Introduction
Lower urinary tract dysfunction may manifest as dysfunctional voiding describing the difficulty to void the urinary bladder due to a dyssynergic striated urethral sphincter-pelvic floor complex but with no clear neurological or anatomical abnormalities [
1]. It can occur in any gender [
2] at any age. Prevalence data vary widely in different studies (4.2 to 32% in children with wetting problems [
3]), mainly because of different study designs and unspecific definitions of the disease. Patients present with difficulty in initiating a void, urinary frequency, urgency, incontinence, and large residual urine volume [
1].This may lead to recurrent urinary tract infections [
4] or, in about one third of the cases, to vesicoureteral reflux [
5]. Severe complications as hydronephrosis or end stage renal failure occur in about 8% [
6].
The causes of voiding dysfunction are heterogeneous. Although learnt and habitual patterns are more frequent [
4], congenital cases are reported [
7].
Here, we studied a father and his dizygotic twin sons, all presenting with early onset of dysfunctional voiding, indicating an inherited mode of the disease. In addition, both sons presented with absence seizures and bilateral spasticity of their lower limbs in the later course. This prompted us to perform whole exome sequencing (WES) in all three, the father and his twin sons.
Results
By WES and subsequent filtering, we identified 13 heterozygous variants segregating between father and sons (Suppl. Table
2). Among these variants, we prioritized a novel, missense variant c.271C>A (p.Leu91Met) in
DSTYK as probably disease-causing. The other 12 variants were either not novel or located in genes that are known to cause diseases not overlapping our patients’ phenotypes. For variants in two genes (
HSF3,
EYA3), a correlation with a certain phenotype is not identified (Suppl. Table
1). Reported patients with
DSTYK variants in literature, however, have similarities with our patients [
9‐
11]. This variant is neither reported in gnomAD v2.1.1./v3.1 nor in dbSNP (last check: 19.02.2021). The amino acid change affects a highly conserved leucine present down to X. tropicalis and zebrafish but absent in birds. The variant was predicted to be deleterious by three different prediction programs (SIFT (0,02), PolyPhen (0,794), and MutationTaster (damaging)) and reached a CADD-PHRED score of 24.5. Sanger sequencing confirmed the variant in all three, the father and his two sons (Fig.
1B). Potentially disease-causing variants in genes, known to be causative for hereditary spastic paraplegia (according to Mackay-Sim) [
8], could not be identified in the brothers.
Discussion
Here, we report a father and his two dizygotic twin sons presenting with early onset lower urinary tract dysfunction due to dysfunctional voiding. Moreover, in the later course of the disease, both sons presented with bilateral spasticity in their lower limbs, brisk reflexes, and seizures. Exome analysis identified a novel, heterozygous missense variant in DSTYK.
DSTYK encodes dual serine/threonine and tyrosine protein kinase. Hitherto, it has been associated with autosomal-dominant congenital anomalies of the kidney and urinary tract (CAKUT) (MIM:610805) comprising solitary kidney, renal hypodysplasia, ureteropelvic junction obstruction, vesicoureteral reflux, and congenital hydronephrosis [
9,
10]. Interestingly, Sanna-Cherchi et al. (2013) reported three patients with epilepsy from a family of seven affected CAKUT patients, all carrying a heterozygous splice side variant in exon 2, and one with early-onset ataxia additional to an ureteropelvic junction obstruction, carrying a heterozygous stop variant in exon 1 (Fig.
1C, Suppl. Table
3) [
9]. However, it must be mentioned that the splice side variant has also been found in patients with no renal pathologies [
12]. If this is explained through incomplete penetrance is not yet fully understood. Therefore, pathogenic
DSTYK variants in CAKUT patients were reclassified as variants of unknown significance [
13]. Here, the two affected sons presented with absence epilepsy. At the time of writing, the father denied an EEG examination for himself, not allowing for a final statement on the presence or absence of epilepsy in the father. As mentioned earlier, mutations in
DSTYK have been described to cause autosomal-recessive hereditary spastic paraplegia 23 (HSP23) (MIM:270750) [
11]. The disease spectrum contains spastic paralysis of the lower limbs, scoliosis, peripheral neuropathy, and abnormal skin and hair pigmentation, but no patients with epilepsy are reported so far [
11,
14‐
17].The phenotypic spectrum occurring in the presented family neither overlaps the classical CAKUT spectrum, as anatomical causes for the voiding dysfunction were excluded, nor does it present with the so far known genetic inheritance model of HSP23. Still, the neurological abnormalities of the lower limbs and the voiding dysfunction are suggestive of a HPS23-related phenotype.
Previous, functional studies on DSTYK revealed that it is expressed in mesenchymal-derived cells of all major organs (heart, lung, liver, colon, kidney, skin) [
9]. In the genitourinary tract, expression was detected in developing mouse kidney as well as in human pediatric renal tubular epithelia and urothelium and smooth muscle of the ureter [
9]. It is also widely expressed in central nervous system structures, but remarkably high in cerebellum and cerebral cortex [
18]. Thus, suggesting that involvement in organ development and organ maintenance could be possible. Functional studies showed that it is a positive regulator of ERK phosphorylation downstream of fibroblast growth factor receptor activation and further is involved in the induction of caspase-dependent apoptosis as well as caspase-independent cell death pathways [
19]. Dstyk KO mouse model are fertile and have no significant morphological defects. While only sections of the brain were conducted and the urinary tract was not assessed separately, homozygous mice show impaired capabilities of learning and memory [
20]. Zebrafish Morpholino knockdown showed growth retardation, abnormal morphogenesis of the tail, cloacal malformations, defects in jaw development, and loss of the median fin fold. Pericardial effusion in 5-day-old morphant larvae was interpreted as attributable to both heart and kidney failure [
9].
While patients expressing HSP23 phenotype only were found to carry deletions of the last two exons, the variants identified in CAKUT patients do not seem to cluster to a certain domain or part of the gene (Fig.
1C). Therefore, further functional studies of DSTYK exploring function and regulation are needed to determine the effects of heterozygous stop and missense variants as well as the homozygous deletions. Until then, the exact pathomecanism that would explain recessive deletions and dominant missense variants to cause the same phenotype remains speculative. Nevertheless, over 30 disease genes have been described so far with both recessive and dominant causative variants within the same gene [
21]. As allelic heterogeneity may be explained in part by the functional consequences of pathogenic variants, we assume that the protein harboring deletions undergo nonsense mediated decay. Therefore, heterozygous carrier of this variant might not develop a phenotype as regulatory processes possibly could increase the amount of transcription, whereas, with missense variants, only half of the transcribed protein is fully functional and therefore already pathogenic in a heterozygous state. Regarding to a possible overlap of the phenotypes, it is also interesting to note that almost 10% of the highly expressed genes in early kidney development (in ureteric bud and metanephric mesenchyme) are also associated with neuronal growth and/or differentiation [
22].
Until now, it remains unclear if CAKUT patients with a variant in
DSTYK might develop neurological problems with late onset as the three patients described by Sanna-Cherchi et al. (2013) were diagnosed for epilepsy in their thirties [
9]. Moreover, incomplete penetrance in this family was suggested (Suppl. Table
3). Also, in the sons of the here described family, besides being diagnosed for epilepsy in first decade of life, further neurological problems hinting towards HSP first became obvious at the end of second decade of life. Therefore, they were not considered as HSP patients until WES was performed and did not receive any specific HSP treatment.
Conclusion
Thus, it might be discussed whether CAKUT patients with potentially causative variants in DSTYK should be additionally investigated and monitored towards neurological symptoms. Further, patients with bladder dysfunction should be investigated for additional neurological symptoms, possibly occurring in the further course.
To the best of our knowledge, this is the first report of early onset dysfunctional voiding, seizures, and bilateral spasticity of the lower limbs associated with a novel heterozygous dominant missense variant in DSTYK.
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