratory failure and severe muscle hypotonia [6, 11]. Later studies describe sparing of the ventral pons and survival into puberty, thereby broadening the clinical and neuroradiological spectrum of PCH1 [12, 13]. Currently, four genes are associated with PCH1 (PCH1A-D). Mutations in the vaccinia-related kinase 1 gene (VRK1, OMIM # 607596)) are a very rare cause of PCH1; VRK1 mutations are described in one consanguineous PCH1 family of Ashkenazi Jewish origin [14]. The phenotype of these patients was atypical for PCH: patients had severe microcephaly from birth on while cognitive development seemed only mildly affected [14]. Later, VRK1 mutations turned out to be predominantly associated with motor neuron disease without PCH [15, 16]. Subsequent studies found pathogenic mutations in EXOSC3 (OMIM # 614678) in roughly half of the patients with PCH1 [12, 13, 17]. EXOSC3 encodes component 3 of the exosome complex, which is involved in mRNA degradation. Mutations in EXOSC3 were clinically associated with prolonged survival (mean age at death was nine months in patients with mutations versus three months in patients without EXOSC3 mutations) [13]. Respiratory failure, congenital contractures and polyhydramnios were rarely reported in the EXOSC3 mutation group [13]. Cerebellar hypoplasia was variably present and the pons was unaffected in a part of the patient group, depending on the genotype (Fig. 2a-d) [12, 13]. Mutations in EXOSC8 (OMIM # 61608), like EXOSC3 encoding a component of the exosome complex, were found to cause psychomotor retardation, severe muscle weakness, spasticity, hearing and vision impairment and motor neuron degeneration in 22 patients from three families. Neuroimaging showed hypoplasia of the cerebellum and corpus callosum, cortical atrophy and immature myelinisation [18]. Mutations in SLC25A46 (OMIM 610826) were shown to cause lethal pontocerebellar hypoplasia [19]. Mutations in this gene were also found to be the underlying genetic cause in the original Dutch PCH1 family, which was described by Barth to delineate the PCH1 phenotype as a distinct subtype of PCH [20]. With the identification of SLC25A46 mutations, for the first time a genetic basis for the most severe spectrum of the PCH1 phenotype was revealed.

Probably PCH2A is the most prevalent and best characterized of all PCH subtypes. PCH2A is caused by homozygosity for a founder mutation (p.A307S) in the TSEN54 gene (OMIM # 277470), which was identified by linkage analysis in 9 affected families from a Dutch genetic isolate [21]. Clinically, PCH2A is distinguished by generalized clonus and incoordination of sucking and swallowing in the neonate. The toddler and young child suffer from spasticity, dystonia/chorea and epilepsy and show a lack of voluntary motor development and grasping and almost complete absence of cognitive development [21‐23]. Patients often suffer from feeding problems requiring PEG feeding. In PCH2A sleeping disorders, recurrent infections, apneas and problems in temperature regulation are reported in the majority of patients. Microcephaly, usually absent in the neonatal period, is progressive, and caused by supratentorial atrophy. Mean OFD at the age of 5 years was − 5.58 SD in a cohort of 33 patients studied by Sánchez and colleagues [24]. Life expectancy is reduced: more than 50% of patients die before puberty [24]. Homozygosity for the p.A307S mutation is strongly correlated with a ‘dragonfly’ configuration of the cerebellum on brain MRI, resulting from severely affected hemispheres (‘the wings’) and relative sparing of the vermis (‘ the body’; Fig. 2e-f) [22]. TSEN54 is part of the transfer-RNA (tRNA) splicing endonuclease complex: a complex involved in splicing of intron containing pre-tRNAs. The TSEN complex consists of four protein subunits: two catalytic (TSEN2 and TSEN34) and two structural (TSEN15 and TSEN54) ones. (Trotta 1997, Cell). Mutations in TSEN2, TSEN34 and TSEN15 rarely occur, resulting in a phenotype similar to PCH2 [22, 25, 26]. These subtypes are classified as PCH2B, PCH2C and PCH2F, respectively (respectively OMIM # 612389, # 612390 and # 617026) [26]. Classification of the PCH2 subtypes was done chronologically, based on the order of the identification of respective disease genes. PCH2D is caused by mutations in the SEPSECS gene (OMIM # 613811) and comprises a clinical spectrum with variable severity and some patients lack pontine hypoplasia [27‐29]. In addition, patients with cognitive impairment and initial normal motor development with pediatric onset ataxia and motor decline have been described [29]. VPS53 mutations (OMIM # 615851) were identified in four families of Jewish Moroccan ancestry and resulted in a PCH-like phenotype (PCH2E) [30]. VPS53 is one of the four subunits composing the Golgi-associated retrograde protein complex, involved in transporting endosomes to the trans-Golgi network. In all ten patients, pregnancy was uncomplicated and head circumference was normal at birth. During the first months of life, a delay in motor and cognitive development became apparent. Progressive spasticity, spastic quadriplegia, multiple contractures, progressive microcephaly and seizures evolved during infancy. Osteoporosis and scoliosis were reported. Initial brain imaging was normal, but cerebellar atrophy was noted in the first year of life followed by cerebral atrophy. All patients had profound mental retardation [30].

Pontocerebellar hypoplasia type 3, in the literature also referred to as cerebellar atrophy and progressive microcephaly (CLAM), is characterized by pontocerebellar hypoplasia/atrophy, thin corpus callosum, progressive microcephaly, seizures, small stature, facial dysmorphism and in some patients optic nerve atrophy. The extrapyramidal movement disorders that are typically seen in PCH2 are absent [31‐33]. In 2015, it was shown that mutations in the PCLO (#608027) gene underlie PCH3 in a sibship form the Sultanate of Oman [31]. In 2009, a Turkish patient was described with a highly similar phenotype to the Omani patients [33]. The locus was mapped to the same region on chromosome 7q, in the region where the PCLO gene is located, but it remains unknown whether aberrations in this gene underlie the phenotype in this patient as well.

Pontocerebellar hypoplasia type 4, previously reported in the literature as olivopontocerebellar hypoplasia due to involvement of the inferior olivary nuclei, is at the severe end of TSEN54-related PCH spectrum. Clinically, PCH4 presents as a severe form of PCH2, with prenatal onset of symptoms including polyhydramnios and congenital contractures, prolonged neonatal clonus, hypertonia and primary hypoventilation requiring prolonged mechanical ventilation. Survival beyond the neonatal period is rare [22]. Neuropathology in PCH4 shows delayed maturation and reduced growth of the cerebral cortex. Pericerebral fluid accumulation is regularly seen, and probably results from shrinkage of the brain due to severe atrophy [22]. Extensive gliosis and a decreased number of neurons in the thalamus and caudate nucleus have been reported [34]. The cerebellar hemispheres were severely hypoplastic and devoid of Purkinje cells, while at most only remnants of the dentate nucleus could be identified [35, 36]. Hypoplasia of the cerebellar vermis is variable; but overall the vermis is more severely affected than in PCH2 (Fig. 2g-h) [22]. An important neuropathological difference with PCH2 is the lack of folding of the inferior olivary nuclei, resulting in a horseshoe shape [35]. The more severe clinical course in PCH4 is reflected by the type of mutation in TSEN54. In PCH2A, patients are homozygous for the p.A307S missense mutation. In PCH4, patients are compound heterozygous with on one allele this hypomorphic missense mutation and a premature stop- or splice site (loss-of-function) mutation on the other allele. The latter combination of mutations is more detrimental for protein function and results in a more severe phenotype of PCH4 compared to PCH2. The distinction between PCH4 and PCH5 was initially based upon the presence of prenatal seizures and a more severely affected vermis compared to the hemispheres in a single PCH5 family [37]. However, Namavar et al. showed that, as in PCH4, compound heterozygous TSEN54 mutations, one being hypomorphic and the other being a loss-of-function mutation, underlie PCH5. The distinction of PCH5 as a separate subtype is therefore redundant [38].

The canonical PCH6 phenotype consists of severe early onset epilepsy, progressive global atrophy including pons and cerebellum, lactic acidosis and/or mitochondrial respiratory chain defects [39]. PCH6 is caused by mutations in the nuclear encoded mitochondrial Arginine tRNA-synthetase (RARS2), which is responsible for catalyzing the specific attachment of Arginine to its cognate mitochondrial tRNA [40]. Currently, 31 patients with RARS2 mutations (OMIM # 611523) have been reported [39‐49]. None of them have biallelic null mutations, implicating that complete abolishment of mitochondrial tRNA-arg would be lethal. Splice site, nonsense or missense mutations are identified throughout the RARS2 gene, while no clear genotype-phenotype correlation could be established [49]. All patients surviving the neonatal period had severe developmental delay, and almost all suffered from refractory epilepsy. The majority of patients had elevated lactic acid in plasma or CSF. Respiratory chain deficiency in fibroblasts or muscle could not be confirmed in all these patients. Brain MRI showed cerebellar hypoplasia/atrophy of variable severity. Some early MRIs were normal or showed mild vermal hypoplasia, with often rapidly progressive atrophy of supra- and infratentorial structures (Fig. 2i-j) [49].

PCH7 is characterized by the rare combination of PCH with disorders of sex development (DSD). Patients show a severe developmental delay, profound truncal hypotonia with hypertonic limbs and brisk deep tendon reflexes, and seizures. Disorders of sex development were most apparent in 46, XY patients, who showed undervirilisation ranging from almost completely differentiated female external genitalia to micropenis with hypoplastic scrotum. In some XY patients, uterine or ovarian remnants could be found, while others had atrophic testes or no gonadal tissue at all [50‐53]. Clinical information on XX patients was provided for three affected individuals. Two had normal external female genitals, and one patient had a prominent clitoris. In one patient the ovaries could not be detected by ultrasound, while the uterus was present [52]. Patients of both sexes manifest profound hypoplasia of pons and cerebellum, a thin corpus callosum and enlarged ventricles with irregular borders as a consequence of reduced cerebral white matter (Fig. 2k-l) [51, 52]. Only one neuropathological report is available, provided by Anderson in 2011, which showed cerebral atrophy with a profound reduction in white matter. The cerebellar hemispheres were almost absent; only rudimentary folia devoid of neurons could be identified. The vermis was relatively spared. The ventral pons lacked pontine nuclei, while the pontine tegmentum was relatively normal. The inferior olivary nuclei could not be identified [51]. In 2017, biallelic mutations in TOE1 (OMIM # 614969) were reported in 12 families with PCH7 [52].

Only three families with PCH8 have been reported. Patients have microcephaly, a severe developmental delay (although some patients were able to walk independently), dystonic posturing and/or choreiform movements. Some patients had (congenital) contractures and seizures. In contrast with the other PCH subtypes, PCH8 seems to be a stable neurologic condition without clear evidence of disease progression. Brain imaging showed profound pontocerebellar hypoplasia, a thin but fully formed corpus callosum and a decrease in cerebral white matter in all patients. In patients who had multiple brain MRIs, no clear signs of ongoing neurodegeneration were noted [54]. PCH8 might be considered as a ‘non-degenerative’ form of PCH and is caused by mutations in the CHMP1A gene (OMIM # 614961). CHMP1A is involved in chromatin modelling and cell proliferation [54].

PCH9 is characterized by progressive microcephaly, profound neurodevelopmental delay, cortical visual impairment, impaired swallowing, spasticity with neonatal clonus, brisk deep tendon reflexes and truncal hypotonia. Facial dysmorphisms are reported with dental abnormalities in a minority of patients. The presence of axonal neuropathy is reported in older patients and is probably age-dependent [55‐57]. Brain MRI showed pontocerebellar hypoplasia, severe hypoplasia of the corpus callosum with, on axial images, a characteristic ‘figure 8’ shape of the mesencephalon, with hypoplastic cerebral peduncles (Fig. 2m-n) [55‐58]. PCH9 is caused by mutations in the AMPD2 gene (OMIM # 615809), which encodes one of the three adenosine monophosphate deaminases. Interestingly, mutations in the same gene have also been associated with spastic paraplegia 63 (SPG63; OMIM #615686).

Nine families of Turkish origin have been reported with PCH10 [59, 60].Mutations in CLP1 has been identified as the causal gene defect (OMIM # 615803). CLP1 interacts with the TSEN complex and is also involved in tRNA splicing (see below). Clinical characteristics are a severe developmental delay, seizures, progressive spasticity, facial dysmorphism, microcephaly and signs of axonal neuropathy of both motor and sensory neurons. Brain MRI showed a thin corpus callosum, delay in myelinisation and hypoplasia of the ventral pons/thinning of the brainstem. Atrophy of the cerebellum is, if present at all, very mild compared to the other subtypes of PCH.

Seven families with PCH11 have been reported. Patients were homozygous for truncating or splice site mutations in the TBC1D23 gene (OMIM # 617695) [61, 62]. PCH11 is characterized as another non-degenerative form of PCH and is characterized by a severe neurodevelopmental delay, microcephaly and hypotonia. Seizures were reported in 2/13 patients. Some patients were able to walk alone. Specific cerebellar symptoms like ataxia were reported in roughly half of the patients; this implies that some degree of voluntary movement was present in those patients. Brain MRI showed non-progressive hypoplasia of pons and cerebellum with hypoplasia of the corpus collosum [61, 62].

No curative treatment is available for any type of PCH. Treatment is symptomatic in all subtypes. PCH2A is the most prevalent subtype of PCH, and the only type in which adequate patient numbers allowed a systematic study of its clinical course. PCH2A patients often suffer from feeding problems starting immediately after birth requiring gavage or PEG feeding. Sleep apnoea, which is a frequently reported and life-threatening complication in PCH, and which may lead to sudden infant death, can be detected by sleep monitoring [24]. Seizures in PCH2A are difficult to treat, but a relatively good effect on phenobarbital and topiramate is reported (resulting in a reduction in seizure frequency in 11 out of 15 and 6 out of 7 patients, respectively) [24]. Choreathetoid movements are present in about 90% of patients. [22, 24] Opisthotonic posturing can be prevented by flexure of the large joints. An ergonomically shaped wheelchair and physiotherapy can be beneficial in achieving this. In one PCH2 patient, an improvement of dystonic crises and choreoathetosis upon treatment with Levodopa was reported [63]. Rhabdomyolysis and strongly elevated serum creatine kinase have been repeatedly reported and should be carefully monitored, especially during episodes of infection [23, 64‐66]. Muscle biopsies and serum CK measurements of some PCH2 patients without any muscular symptoms showed evidence for a subclinical myopathy in a part of the PCH2 population [65].

PCH9 has been considered to be a potentially treatable disorder, because administration of a purine nucleotide precursor (AICAr) could rescue the phenotype at a cellular level. Follow-up experiments are needed, because it was shown that administering of AICAr tends to inhibit axonal growth in mouse cultures neurons [55].

Cerebellar hypoplasia and cerebellar atrophy are both relatively common and nonspecific findings occurring in a very heterogeneous group of disorders [67]. Hypoplasia refers to a static condition in which the cerebellum has a normal shape but is too small, while atrophy refers to progressive loss of cerebellar tissue, resulting in progressive widening of interfolial spaces. The distinction between hypoplasia and atrophy may be easy to make on theoretical grounds; in practice however this can be challenging or even impracticable when based on a single brain imaging study.

Neuropathology reports in PCH2A mention both hypoplastic features with shortening of cerebellar folia with reduced branching (‘stunted folial growth’) and degenerative changes like thinning of the cerebellar cortex [35]. A mechanism of prenatal onset neurodegeneration is assumed, which is reflected by the ongoing postnatal neurodegeneration of the cerebral cortex, resulting in progressive microcephaly and widening of the lateral ventricles. The use of the term ‘hypoplasia’ in PCH is explained by historic reasons: at the time of the first descriptions of PCH, the concept of prenatal onset neurodegenerative disorders was yet unknown. Therefore, it was common to describe this congenital defect as ‘hypoplasia’.

Many metabolic disorders and genetic syndromes display cerebellar hypoplasia/atrophy. Careful assessment of the neuroimaging pattern is essential to evaluation. In PCH vermal growth is relatively spared compared to the cerebellar hemispheres, while in disorders like Joubert syndrome and Dandy-Walker malformation the vermis is mainly affected. The molar tooth sign, caused by elongation and a more horizontal course of the superior cerebellar peduncle, is characteristic of Joubert syndrome. While cerebellar hypoplasia has a broad differential diagnosis, the presence of pontine hypoplasia significantly narrows down the diagnostic options. Pontine hypoplasia affects the basis of the pons, and is mainly due to loss of ventral pontine neurons and transverse pontine fibers. However, the combination of pontine and cerebellar hypoplasia, does not automatically result in a diagnosis of PCH as discussed here. The use of the term ‘PCH’ is not uniform which may give rise to confusion. Some of the disorders that share pontine and cerebellar hypoplasia as a feature on MRI are discussed below (and Table 2).

PCH is a very rare disorder. The exact incidence of PCH is unknown. PCH2A caused by homozygosity for the p.A307S mutation in TSEN54 is the most frequent type of PCH (Fig. 4). The estimated incidence of PCH2A is < 1:200.000 [24]. We identified mutations in PCH related genes in 42 patients in routine diagnostic care between 2012 and 2017. Twenty out of those 42 patients were homozygous for the p.A307S mutation, 13 patients had mutations in EXOSC3, four patients harboured mutations in RARS2, one patient had SEPSECS mutations and one had mutations in AMPD2. We also identified hetero- or hemizygous CASK mutations in nine patients, indicating that this is a frequent cause of a condition similar to PCH.

Counselling regarding recurrence risk should be provided and reproductive options discussed. Preconception carrier screening is offered for PCH2A in a closed community in the Netherlands [111]. Since 2010, seven carrier couples were identified in this community. In families were the causal genetic defect is identified, preimplantation genetic diagnosis (PGD) or invasive prenatal testing should be offered for family planning. Prenatal detection of PCH by ultrasound is unreliable, since it often fails to detect cerebellar abnormalities at time of the routine 20 weeks screening for structural abnormalities [111‐113].