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Improving the definition of the structure of the isochromosome i(7)(q10) in Shwachman‐Diamond Syndrome

2010; Wiley; Volume: 150; Issue: 5 Linguagem: Inglês

10.1111/j.1365-2141.2010.08243.x

1365-2141

Barbara Pressato, Cristina Marletta, Giuseppe Montalbano, Roberto Valli, Emanuela Maserati,

Pneumocystis jirovecii pneumonia detection and treatment

Shwachman-Diamond Syndrome (SDS; On-line Mendelian Inheritance in Man database number 260400) is an autosomal recessive disorder caused by mutations in the SBDS gene in at least 90% of cases (Boocock et al, 2003). It is characterized by exocrine pancreatic insufficiency, skeletal anomalies, and bone marrow failure with variable severity of neutropenia, thrombocytopenia and anaemia (Rothbaum et al, 2002). Acquired clonal chromosome anomalies are commonly found in the bone marrow (BM), being an isochromosome for the long arms of a 7, i(7)(q10), and a deletion of the long arms of a 20, del(20)(q11), the most frequent. The relationship between these chromosome changes and the risk of patients with SDS to develop myelodysplastic syndromes and acute myeloid leukaemia (MDS/AML) has been discussed (Dror, 2005). This risk increases with the age (Shimamura, 2006), and we have also shown that the acquisition of BM clonal anomalies is age-related (Maserati et al, 2009). The i(7)(q10) was shown to carry a double c. 258 + 2t>c mutation of SBDS in all the compound heterozygous patients tested (Minelli et al, 2009): wild-type SBDS mRNA can be found in the BM of these patients (Nicolis Elena et al, unpublished observation), and this may be related to the low incidence of MDS/AML in this subset of patients. This context warrants the need for the best definition of how the i(7)(q10) is precisely built up. The centromeric region of chromosome 7 contains two arrays of alpha satellite DNA: D7Z1, ranging in size from 1500 to 3800 Kb, orientated towards the long arms, and D7Z2, 100–550 Kb long, orientated towards the short arms with the interval between D7Z1 and D7Z2 being <1 Mb (Wevrick & Willard, 1991). In our cohort of patients, we have previously selected i(7)(q10)-positive cases, and performed fluorescent in situ hybridization (FISH) with the probe D7Z1 (Cytocell Technologies, Cambridge, UK) in two of them, and comparative genomic hybridization on microarray (a-CGH) with the 244K genome-wide system (Agilent Technologies Inc., Santa Clara, CA, USA) in five cases (Maserati et al, 2009): the conclusion was that the i(7)(q10) was made up linearly by the entire long arms, the D7Z1 alphoid sequences, a portion of non alphoid material, and than again D7Z1 and the long arms. We report here the results of further investigations on the i(7)(q10) by FISH with probes aimed at clarifying, with the utmost precision, its centromeric structure: pα7t1 and pMGB7 detecting respectively the sequences of D7Z1, and D7Z2 (kindly provided by Professor H. F. Willard, Institute for Genome Sciences and Policy, Duke University Medical Center, Durham, NC, USA). We tested BM cells of five i(7)(q10)-positive SDS patients, identified as UPN (unique patient number) 24, 25, 32, 36, and 40 (the last patient was not included in Maserati et al, 2009). The results of the dual colour FISH showed that both the probes hybridized the centromeric region of the i(7)(q10) in all the patients, with different patterns: the probe D7Z1 showed two distinct separate signals on the i(7)(q10), exactly as we had already detected by FISH with the commercial probe, but also a signal referable to the probe D7Z2 was present. In conclusion, our present results demonstrated that the centromere of the i(7)(q10) is in fact composed linearly by D7Z1, an interval of up to 1 Mb, D7Z2, another similar sized interval, and D7Z1 (Fig 1), at partial variance with our previous interpretation (Maserati et al, 2009). It was not possible to determine if the precise rearrangement site in D7Z2 is identical in all the patients due to the individual variability in length of alpha satellite arrays. Ideograms showing the localization on the normal chromosome 7 of the alphoid arrays sequences, used as informative probes to investigate the mechanism of origin of the i(7)(q10) (left), and the structure of the i(7)(q10) according to the results obtained (right). This work was supported by grants from Associazione Italiana Sindrome di Shwachman, AISS, and Fondazione Banca del Monte di Lombardia.