POLR3B-associated leukodystrophy: clinical, neuroimaging and molecular-genetic analyses in four patients: clinical heterogeneity and novel mutations in POLR3B gene.

INTRODUCTION AND AIM OF THE STUDY: White matter disorders represent a spectrum of neurological diseases frequently associated with an unfavourable prognosis and a delay in diagnostics. We report the broad phenotypic spectrum of a rare hypomyelinating leukodystrophy and three novel mutations. Further, we aim to explore the role of the combined clinical and neuroimaging diagnostic approach in the era of whole exome sequencing. MATERIALS AND METHODS: We present a clinical, neuroimaging and molecular-genetic characterisation of four patients from three families suffering from a rare genetic leukoencephalopathy. Two severely affected siblings (P1, P2) manifested a profound developmental delay, cerebellar symptomatology, microcephaly, failure to thrive, short stature and delayed teeth eruption with oligodontia. The other two patients (P3, P4), on the contrary, suffer from substantially less serious impairment with mild to moderate developmental delay and cerebellar symptomatology, delayed teeth eruption, or well-manageable epilepsy. In all four patients, magnetic resonance revealed cerebellar atrophy and supratentorial hypomyelination with T2-weight hypointensities in the areas of the ventrolateral thalamic nuclei, corticospinal tract and the dentate nuclei. RESULTS: Using whole-exome sequencing in P1, P2 and P3, and targeted sequencing in P4, pathogenic variants were disclosed in POLR3B, a gene encoding one of 17 subunits of DNA-dependent RNA polymerase III - all patients were compound heterozygotes for point mutations. Three novel mutations c.727A>G (p.Met243Val) and c.2669G>A (p.Arg890His) (P1, P2), and c.1495G>A (p.Met499Val) (P3) were found. Magnetic resonance revealed the characteristic radiological pattern of POLR3-leukodystrophies in our patients. CONCLUSION AND CLINICAL IMPLICATIONS: The diagnosis of POLR3-associated leukodystrophies can be significantly accelerated using the combined clinical and neuroradiological recognition pattern. Therefore, it is of crucial importance to raise the awareness of this rare disorder among clinicians. Molecular-genetic analyses are indispensable for a swift diagnosis confirmation in cases of clear clinical suspicion, and for diagnostic search in patients with less pronounced symptomatology. They represent an invaluable tool for unravelling the complex genetic background of heritable white matter disorders.

Mutations in ANTXR1 cause GAPO syndrome.

The genetic cause of GAPO syndrome, a condition characterized by growth retardation, alopecia, pseudoanodontia, and progressive visual impairment, has not previously been identified. We studied four ethnically unrelated affected individuals and identified homozygous nonsense mutations (c.262C>T [p.Arg88*] and c.505C>T [p.Arg169*]) or splicing mutations (c.1435-12A>G [p.Gly479Phefs*119]) in ANTXR1, which encodes anthrax toxin receptor 1. The nonsense mutations predictably trigger nonsense-mediated mRNA decay, resulting in the loss of ANTXR1. The transcript with the splicing mutation theoretically encodes a truncated ANTXR1 containing a neopeptide composed of 118 unique amino acids in its C terminus. GAPO syndrome's major phenotypic features, which include dental abnormalities and the accumulation of extracellular matrix, recapitulate those found in Antxr1-mutant mice and point toward an underlying defect in extracellular-matrix regulation. Thus, we propose that mutations affecting ANTXR1 function are responsible for this disease's characteristic generalized defect in extracellular-matrix homeostasis.

Fatal neonatal nephrocutaneous syndrome in 18 Roma children with EGFR deficiency.

Epidermal growth factor receptor (EGFR) is a transmembrane glycoprotein with tyrosine-kinase signaling activity, involved in many cellular functions including cell growth and differentiation. Germ line loss-of-function mutations in EGFR lead to a severe neonatal skin disorder (Online Mendelian Inheritance in Man #131550). We report 18 premature Roma children from 16 families with birthweights ranging 440-1470 g and multisystem diseases due to the homozygous mutation c.1283G˃A (p.Gly428Asp) in EGFR. They presented with thin, translucent, fragile skin (14/15), skin desquamation (10/17), ichthyosis (9/17), recurrent skin infections and sepsis (9/12), nephromegaly (10/16) and congenital heart defects (7/17). Their prognosis was poor, and all died before the age of 6 months except one 13-year-old boy with a severe skin disorder, dentinogenesis imperfecta, Fanconi-like syndrome and secondary hyperaldosteronism. Management of ion and water imbalances and extremely demanding skin care may improve the unfavorable outcome of such patients.