PMC4359772_01
Cytogenetic and molecular results. (a) G-banding partial karyotypes of the patient, indicating a terminal deletion of 2q, accompanying with a marker chromosome. (b) C-banding showing no centromeric heterochromatin. (c) Copy number variation analysis for individual SNP loci along chromosome 2 using Illumina Human OmniZhongHua-8 Beadchip. B Allele Frequency and Log R show a gain of 34,268,291 bp (nt. 208775856-243044147) in terminal end of 2q (Red arrow ). (d) FISH results of the patient. FISH using RP11-526 L8 (orange) at 2q37.3 as a test probe and RP11-119B15 (green) at 2p22.3 as a control probe, showing an orange signal and a green signal on each of the two copies of only one chromosome 2, while the other one lacking the orange signal. Two orange hybridization signals for test probe on the SMC corresponding to an inverted duplication marker. (e, f) FISH results of the parents showing one hybridization signal for each probe on the two normal chromosomes 2.
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Fig1
Analysis of GTG-banded chromosomes in the proposita revealed a supernumerary marker in all analyzed metaphases; however, a 2q deletion was observed (Figure 1a). C-banding (Figure 1b) showed the absence of centromeric heterochromatin in the marker chromosome, suggesting the presence of a neocentric sSMC. SNP array analysis detected a 34.3 Mb-duplication (nt.208775856-243044147) of the 2q33.3 to q37.3 region (Figure 1c). FISH analysis using a BAC clone RP11-526 L8 (orange) within the duplicated region in the patient and her parents (Figure 1d, e, and f). FISH studies also showed that the marker chromosome has some features of an isochromosome.
PMC4359772_02
Possible mechanisms for the formation of neocentric marker chromosome in the patient. Chromatids breakage (red scissors) and a distal U-type exchange happened before sister chromatid separation during the meiosis II. Neocentromerization occurred once breakage happened. Deep color ovals are the centromeres, and the same one on the acentric fragment is neocentromere. The broken ends (red wave) of the centric fragment could be stabilized by telomere restitution. After zygogenesis and subsequent replication the result would be partial trisomy for the duplicated fragment.
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Fig2
In view of the above facts, we speculate that the most plausible explanation is a distal U-type exchange occurring between two chromatids of the same chromosome at meiosis II (Figure 2).
Microsatellite analysis of markers D2S1279 and D2S1338 on ABI 3100 genetic analyser. The 2:1 ratio between two fluorescent peaks areas in the proband indicated that she inherited a double dosage of one of the paternal alleles.
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Fig3
To validate our speculation, we used simple quantitative fluorescent polymerase chain reaction (QF-PCR) to perform short tandem repeat analysis (Figure 3).