RNA polymerase III leukodystrophy (POLR3-HLD), also known as 4H leukodystrophy, is a hypomyelinating leukodystrophy resulting in a spectrum of neurological and extra-neurological manifestations with an age of onset typically in early childhood. Several leukodystrophies described in the 2000s are now grouped under this leukodystrophy since they have similar clinical characteristics and are caused by mutations in the same genes: the 4H syndrome (Hypomyelination, Hypodontia and Hypogonadotropic Hypogonadism), ADDH (Ataxia, Delayed Dentition and Hypomyelination ), TACH leukodystrophy (Tremor-Ataxia with Central Hypomyelination ), leukodystrophy with oligodontia and HCAHC syndrome ( Hypomyelination with Cerebellar Atrophy and Hypoplasia of the Corpus Callosum ).

A large study on more than 100 patients has allowed us to better understand the spectrum of this disease.

The neurological clinical features of POLR3-HLD include:

significant cerebellar manifestations (ataxia or problem with balance, dysarthria or difficulty pronouncing words correctly, dysmetria  or imprecision of movements), with or without tremor, pyramidal manifestations such as spasticity ( stiffness) and brisk reflexes, as well as so-called extra-pyramidal dystonia-like manifestations (stiffness in the arms and legs, which fluctuates with associated abnormal emotions and postures). Non-neurological features of the disease include:

• dental abnormalities (examples: small teeth, missing teeth, delayed eruption of teeth abnormalities in the order of tooth eruption, etc.),
• eye abnormalities (myopia),
• endocrine abnormalities (example: small size) and pubertal abnormalities (puberty arrest or absence of puberty). POLR3-HLD is caused by recessive mutations in the POLR3A and POLR3B genes. To date, more than 100 patients with this disease have mutations in one or the other of these genes. The POLR3A and POLR3B genes encode the two largest subunits of an enzyme called RNA polymerase III, and together form the active center of the 17-subunit complex. No patient has two null mutations, that is, two mutations that would result in the complete absence of the protein for which the gene codes. This is not surprising given the essential role of RNA polymerase III: the transcription of DNA encoding small RNAs such as transfer RNAs, 5S, U6 and 7SK. These small RNAs are important for the survival of the cell.

Work has begun on the link between mutations in the POLR3A or POLR3B / hypomyelinating leukodystrophy gene. The representation of the mutations found on three-dimensional modeling of polymerase III suggests that the mutations may have an effect on the assembly of the enzyme by altering the interactions between the subunits, or even have an effect on the binding of DNA with the complex, thereby resulting in abnormal transcription of DNA into RNA.

Preliminary results suggest that, at least for one mutation, the first hypothesis seems correct, that a mutation in the POLR3A gene leads to deficient assembly of the complex and, in this case, to migration failure of the complex in the nucleus of the cell, where the polymerase does its job.

Our hypothesis is that mutations in POLR3A or POLR3B lead to an abnormal transcription of certain RNAs, such as transfer RNAs, important for the development of myelin (myelination). We have set up several types of experiments to study these small RNAs from fibroblasts (skin cells from patients and healthy controls) and it seems that there are small variations in the transcription of some RNAs. However, as the skin was unaffected in our patients, and now having mice with mutations in POLR3A, we will now repeat these experiments with myelin and gray matter from diseased mouse brains.

Note that the involvement of transfer RNAs is also suspected in other hereditary diseases involving the cerebral white matter such as LBSL (Leukoencephalopathy with Brainstem and Spinal cord involvement and Lactate elevation) and in two other hypomyelinating leukodystrophies called HBSL ( Hypomyelination with brainstem and spinal cord abnormalities and leg spasticity) and RARS-associated hypomyelination, caused by mutations in the DARS and RARS genes, respectively.

Finally, we have now started the experiments in POLR3A mice, exhibiting motor difficulties. We are going to study its characteristics, its brain. This will allow us to advance our understanding of the pathophysiology of the disease.

The discovery of the associated POLR3-HLD genes has enabled many patients and their families to obtain molecular diagnosis and appropriate genetic counseling. Clinical, radiological, and pathophysiological studies are still ongoing in order to better understand the extent of clinical and radiological manifestations, genetic abnormalities and of course, the pathophysiology of this group of diseases in order to be able to develop therapeutic strategies.