Grand paternal inheritance of X-linked myotubular myopathy due to mosaicism, and identification of necklace fibers in an asymptomatic male

Father-to-daughter transmission in late-onset OTC deficiency: an underestimated mechanism of inheritance of an X-linked disease

BACKGROUND

Ornithine Transcarbamylase Deficiency (OTCD) is an X-linked urea cycle disorder characterized by acute hyperammonemic episodes. Hemizygous males are usually affected by a severe/fatal neonatal-onset form or, less frequently, by a late-onset form with milder disease course, depending on the residual enzymatic activity. Hyperammonemia can occur any time during life and patients could remain non- or mis-diagnosed due to unspecific symptoms. In heterozygous females, clinical presentation varies based on the extent of X chromosome inactivation. Maternal transmission in X-linked disease is the rule, but in late-onset OTCD, due to the milder phenotype of affected males, paternal transmission to the females is possible. So far, father-to-daughter transmission of OTCD has been reported only in 4 Japanese families.

RESULTS

We identified in 2 Caucasian families, paternal transmission of late-onset OTCD with severe/fatal outcome in affected males and 1 heterozygous female. Furthermore, we have reassessed the pedigrees of other published reports in 7 additional families with evidence of father-to-daughter inheritance of OTCD, identifying and listing the family members for which this transmission occurred.

CONCLUSIONS

Our study highlights how the diagnosis and pedigree analysis of late-onset OTCD may represent a real challenge for clinicians. Therefore, the occurrence of paternal transmission in OTCD should not be underestimated, due to the relevant implications for disease inheritance and risk of recurrence.

Update on the Genetics of Congenital Myopathies

The congenital myopathies form a large clinically and genetically heterogeneous group of disorders. Currently mutations in at least 27 different genes have been reported to cause a congenital myopathy, but the number is expected to increase due to the accelerated use of next-generation sequencing methods. There is substantial overlap between the causative genes and the clinical and histopathologic features of the congenital myopathies. The mode of inheritance can be autosomal recessive, autosomal dominant or X-linked. Both dominant and recessive mutations in the same gene can cause a similar disease phenotype, and the same clinical phenotype can also be caused by mutations in different genes. Clear genotype-phenotype correlations are few and far between.

New variant of necklace fibres display peculiar lysosomal structures and mitophagy

Here, we describe a new variant of necklace fibres with specific myopathological features that have not been described thus far. They were observed in two patients, from two independent families with identical DNM2 (dynamin 2) mutation (c.1106 G > A (p.Arg369Gln)), displaying mildly heterogeneous clinical phenotypes. The variant is characterized by lysosomal inclusions, arranged in a necklace pattern, containing homogenous material, devoid of myonuclei. The so-called necklace region has a certain characteristic distance to the sarcolemma. Electron microscopy, including three dimensional reconstructions of serial section images highlights their ultrastructural properties and relation to neighbouring organelles. This new pattern is compared to the previously reported patterns in muscle biopsies containing necklace fibres associated with MTM1- and DNM2-mutations.