Application of Next-Generation–Sequencing to the Diagnosis of Genetic Disorders: A Brief Overview - Next Generation Sequencing Technologies in Medical Genetics - page 35

evaluation, and muscle biopsy for histologic or histochemical evidence of
mitochondrial disease. If a specifi c phenotype has been recognized, such as Leber
hereditary optic neuropathy (LHON), Kearns-Sayre syndrome (KSS), chronic pro-
gressive external ophthalmoplegia (CPEO), mitochondrial encephalomyopathy
with lactic acidosis and stroke-like episodes (MELAS), myoclonic epilepsy with
ragged-red fi bers (MERRF), neurogenic weakness with ataxia and retinitis pigmen-
tosa (NARP), or Leigh syndrome (LS), the diagnosis can be confi rmed by molecular
genetic testing.
Approximately 70 nuclear genes have been implicated in mitochondrial diseases.
However, many other genes are unknown. Mutations in mtDNA are suspected to
cause adult onset disease. Mutations in nDNA usually cause pediatric disease. As
the more common presentation of mitochondrial disease can be very nonspecifi c, it
makes sense to test for mutations in both mtDNA and nDNA. However, traditional
Sanger sequencing makes the cost very prohibitive. Hence NGS, which allows par-
allel sequencing, can dramatically reduce cost (Calvo et al. 2012 ). To explore its
diagnostic use, a mitochondrial gene panel consisting of all the mitochondrial
genome and additional 1,000 nuclear genes was assessed by examining 42 unrelated
infants with clinical and biochemical evidence of mitochondrial oxidative phos-
phorylation disease (Vasta et al. 2009 ). In total, 23 of 42 (55 %) patients harbored
mutations in recessive genes or pathogenic mtDNA variants. Firm diagnoses were
enabled in ten patients (24 %) who had mutations in genes previously linked to
disease. Thirteen patients (31 %) had mutations in nuclear genes not previously
linked to disease. The results underscore the potential and challenges of deploying
NGS in clinical settings.

4.5 Cardiovascular Diseases

The impact of NGS on inherited cardiomyopathies is tremendous (Meder et al.
2011 ). The major clinical forms of inherited cardiomyopathy are hypertrophic
(HCM) and dilated (DCM; Meder et al. 2011 ). HCM usually presents as a sudden
cardiac death and is a major cause of morbidity in young people and it can also
cause heart failure. It presents with a left ventricular hypertrophy, usually asym-
metric. Functionally, it causes diastolic dysfunction and histologically it can show
up as myofi brillar disarray. In contrast, dilated cardiomyopathy is manifested as
diastolic dysfunction caused by dilated ventricular cavity, leading to heart failure as
its most common manifestation. HCM is caused by mutation in genes responsible
for sarcomere formation. More than 450 mutations have been identifi ed in 16 genes
(Meder et al. 2011 ). DCM is caused by mutations in genes encoding proteins in
sarcomere, Z-disk, nuclear lamina proteins, intermediate fi laments, and the glyco-
protein complexes.
Meder et al. established a microarray-based subgenomic enrichment followed
by NGS to detect mutations in patients with HCM and DCM (Meder et al. 2011 ) .
Interestingly, NGS sequencing of patients with an unknown genetic cause of