الفهرس 
Inborn errors of metabolismOnly 14 pages are availabe for public view
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Abstract
Maple syrup urine disease (MSUD, OMIM #248600)
is an autosomal inborn disorder, triggered by a mutation in
BCKDHA, BCKDHB and DBT genes that encoding E1α,
E1β and E2 of branched chain ketoacid dehydrogenase
(BCKDH) complex. This multi-enzyme complex involves
in the metabolism of branched chain amino acids (BCAAs):
leucine, isoleucine and valine via the oxidative
decaboylation. The deficiency of BCKDH causes the
accumulation of BCAAs and their crossponding α-
ketoacids in the blood leading to brain encephalopathy.
The worldwide incidence of MSUD is estimated
1:185000 and the rate elevated in population characterized
by consanguineous marriage, but the incidence in Egyptain
population has not yet been reported.
The MSUD genotype is determined according to the
affected loci: type IA for the BCKDHA gene (E1α
subunit); type IB for the BCKDHB gene (E1β subunit) and
type II for the DBT gene (E2 subunit). MSUD patients are
categorized based on the severity of symptoms into classic
form which represents 75% of patients and is considred the most severe phenotype and the less severe variant
phenotypes: intermediate, intermittent and thiamine
responsive.
MS/MS is widely used in neonatal screening in many
countries such as India, China, Turkey, Iran, Lebanon,
Spain, Portugal and Isreal which enables MSUD diagnosis.
The results of biochemical testing using MS/MS provide
the concentration of BCAAs in the blood but it do not
introduce which gene has been affected.
In this present study, RNA was extracted from the
whole blood from twenty patients (10 males & 10 females)
who were shown an elevation in blood BCAAs
concentration by LC/MS/MS screening.
The cDNA was synthesized to amplify the entire
BCKDHB and DBT coding regions by conventional PCR
and amplification products were sequenced. Finally, the
pathogenicity of nucleotide alteration and their effect in
protein function were examined using in silico analysis
tools.
The mutational identification of twenty MSUD
Egyptian patients showed tweleve mutations (eight novel &
four previously published) in homozygous state in the
coding region of two genes. The mutations spanned across BCKDHB coding region were found in exons 3, 4, 7, 8, 9,
10. The novel mutations (4/12), missense mutations:
c.388G>T/p.V130F (exon 4), c.806G>A/p.G269E (exon 7),
c.995C>T / p.P332L (exon 9),c.1091A>G/p.D364G (exon
10), deletion mutation (1/12): c.288_288delA/
D97Mfs*133 (exon 3), insertion mutation: (1/12)
c.908_909insA/ p.D303Efs*15 (exon 8) in addition to
nonsense reported mutations (2/12): c.853C>T/p.R285X
(exon 8) & c.970C>T/p.R324X (exon 9) while DBT
mutations were identified in exons 1, 3, 6, 9 as follows: the
novel mutations (2/12), deletion mutation (1/12):
c.241_242delGT/p.V81X (exon 3) and nonsense mutations
(1/12): c.30G>A, p.W10X (exon 1), in beside two reported
nonsense mutations (2/12): c.1291C>T, p.R431X (exon 9)
& c.670G>A, p.E224K (exon 6).
The novel missense mutations of BCKDHB gene were
predicted to cause MSUD and had a local structural effect
on E1B subunit while the pathogenicity of nonsense
mutations in both BCKDHB and DBT genes returned to the
production of truncated E1B and E2 subunits.