HLD7 is an autosomal recessive neurodegenerative disorder characterized by childhood onset of progressive motor decline that manifests as spasticity, ataxia, tremor, and cerebellar symptoms, as well as mild cognitive regression. Other features include hypodontia or oligodontia and hypogonadotropic hypogonadism. Initially POLR3-related leukodystrophy were included originally, such as 4H; ataxia, delayed dentition, and hypomyelination (ADDH); leukodystrophy with oligodontia (LO); tremor-ataxia with central hypomyelination (TACH); and hypomyelination with cerebellar atrophy and hypoplasia of the corpus callosum (HCACH) syndromes, because of different clinical phenotypes. They are now known as the same entity and referred to as POLR3-related leukodystrophy, and also known as 4H leukodystrophy because they have phenotypes in common and the same molecular foundation, which are commonly caused by mutations in POLR3A, POLR3B, and POLR1C [7, 8, 12–14]. Patients exhibit a wide range of severities and onset time ranges from the neonatal period to late childhood. Mean age with gross motor delay or regression of the majority is before 6 years. Ten percent of the patients had an onset of beyond 10 years [1, 15].
In this study, we present a female child from the southern district of China who displayed severe neurological manifestations. Before 1 year of age, she presented with reduced motor ability, and then started to show prominent cerebellar signs, including nystagmus, motor ataxia, dysarthria, and spastic paralysis of the upper and lower limbs. Delayed dentition and development figures, prominent body hair, and hypertonia of both the upper and lower limbs were also observed. She had not attained complete head control and required assistance to sit. To date, she is two and a half years of age, does not have ambulation ability, and is bedridden. Brain MRI indicated loss of white matter and hypomyelination of white matter in the anterior angle of the bilateral posterior ventricle. Since hypomyelination is a nonspeciﬁc ﬁnding that affects patients, brain MRI of approximately one-third of patients with hypomyelination did not result in a deﬁnitive diagnosis . The identiﬁcation of further clinical and genetic characteristics will be fundamental for the classiﬁcation of this group of diseases . Therefore, we performed medical exome sequencing and found novel compound heterozygous mutations of the POLR3A gene. We therefore concluded the diagnosis and identified the compound heterozygous variants as the causative variants for the disease in this patient. It is noteworthy that this disease has mostly been reported in European populations, including French-Canadian, Caucasian, and Syrian individuals [1, 4]. Occasional cases have been reported in the Indian population [18–20]. However, this is the first case reported in a Chinese family.
POLR3-related leukodystrophy is a hypomyelinating leukodystrophy characterized by neurological (cerebellar, extrapyramidal, pyramidal, and cognitive) and non-neurologic (dental, endocrine, and ocular) features. Neurological impairment of our case started in the infantile period with marked hypotonia, global developmental delay, and congenital ataxia. Although cerebellar signs of this case became progressively obvious, cerebellar atrophy was not observed, which may be due to the young age. Previous studies have found that cerebellar anomalies were more severe in patients with POLR3B defects while the pattern of hypomyelinization was more evident in the MRI of patients with POLR3A mutations [1, 3]. This may be another explanation for our case. Other than neurological symptoms, patients also show extra neurologic symptoms, including hypodontia with abnormal tooth eruption shape, hypogonadotropic hypogonadism, myopia, and short stature. Our patient also showed classical extraneurologic features, characterized by hypodontia with delayed tooth eruption and short stature. She also displayed polytrichia, an atypical feature of HLD7, which may be due to aberrant gonadal function or other reasons; gonadal function was not assessed. Hypogonadotropic hypogonadism was not detected because she was too young. Previous studies have also shown that the syndrome may or may not be associated with hypodontia and/or hypogonadotrophic hypogonadism in many cases [6, 7]. She did not show myopia and optic atrophy; this is inconsistent with most cases, which are usually accompanied by myopia . Her dysphagia phenotype was striking, which is rare. She had obvious difficulty with tube feeding and forceful vomiting occurred frequently. This is likely due to the incoordination of swallowing of cerebellar syndrome, or due to other unpredictable reasons. Bronchodysplasia is another feature first observed in HLD7, suggesting that it was not recognized previously in the HLD7 spectrum. Thus, in addition to the classical extraneurological features, abnormal body hair and visceral smooth muscle features should be carefully looked for in patients with POLR3-related disorders. Our case presented with severe manifestation at early onset and diverse manifestations among those of patients with Pol III-related leukodystrophy, which may be a result of the genotype identified in this patient; further analysis is necessary.
POLR3A, POLR3B, and POLR1C mutations are associated with Pol III-related leukodystrophies. The former two genes encode the two largest subunits of Pol III, which is composed of 17 subunits, while defects in the POLR1C gene impair assembly and nuclear import of POLR3 and thereby lead to decreased binding to its target genes . Pol III transcribes a series of small non-coding RNAs (i.e., tRNAs, 5S RNA, 7S RNA, U6 RNA), which participate in the regulation of vital cellular processes, such as transcription, RNA processing, and translation , leading to a series of neuronal and non-neurological features. There are 62 pathogenic variants associated with POLR3-related leukodystrophy, and 14 different mutations in the POLR3A gene have been reported to be associated with 4H syndrome patients [7, 10]. To date, most of the identified mutations are point mutations in the codon region; however, non-coding DNA variants are suspected to account for a substantial portion of undiscovered causes of rare diseases . Minnerop et al. identified mutations in deep intronic regions of POLR3A as a common cause of hereditary spastic paraplegia and cerebellar ataxia, and > 80% of POLR3A mutation carriers presented the same deep intronic mutation (c.1909+22G>A), which leads to a novel, distinct, uniform, and severe phenotype . Jay AM et al. also reported alteration of mRNA splicing in POLR3A causing neonatal progeroid syndrome with severe clinical manifestations . In this study, we identified the c.1771-6C>G (NM_007055) mutation adjacent to the mRNA splice site demonstrating that exploring non-coding genomic regions was helpful in revealing the causes of related hereditary diseases.
The complexity of clinical phenotypes and the heterogeneity of genotypes raise new challenges in genetic diagnoses. In the present study, medical exome sequencing was used to explore the possible genetic defects resulting in the disease of the patient. Compared to whole genome and whole exome sequencing, medical exome sequencing focuses on clinical interpretable regions of genes; less variants of uncertain significance in medical exome sequencing greatly improve the diagnostic yield and increase the coverage depth of sequencing, improving the accuracy of sequencing and broadening the spectrum of variants. In the present study, we identified novel heterozygous mutations of POLR3A that caused HLD7 disease for the first time in a Chinese family. This study will further our understanding of the molecular mechanisms of HLD7 and contribute to further analysis of phenotype–genotype correlations of related disorders.