Introduction
Hypomyelination, hypogonadotropic hypogonadism, and hypodontia is an autosomal recessive hypomyelinating disorder first reported in children and adults by Wolf et al. and Timmons et al. [
1,
2]. These patients had a hypomyelinating leukodystrophy with prominent cerebellar features and followed a progressive course. At that time, no molecular cause was identified. In 2006, Timmons et al. [
2] suggested the term “4 H syndrome”, although it should be noted that the non-neurological features hypodontia and/or hypogonadism are not always present [
2,
3].
Several syndromes causing diffuse hypomyelination have been documented, including hypomyelination, hypodontia, and hypogonadotropic hypogonadism (4H) syndrome (MIM 612440) [
1,
2,
4], hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC) (MIM 612438) [
5,
6], diffuse cerebral hypomyelination with cerebellar atrophy and hypoplasia of the corpus callosum [
7,
8], tremor–ataxia with central hypomyelination [
9], Pelizaeus–Merzbacher disease and Pelizaeus–Merzbacher-like syndrome [
10,
11], among others.
The diagnosis of 4H syndrome is based on clinical findings and magnetic resonance imaging (MRI). Recently, mutations of the genes encoding POLR3 (RNA polymerase III) subunits POLR3A (
POLR3A), subunit POLR3B (
POLR3B) and POLR1C (
POLR1C) have been identified as the genetic causes of this disorder [
3,
8,
12‐
18]. POLR3 is an enzyme responsible for transcription of specific noncoding small RNAs involved in the regulation of essential cellular processes (transcription, RNA processing, translation) [
19]. It is suggested that mutations in
POLR3A, POLR3B, and
POLR1C lead to abnormal POLR3 function and abnormal production of proteins important for development of the central nervous system white matter [
18,
20].
In 2014, Wolf et al. published a multinational cross-sectional observational study on the clinical, molecular, and MRI characteristics of 105 patients with mutation-proven 4H leukodystrophy caused by mutations in
POLR3A or
POLR3B. A total of 62 patients had mutations in the
POLR3B gene [
21]. In our publication we report two Polish female siblings diagnosed and treated at the Children’s Memorial Health Institute in Warsaw, Poland, with compound heterozygous mutations in
POLR3B. They both presented with compatible clinical and MRI features of 4H leukodystrophy, together with polymicrogyria (PMG) and cataracts, which have been never reported in 4H patients before.
Discussion
4H or POLR3-related leukodystrophy is inherited in an autosomal recessive fashion and is caused by mutations in one of three genes encoding RNA polymerase III (POLR3) subunits, that is,
POLR3A, POLR3B, and
POLR1C [
8,
12‐
17]. Daoud et al. [
3] noticed that POLR3A mutations are more frequent but a multinational cross-sectional observational report published in 2014 by Wolf et al. [
21] concluded that patients from European backgrounds were more likely to have
POLR3B mutations than other populations. The two patients we investigated with mutations in
POLR3B genes showed diffuse hypomyelination of the cerebral and cerebellar white matter, hypodontia, and cataracts. Children were prepubertal, thus hypogonadotropic hypogonadism could not be assessed. Interestingly, pituitary volume in the older sister is below normal and in the younger sister is in the borderline small range [
25]. All genetic, neuroimaging, and clinical features were characteristic for 4H leukodystrophy, except for PMG and cataracts. Mild atrophy of cerebellar hemispheres and vermis together with enlarged folia was observed in the older girl, while diffuse hypomyelination of cerebral white matter with unmyelinated corpus callosum were seen in both sisters. Such imaging findings are typical for 4H leukodystrophy and have been previously described in the literature [
3,
17,
21].
In addition to the characteristics of 4H leukodystrophy, both patients presented bilateral frontoparietal PMG. PMG is a common cortical malformation characterized by an excessive number of abnormally small gyri. PMG may be uni- or bilateral, symmetrical or asymmetrical, focal or diffuse. Any region of the cerebral cortex can be affected. It can be caused by congenital infections,
in utero ischemia or could be genetic in origin [
26]. Several genes have been associated with PMG, including
GPR56, SRPX2, TUBB2B, TUBB3, PAX6, TBR2, KIAA1279, NHEJ1, RAB3GAP1, EOMES, COL18A1, and
TUBA8 [
26‐
29].
According to previously published data [
3,
21,
31] myelination of the optic radiation was seen in almost all 4H patients (95 % of patients in a paper published by Wolf et al. [
1] in 2014 [
21]. We also noticed narrow, slightly T2-hypointense and poorly visible signal of the optic radiation. Relative T2 hypointensity of the anterolateral thalamus was seen in 91 % of the patients [
21] and relative myelin preservation in the dentate nucleus was visualized in 93 % of the patients [
20]; such features were also present in both our patients. We did not observe myelination of the corticospinal tracts at the level of the PLIC. Our results are in contrast to previously published papers, which reported hypointense dots in the PLIC. Focal myelination of the corticospinal tracts at the level of internal capsules was seen in 70 % of the patients with
POLR3B mutations [
3,
21,
31]. Mild atrophy of cerebral hemispheres, seen only in the older sister, was described by Wolf et al. [
21] who had mentioned that supratentorial atrophy was rarely seen before the age of 10 years. We observed unmyelinated corpus callosum in both our patients with overall thinning in the older and partial thinning in the younger patient (splenium and isthmus). Thinned corpus callosum was found in all ten patients with
POLR3B mutations in a paper published by Daoud et al. [
3]. Wolf et al. [
1] noted thinning of corpus callosum in all patients above 17 years of age, and concluded that among children below the age of 10 years thin corpus callosum is a more frequent finding in
POLR3A than
POLR3B mutations [
21]. Mild atrophy of the vermis and cerebellar hemispheres were seen in the older patient while a deep primary fissure, although within normal limits, was noted in the younger patient. This is somewhat in contrast with Wolf et al. [
21], who had found cerebellar atrophy in all
POLR3B (except for one 3-year-old), although median age at last MRI examination among those patients was 10 years. Our investigations did not fully corroborate the observations published by Takanashi et al. [
17] in a small group of patients concerning different patterns of cerebellar abnormalities and hypomyelination between
POLR3A and
POLR3B mutations. The authors found small vermis and cerebellar hemispheres with thin folia and enlarged fissures in all the patients with
POLR3B mutations. They also suggested that patients with
POLR3B mutations were affected by milder hypomyelination than those with
POLR3A mutations. These observations stand in opposition to our findings, since our patients presented diffuse hypomyelination. The discrepancy in the size and atrophy of cerebellar hemispheres and vermis may be possibly explained by the ages of our patients (5 and 10 years), compared with older patients (16, 28, 31 years) investigated by Takanashi et al. [
17]. It will be interesting to investigate the sisters when they get older.
PMG is a new neuroradiological finding in both our cases as it has been never reported in association with 4H leukodystrophy. It is our opinion that the PMG in these girls is most likely not due to the
POLR3B mutations. Indeed, Wolf et al. has reviewed over 100 cases of POLR3-related leukodystrophy and has never encountered another case with PMG. However, since subtle PMG may be difficult to detect on MRI, it needs meticulous reading and is better seen with MRI protocols not typically performed in leukodystrophy patients, it is not possible to completely rule out that mutations in
POLR3B are also causative for the PMG [
17,
21]. PMG can be due either to genetic aberrations or to other causes (e.g., infections such as cytomegalovirus (CMV)). PMG has not been described in association with
POLR3 mutations in a paper published by Bizotto et al. [
30] in 2015. We, therefore, suspect that patients have mutations in two different genes
(POLR3B and another gene causing PMG
).
Both girls were hypotonic at birth, suffer from microcephaly and their developmental delay was observed during the 1st year of life. However, other neurological symptoms, such as ataxia and very intense tremors occurred later (Table
1). The age of onset ranged from 1 to 13 years in papers published by Bernard et al. and was estimated to be below 4–5 years in cases published by Terao et al. [
12,
15]. Severe clinical symptoms and neurological deterioration were present, and developed so early in childhood that the older sister had to use a wheelchair at the age of 8 years. The younger patient could still walk at the age of 6 years despite progressive gait disturbances. The same observation was made by Tetreault et al. [
13], who reported three European patients with
POLR3B mutation presenting with mild developmental delay in early childhood and developed dysarthria as well as progressive motor dysfunction, including cerebellar ataxia. In all, two of these three patients showed progressive spasticity. In an article previously published by Saitsu et al. [
8], the patient began to walk unsteadily at the age of 11 months, but retained her ability to walk as a teenager. In a paper published by Takanashi et al. [
17] three patients with
POLR3B mutations aged 16, 28, and 31 years could walk. In the multinational cross-sectional observational study published by Wolf et al. [
21] in 2014, wheelchair dependence appeared at a mean age of 14 years. Only three patients with cataracts have been described to date [
21,
32]. From a clinical point of view, our observations concerning cataracts are very important. Both sisters’ cataracts developed very rapidly. The cause of such rapid cataract development has not been yet identified. Furthermore, as our patients are hypothesized to have mutations in two different genes, it is unclear whether the cataracts are caused by the
POLR3B mutations or are rather caused by the second gene mutation(s) leading to PMG, microcephaly, and dysmorphic features. Our female patients demonstrated dental abnormalities very typical for 4H leukodystrophy. Wolf et al. [
21] described dental abnormalities in 87 % of patients, the majority (72 %) presenting with hypodontia. Our sisters did not have neonatal teeth, but they do suffer from hypodontia. Hypodontia and/or hypogonadism are not always present, as reported in previous articles [
3,
8]. In 4H leukodystrophy developmental delay was noted in half of cases (52 % according to the data published by Wolf et al. [
21]) and intellectual disability is also very common. We noted developmental delay in our patients at the age of 1 year and intellectual disability at 4 years. These two Polish girls presented with characteristic cerebellar features, such as ataxia and severe tremors. Pyramidal signs were not observed. However, our patients are young and, according to Wolf et al. [
21], pyramidal signs are usually absent in young children. We did not observe epileptic seizures, although the presence of PMG and the abnormal EEG pattern observed, clearly indicate that they are at risk. Epilepsy is not a characteristic feature of 4H leukodystrophy; it was noted in 19 % of the patients [
21]. Musculoskeletal signs are sometimes observed in 4 H leukodystrophy, such as hip dysplasia and flat valgus feet, as seen in our patients. At their last evaluations, both patients had (at the age of 11 years and 6 years) short statures and low weights (endocrine tests are normal in the older sibling), but hypogonadotropic hypogonadism as well as delayed puberty will be monitored when the time comes even if hypogonadotropic hypogonadism is sometimes absent. Indeed, delayed puberty was found in 69 % of the patients with
POLR3B mutations [
21].