Abstract
Alu-mediated tandem duplication of exons 4 and 5 (g.15815_22218dup6404) is a novel mutation that has been detected in the LAMP2 gene (Xq24). This exon copy number variation was found in two brothers with the typical phenotype of Danon disease, including characteristic myocardial changes on magnetic resonance imaging. The 6.4 kb duplication was identified in both boys by a combination of exon dosage qPCR analyses and duplication breakpoint/junction mapping. The rearrangement results in a plethora of abnormal LAMP2 splicing variants and also in use of likely cryptic splice sites in the 3′ terminus of LAMP2 gene. Although we found minute amounts of normal LAMP2B and LAMP2A mRNAs, no protein was detectable in peripheral blood leukocytes by flow cytometry in both brothers. Uniquely, the fraction of LAMP2-deficient granulocytes (0.06 %) assessed by flow cytometry in the patients’ asymptomatic mother substantially differed from the random distribution of X-chromosome inactivation in her leukocytes. This discrepancy was later explained by molecular genetic methods as a consequence of mosaic distribution of the mutation in her somatic tissues. Altogether, we report a novel and mosaically distributed exon copy number rearrangement in the LAMP2 gene and comment on obstacles this genetic setup presents to the overall cellular and molecular diagnostic algorithm of Danon disease. Our observations of the mosaicism in the asymptomatic mother suggest that similarly affected females could be a potentially under-diagnosed Danon disease carrier group and that LAMP2 flow cytometry, because of its supreme sensitivity, can be an efficient method for pedigree screening.
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Abbreviations
- bp:
-
base pair
- CDS:
-
coding sequence
- CMR:
-
cardiac magnetic resonance
- CNV:
-
copy number variation
- CP:
-
crossing point value
- DD:
-
Danon disease
- ECG:
-
electrocardiography
- LGE:
-
late gadolinium enhancement
- LAMP:
-
lysosomal-associated membrane protein
- LV:
-
left ventricle
- WBC:
-
white blood cells
- qPCR:
-
quantitative polymerase chain reaction
- wt:
-
wild-type
- XCI:
-
X-chromosome inactivation
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Acknowledgements
Milan Elleder, sadly deceased in September 2011 fundamentally contributed to the LAMP2 protein studies and thus to DD diagnosis in the family described in this manuscript. This work was primarily supported by Research Project no. 0021620806 and the PRVOUK-P24/LF1/3 and MH CZ–DRO VFN64165 projects. OP and TK are supported by MH CR, DRO 00064203 and OPPK CZ.2.16/3.1.00/24022 (University Hospital Motol, Prague, Czech Republic) projects. TP and PK are supported by the PRVOUK-P35/LF1/5 project.
We would like to acknowledge Josef Zamecnik M.D., Ph.D. (Department of Pathology and Molecular Medicine, 2nd Faculty of Medicine, Charles University in Prague, University Hospital Motol in Prague, Czech Republic).
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Communicated by: John Christodoulou
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Majer, F., Pelak, O., Kalina, T. et al. Mosaic tissue distribution of the tandem duplication of LAMP2 exons 4 and 5 demonstrates the limits of Danon disease cellular and molecular diagnostics. J Inherit Metab Dis 37, 117–124 (2014). https://doi.org/10.1007/s10545-013-9617-z
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DOI: https://doi.org/10.1007/s10545-013-9617-z