Beatrice Saposnik

Mutation spectrum and genotype-phenotype correlations in a large French cohort of MYH9-Related Disorders

MYH9‐Related Disorders are a group of rare autosomal dominant platelet disorders presenting as nonsyndromic forms characterized by macrothrombocytopenia with giant platelets and leukocyte inclusion bodies or as syndromic forms combining these hematological features with deafness and/or nephropathy and/or cataracts. They are caused by mutations in the MYH9 gene encoding the nonmuscle myosin heavy chain II‐A (NMMHC‐IIA). Until now, at least 49 MYH9 mutations have been reported in isolated cases or small series but only rarely in large series. We report the results of an 8‐year study of a large cohort of 109 patients from 37 sporadic cases and 39 unrelated families. We have identified 43 genetic variants, 21 of which are novel to our patients. A majority, 33 (76.7%), were missense mutations and six exons were preferentially targeted, as previously published. The other alterations were three deletions of one nucleotide, one larger deletion of 21 nucleotides, and one duplication. For the first time, a substitution T>A was found in the donor splice site of intron 40 (c.5765+2T>A). Seven patients, four from the same family, had two genetic variants. The analysis of the genotype‐phenotype relationships enabled us to improve the knowledge of this heterogeneous but important rare disease.

Apparent homozygosity for p.E1841K mutation in a patient with MYH9-related disorder: a misdiagnosis for an autosomal dominant disorder?

To the Editor: Poopak B. et al. have reported (Eur J Haematol, 2011, 86(4):357) the case of a patient (P1) with MYH9-related disorder (MYH9-RD) that they have considered to be homozygous for p.1841E>K mutation in the non-muscle myosin heavy chain IIA (NMMHC-IIA). The homozygosity of P1, father of a 15-year-old boy (P2) heterozygous for p.1841E>K, was assessed by DNA sequence analysis. As all patients published with MYH9-RD, an autosomal dominant disorder, are heterozygotes, such a result deserves to be demonstrated by unquestionable data in order to confirm the results of the chromatogram shown by Poopak et al. Several causes of false homozygosity should have been examined. One of the most frequent of these causes is the inhibition or absence of PCR amplification of one allele. Could the authors precise the sequences of the primers used and their corresponding positions on MYH9 gene? Did they check the sequencing result using several couples of different primers? Did they control their result with several different Taq polymerases, to exclude possible error because of this enzyme? Did they analyse in detail the sequence of exon 38 and its junctions with introns 37 and 38, to detect regions difficult to amplify or containing SNPs and possibly preventing the hybridisation of the primers with the patient’s DNA? We have analysed the MYH9 gene in a cohort of 109 MYH9-RD patients with mutations in the MYH9 gene. We have had to face a problem of false homozygosity in exon 1 of MYH9 gene for 5 patients (Fig. 1), because of the presence of a poly G repeat in intron 1 preventing the PCR amplification of exon 1. If the primer was located downstream of the poly G repeat, only five samples harboured a mutation, two heterozygous, but three homozygous. If the primer was located upstream of the poly G repeat, seven samples harboured a mutation, heterozygous in the seven cases. These examples show that if one of the two alleles is not amplified, a heterozygous patient may be found either homozygous for the mutation (amplification of the mutated allele only) or normal (amplification of the normal allele only).