Background
Hereditary spastic paraplegia (HSP) is a large, genetically heterogeneous group of neurodegenerative diseases characterized by retrograde degeneration of axons of motor neurons of the corticospinal tract [
1]. A rare subgroup of the HSP recessive-forms was described with a brain-imaging marker of thin corpus callosum (TCC), for which a set of few genes has been identified so far [
2]. Genetic defects in nucleotide metabolism related genes have been only recently implicated in HSP pathogenesis [
3]. Nucleotidases are a group of hydrolases classified according to their subcellular localization and sites of cleavage. The 5’ nucleotidases catalyze the hydrolysis of 5’ ribo- and deoxyribo-nucleotide monophosphate into the corresponding nucleoside. The 5’ nucleotidase cytosolic II (NT5C2) enzyme has a critical role in maintaining the balance of nucleotides, nucleosides and free nucleobases of purine’s pools in the brain and spinal cord [
4]. NT5C2 catalyzes the hydrolysis of Adenosine Monophosphate and Inosine Monophosphate releasing Adenosine. Adenosine has been recently recognized to have a key role in promoting myelin formation in the central nervous system (CNS) [
5]. Adenosine inhibits proliferation of oligodendrocytes progenitor cells (OPC), whereas it stimulates their differentiation into mature oligodendrocytes and modulates the communication of the neuron and glial cells with the axons [
6‐
8]. This study describes a consanguineous Qatari family with a provisional clinical diagnosis of complex-HSP, in which the HSP-genes panel testing failed to detect the underlying gene defect. By using whole genome sequencing (WGS) and ingenuity variant data analysis (IVA) we were able to identify a novel homozygous splice site mutation in
NT5C2 that recessively segregates in the family. The
NT5C2 gene involvement in HSP families from Middle East has been reported only once before, however; this is the first report in Qatari patients and demonstrates new clinical findings and interesting observations.
Discussion
The NT5C2-related phenotype has previously been clinically assigned to SPG45 (MIM: 613162). This report describes a homozygous novel
NT5C2 splice-site mutation in two Qatari siblings with AR-HSP. This mutation resulted in markedly altered stability of the enzyme-protein.
NT5C2 involvement in recessive HSP was reported once before in a large HSP-cohort study [
3]. The splice mutation identified in this study is located at the C-terminal half of the protein and led to in-frame skipping of exon 14. The significantly deficient expression of mutant NT5C2 shown in in-vitro overexpression experiment highlights the substantial impact of exon 14 skipping on protein stability and/or its proper folding; hence a loss of function mutant is assumed.
The six reported
NT5C2 mutations, so far, (3 and this report) (Additional file
3: Table S1) seems to lead to the same path of loss of function. Even so the impact of previously described
NT5C2 mutations on the encoded protein was not tested, however they were predicted to be deleterious and impair the protein function [
3].
Phenotypic features of delayed and dysarthric speech, persistent truncal hypotonia, variable-sized patches of skin brownish discoloration and the early-onset, markedly severe spasticity were first described in our patients, expanding the phenotypic spectrum of SPG45.
The skin patches started to appear at 6 years of age; hence it was detectable only in the older patient. There was no clinical reason found for those patches. Altered nucleotide metabolism might be an explanation; follow up with the younger patients might provide a clue.
The capacity to maintain the walking ability despite the marked spasticity (this report) and even in the longest reported disease duration [
3,
10] is of a good prognostic value to SPG45 families with NT5C2 mutations.
Studying the brain imaging of the two patients versus the unaffected carrier brother was quite interesting. Brain MRI findings strongly support the observation that homozygous mutation involving the two copies of
NT5C2 is essential to producing the developmental defects in cerebral white matter with dysgenic/TCC. By contrast, the heterozygous status of the mutation in the unaffected brother was associated with normal development of cerebral white matter (Fig.
1.2). Brain images in our cases backed by reports of previously described families with available MRI [
3] distinguish SPG45 (
NT5C2) as an additional member of SPG-TCC subgroup.
Taken together, the mature oligodendrocytes are the main myelin forming cells in the brain and Adenosine is both a modulator of OPC development and a potent neuron-glial-axonal transmitter, it is likely that the normal adenosine and purines’ pool concentrations in the brain has an influence on the normal developmental process of myelin formation in the CNS. Further experiments are necessary to verify this assumption.
We recognized a striking similarity in the key clinical features involving TCC, markedly severe spasticity and long disease duration with maintained ability of unsupported walking in families with NT5C2 (SPG45) or DDHD2 (SPG54) mutations [
11,
12]. Mutated phospholipase DDHD2 was suggested to affect both the dynamics and/or morphology of Golgi and ER in a retrograde or anterograde transport mechanism [
13,
14] as well as the complex lipid metabolism [
15,
16]. Whether there might be a common mechanism potentially linking the NT5C2 loss of function, disturbed purines’ signaling to the transport along the corticospinal tract and/or membrane trafficking, remains a hypothesis awaiting further studies.
Conclusions
The present report confirms the critical role of cytosolic 5’nucleotidase and nucleotide metabolism in the normal development of central white matter structures and warrants further experiments to explore a potential role of 5’ nucleotidase in transport and/or maintenance along the corticospinal tract. The NT5C2 c.1159 + 1G > T splicing mutation presents a phenotype of markedly severe and early onset spasticity, persistent truncal hypotonia, delayed-dysarthric speech and skin patches of darkened discoloration. These characteristic features expanding the phenotypic spectrum of SPG45. The present family emphasizes SPG45 with NT5C2 mutations as a member of the TCC-SPG subgroup. Of the notable observation is the good prognosis of the recessive complex SPG45 in terms of mild cognitive impairment with some learning difficulties and the maintained unsupported walking ability, however with abnormal gait.
Acknowledgements
The authors are thankful to family members for their participation in the study. We thank Ms. Amira Assad, the study research coordinator and Dr. Adam Larson for his critical reading of the manuscript.