Clinical report:
The patient presented at 8-month-old boy, who was the fifth pregnancy of
non-consanguineous Canadian (European) parents, with an uneventful
pregnancy and delivery. He presented with a history of hypotonia,
developmental delay, and failure to thrive. He was severely delayed in
motor milestones but appropriate in social and language
development.There was a significant family history of a paternal first
cousin with a undifferentiated muscular dystrophy, the details or
diagnosis of which could not be confirmed. There were no other familial
neuromuscular, genetic or congenital diseases.
The patient had shown signs of hypotonia since birth and at three months
he was admitted to hospital for failure to thrive. Upon presentation to
the neuromuscular clinic, he continued to demonstrate difficulty with
head movement, including holding up his head. He was however, able to
spontaneously move all extremities against gravity.
His general exam was normal. On neurological examination, he was
hypotonic with evidence of poor muscle bulk to his extremities, truncal
hypotonia and head-lag. There were no obvious dysmorphic features or
evidence of tongue fasciculations. Sensation appeared intact. He was a
reflexic with downgoing toes. His mobility was limited to side-to-side
rolling.
The patient underwent multiple admissions for respiratory difficulties
and additional investigations. Given the significant hypoventilation and
inability to increase respiratory effort, the decision was made for the
patient to initiate nighttime BiPAP. At age 9 months he had a
tracheostomy placed and has been stable on intermittent BiPap via
tracheostomy.
The creatine kinase (CK) level was normal. Echocardiography was
normal.Sequencing of the SMN1 gene showed no pathogenic
mutations,andalpha-Glucosidase DBS(dried blood spot) enzyme test was
normal.EMG and nerve conduction studies demonstrated spontaneous
activity in proximal muscles, consistent with possible myopathy. An MRI
of the thigh and pelvic girdle muscles showed no obvious abnormalities.
The patient underwent a quadriceps skeletal muscle
biopsy.Histologically,the muscle demonstrated fairly dramatic fibre-size
variability with both atrophic/hypotrophic (‘small’) and hypertrophic
fibres seen (including scattered fibre-splitting). Fibrosis was
increased among both the endomysium and perimysium. The Gomoritrichrome
staining revealed numerous subsarcolemmal and sarcoplasmic ‘red’
granules and rods, consistent with nemaline rods, primarily in the small
fibres but also noted in the histologically ‘normal’ fibres.
Enzyme histochemistry demonstrated that the small fibres were type 1
(ATPase 4.2 and MHC-s immunohistochemistry) and that the type 2 fibres
(and scattered type 1 fibres) were of relatively normal size. Overall,
there was type 2 fibre predominance with many fibres co-expressing both
slow (MHC-s) and fast (MHC-f) myosin heavy chains.
Prominent sarcoplasmic inclusions were noted among the atrophic fibres
on semi-thin Toluidine blue-stained sections. Ultrastructural analysis
demonstrated abundant and robust Z-line-like electron densities with
lattice morphology. These were seen in highest density among the small
fibres, but also noted among the more ‘normal’ appearing fibres.
. Generally, it is thought that in alpha-tropomyosin NEM cases, the
nemaline inclusions are largely restricted to Type-1 fibres and that the
majority of these fibres are atrophic/hypotropic (small).
Molecular genetics :
Clinicopathological correlation via genetic testing (Nemaline myopathy
panel, Invitae) showed three Variants of Uncertain Significance (VUS)
identified in MEGF10 , NEB and TPM3 , and all of them
were heterozygous. The first was a missense VUS, c.2974G>A
(p.Glu992Lys) identified in MEGF10 .The MEGF10 gene is
known to be associated with autosomal recessive early-onset myopathy,
areflexia, respiratory distress and dysphagia (EMARDD), but not nemaline
rod histology on muscle biopsy. The second was asynonymous VUS,
c.17619C>T (p.Gly5873=) identified in NEB . TheNEB gene is only known to be associated with autosomal recessive
nemaline myopathy 2 (NEM2). The third was a missense VUS,
c.43G>C (p.Asp15His) identified in TPM3 . This
variant was not present in population databases including ExAC and
gnomAD. Algorithms developed to predict the effect of missense changes
on protein structure and function (SIFT, PolyPhen-2,Align-GVGD
andMutationTaster) all suggest that this variant is likely to be
disruptive. The TPM3 gene is associated with autosomal dominant
or recessive NEM1and congenital myopathy with fibre-type disproportion
(CFTD). Absence of a second causative variant in the autosomal recessive
MEGF10 and NEB genes, suggested it is unlikely that these are causative
variants. In contrast, genetic analysis confirmed that neither of the
parents were carriers for the pathogenic TPM3 mutation in
peripheral blood, suggesting a de novo mutation in the affected
child in a gene with known dominant inheritance.