In this article, we report the case of a Chinese patient with a novel homozygous mutation, c.1549C>T, in RNF216 and review the clinical, neuroimaging and genetic data of all patients with RNF216 mutations published in the literature thus far.
RNF216 disorders span a phenotypic continuum characterized by variable combinations of cerebellar ataxia, chorea, cognitive decline and hypogonadotropic hypogonadism. According to the clinical features, patients with RNF216 mutations were categorized as having GHS or HLD, except for two with 4H syndrome or CHH. Four genes related to GHS include RNF216, either alone or in association with OTUD4, STUB1 and PNPLA6.(2, 11, 12) RNF216-related GHS is characterized by dementia and confluent white matter lesions, which are rare in GHS patients with other gene mutations.(12, 13) In our review, GHS patients with RNF216 mutations developed neurological symptoms in their 20s. Over 90% of those patients typically presented with ataxia, cognitive decline and dysarthria. HLD was another common clinical phenotype of RNF216-related disorders and characterized by chorea, cognitive decline, and psychological and behavioural abnormalities. Interestingly, we found that the average age at the onset of neurological symptoms in HDL patients was approximately 12 years older than that in GHS patients. The HLD group was predominantly female, whereas the GHS group was predominantly male. Despite the clinical heterogeneity, there was also an overlap between different phenotypes. For instance, cognitive decline was very common in both groups. More than half of HLD patients developed ataxia during the course of disease, and one GHS patient had accompanying chorea. Apart from GHS and HLD, a male patient with the homozygous mutation c.2453-2A>G in RNF216 was reported to present with 4H syndrome.(3) However, it is still controversial to classify the patient as having 4H syndrome on the basis of atypical dental abnormalities and imaging manifestations. Nicole et al considered that this patient should be diagnosed with GHS rather than 4H syndrome.(14) Furthermore, a 19-year-old male patient, in whom digenic mutations in RNF216 (c.2374G>A) and SRA1 (c.2374G>A, NM_001035235.3) were observed, presented with CHH without obvious neurological symptoms or MRI abnormalities.(10) Considering the age at the onset of neurological symptoms in other patients with RNF216 mutations, we thought this patient might be too young to show neurological symptoms.
Another prominent feature in patients with RNF216 mutations is hypogonadotropic hypogonadism due to diminished GnRH secretion and pituitary dysfunction.(2) Interestingly, male patients seemed to be more vulnerable than female patients. Most male patients suffered from poor pubertal development. In contrast, the majority of female patients had normal puberty, and three of them delivered successfully.(4, 8) This phenomenon was consistent with the results of previous animal experiments. The targeted deletion of the RNF216 gene in mice resulted in disruption in spermatogenesis and male infertility, but RNF216 was not required for female fertility.(15)
White matter lesions and cerebellar atrophy were the most common imaging findings and were present in over 90% of patients. Cerebral atrophy and the involvement of the brainstem, basal ganglia and thalami were also observed in some patients. However, gadolinium-enhanced lesions or diffusion-restricted lesions were not detected in any patient. Characteristic bilateral white matter lesions surrounding the basal ganglia, which have been reported to be related to Huntington’s disease,(16) were exclusively found in patients with chorea.(4, 6, 9)
RNF216 is a ubiquitin ligase of the RBR class that attaches ubiquitin chains to substrates. For RNF216, various substrates have been reported, including TOLL-like receptors, tumour necrosis factor-receptor associated factor 3 (TRAF3), the autophagy regulator Beclin and the synaptic regulator activity regulated cytoskeleton associated protein (ARC).(1, 17–19) The ubiquitin ligase activity of RNF216 requires not only the RBR domain but also an additional C-terminal extension of the RBR domain.(19, 20) All mutations in patients with GHS, 4H or CHH resulted in amino acid changes or the truncation of the RBR domain/C-terminal extension of the RBR domain, which was presumed to abrogate ubiquitin E3 ligase activity. However, three of seven mutations (c.1367G>A, c.1616A>G and a deletion in exon 2) in HLD patients did not affect the RBR region/C-terminal extension of the RBR domain. Further functional studies confirmed that the two missense mutations (c.1367G>A and c.1616A>G) in HLD patients reversed the E3 ubiquitin ligase activity of RNF216, implying that these mutations caused disease by mechanisms other than by influencing ubiquitin E3 ligase activity.(19) These findings indicated that RNF216-related GHS was strongly associated with mutations affecting the ubiquitin E3 ligase activity of RNF216. However, the pathogenic mechanism underlying RNF216-related HLD is more complicated and remains to be explored in future studies.
In summary, patients with RNF216-related disorders typically presented with GHS or HLD in a monogenic autosomal-recessive pattern or an oligogenic pattern. Mutations in GHS patients were closely related to the RBR domain and ubiquitin E3 ligase activity of RNF216. However, the pathogenic mechanism underlying RNF216-related HLD remains to be explored in future studies.