To the Editor
We read with great interest the article by Filocamo et al. [1] in the November 2018 issue of Italian Journal of Pediatrics. In this manuscript, the authors mention that a complication for molecular analysis of NAGLU gene (associated with Sanfilippo B syndrome or also called mucopolysaccharidoses type IIIB) is the presence of a pseudodeficiency allele, constituted by p.Ser141Ser and p.Arg737Gly polymorphisms simultaneously, which leads to a reduced level of the alpha-N-acetyl-D-iglucosamnidase activity.
Anzeige
We have been working on the diagnosis of mucopolysaccharidoses (MPS) over the past 35 years, and during this time, MPS diagnosis was confirmed in 1184 Brazilian patients, of which, 192 were affected by one of the four subtypes of MPS III [2]. As part of our MPS III investigation, after detection of heparan sulphate glycosaminoglycan in urine, we performed different enzyme activity assays in leukocytes in order to reach the definitive diagnosis and to determine the MPS III subtype [2]. Following this step, approximately 40% of our MPS III patients, were analyzed by targeted-next generation sequencing (NGS), using a customized gene panel containing SGSH, NAGLU, HGSNAT and GNS which was designed to simultaneous sequence the entire coding regions plus 20pb of intron-exon junction of these genes. Pathogenic variants were detected in all the cases. Therefore, we have extensive biochemical and molecular data of our group of patients.
Based in our data, p.Ser141Ser (rs659497) and p.Arg737Gly (rs86312) variants were present in homozygosis in all our MPS IIIB patients analyzed by NGS. For these patients, the enzyme activities were below the reference range, as they are clinically affected by MPS IIIB as proven by mutation analysis.
On the other hand, we have patients with other types of MPS III (IIIA, IIIC and IIID) where the same alterations that are said to cause the pseudodeficiency allele were present in homozygosis. These variants were detected because patients had clinical phenotype of MPS III and were analyzed in the same NGS panel used for the 4 types of MPS III. For those, the NAGLU enzyme activity was within the normal range when compared to normal controls as shown in Table 1.
Table 1
Genotype and alpha-N-acetyl-D-glucosaminidase activity for MPS III patients
n | p.Ser141Ser (c.423 T > C) | p.Arg737Gly (c.2209C > G) | alpha-N-acetyl-D-glucosaminidase activity in leukocytes (reference value: 10–34 nmol/17 h/mg protein) | |
|---|---|---|---|---|
MPS IIIB patients | 12 | C/C | G/G | 0,25 ± 0,17 |
Other MPS III patients | 18 | C/C | G/G | 14,5 ± 3,5 |
2 | C/C | C/G | 19,8 ± 3,2 | |
2 | T/C | C/G | 16,5 ± 1,5 |
These two variants were also present in homozygosis in one MPSIIIA patient from Turkey and two MPS IIIB from Ecuador.
Anzeige
For our group of MPS III patients, allele frequencies of p.Ser141Ser and p.Ar737Gly were 0.9706 and 0.9412, respectively. In addition, Exome Aggregation Consortium (ExAC) reported that frequency for p.Ser141Ser is 0.9947 and 0.9055 for p.Arg737Gly [3]. Furthermore a Brazilian mutation database (ABraOM), which comprised the exomic variants of a cohort of 609 healthy Brazilian elderly, reported 602 homozygotes for p.Ser141Ser (0.9942) and 459 homozygotes for p.Arg737Gly (0.8686) [4].
Pseudodeficiency alleles are not rare in Lysosomal Disorders, including several MPS disorders [5‐10], nevertheless the high frequency of both polymorphisms in worldwide populations and the fact that the presence of these alleles does not alter the MPS IIIB enzyme activity when present, suggests that these two variants are not producing a pseudodeficiency allele. Thus, no false positive results and no alteration on the protein product or changes in gene expression are detected in the presence of these two variants.
Acknowledgements
Not applicable.
Funding
This study was partially supported by FIPE-HCPA (grant # 2013–0254) and Brazilian Institute for Population Medical Genetics (INAGEMP).
DRM is supported by CAPES post-doc scholarship. SLS receives a Research Grant from CNPq.
Availability of data and materials
Data sharing is not applicable to this article as no datasets were generated or analysed during the current study.
Ethics approval and consent to participate
This study was approved by the institutional ethics committee of Hospital de Clínicas de Porto Alegre, Brazil which is recognized by the Office for Human Research Protections as an Institutional Review Board (IRB0000921). All patients provided written informed consent (protocol number 2003–0066).
Consent for publication
Not applicable.
Anzeige
Competing interests
The authors declare that they have no competing interests.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.