The human myophosphorylase gene (PYGM
, MIM #608455), assigned to chromosome 11 in 1984 (7
), was identified in 1993, as the GSD-V causing gene (3
gene spans about 14.2 kb of genomic sequence made of 20 exons, and contains a coding region of 2529-bp in length that encodes for a protein of 842 amino acids (3
At the last count (December 2006), 67 different mutations have been identified in the PYGM
gene: 12 nonsense mutations, 33 missense mutations, 12 deletions, 3 deletion/insertions, one silent mutation affecting the splicing and 5 intronic mutations (8
) (Table ).
PYGM Mutations reported up to December 2006.
Among the mutations located at the codifying region, 27 variants lie within the N-terminal region and 34 in the C-terminal domain, indicating that the regulatory and catalytic domains are equally affected. Since mutations are described in almost every exon of the PYGM gene, we can conclude that there is no a real mutational “hot spot” region.
Mutations in PYGM reduce or abolish the myophosphorylase enzyme activity in muscle. Missense mutations may affect contact dimer pairs, or can disrupt hydrogen bond interactions thus affecting substrate or effector/inhibitor binding sites. Nonsense mutations lead to truncated proteins, but may also produce severe effects at the transcriptional level. Interestingly, no insertions or large rearrangements have been reported in the PYGM gene. In addition, no mutations have been found in the PYGM promoter region.
We have recently showed that an apparently silent PYGM
polymorphism (p.K609K) in a patient led to a severe alteration in mRNA splicing, thus resulting to be pathogenic (41
Additionally, in two unrelated patients, with no changes identified in the DNA sequences, we have detected at the cDNA level, the complete deletion of exon 17, suggesting the presence of an intronic mutation affecting the splicing of exon 17 or a large genomic deletion (39
The “common” p.R50X mutation.
A single base pair substitution (G > T) at nucleotide 148 in exon 1 is the most frequent mutation identified in Caucasian McArdle patients. This common mutation, initially reported as p.R49X has been now recalled as p.R50X, following the recommendations of the Nomenclature Working Group (42
), http://hgvs.org/mutnomen. Several studies of large series of patients suggest an homogeneous distribution of this nonsense mutation among different countries, having the highest frequency in British and North-American patients (81% and 63% of the alleles, respectively) (8
). In other European countries the relative percentage of affected alleles bearing the p.R50X mutation is rather uniform (Germany 56%, France 56%, Spain 55%) with the lowest frequency in Italy (17
) (Table ).
p.R50X allele frequency in different Caucasian populations.
This observation has important diagnostic consequences as this is the mutation that should be studied first. This can be made easily by using the restriction fragment length polymorphism (RFLP) analysis (Nla
III restriction enzyme) (3
), or by a specific primer extension technique, using minisequencing analysis (39
Other PYGM Mutations.
The second most common mutation identified in the PYGM
gene is the missense p.G205S mutation that accounts for 10% of alleles in American patients and 9% of alleles in Spanish patients (3
). However, both the p.R50X and the p.G205S mutations have never been identified in Japanese patients. Interestingly, two frequent population-related mutations have been exclusively found in Japan and in Spain: the p.F710del is the most common mutation in Japanese McArdle cases (11
), and the p.W798R has been found in 16.5% of Spanish patients (29
). Few examples of less frequent mutations reported by different groups are: p.R270X (27
), p.L397P (12
), the intron 14 mutation c.1768 + 1G > A (10
) and c.2262delA in exon 18 (10
Most of the other mutations reported, have only been found in a single patient.