Variation in pattern and frequency of acrocentric association in normal and trisomy-21 individuals

Linear dose response of acrocentric chromosome associations to gamma irradiation in human lymphocytes

PURPOSE

The frequency of acrocentric chromosome associations (ACA) was studied to determine the possible dose-response relationship of gamma irradiation in human lymphocytes.

METHODS

Peripheral blood collected from three healthy donors was irradiated with 0, 0.1, 0.25, 0.5, 0.75, 1, 2, 3, 4, and 5 Gy of gamma radiation. Chromosomal preparations were made after 48 h of culture as per standard guidelines. The experiment was repeated three times, with a different donor each time.

RESULTS

The ACA frequency in irradiated lymphocytes increased with radiation dose. The D-G type of association was most prominent and showed a significant dose-dependent increase in frequency. The dose response of ACA frequency to radiation was found to be linear: ACA frequency = 0.2923 (±0.0276) + 0.1846 (±0.0307) × D (correlation coefficient r = 0.9442). As expected, dicentric chromosome (DC) frequencies followed the linear quadratic fit model, with DC frequency = 0.0015 (±0.0013) + 0.0220 (±0.0059) × D + 0.0215 (±0.0018) × D^2 (correlation coefficient r = 0.9982). A correlation curve was prepared for ACA frequency versus DC frequency, resulting in the regression equation y = 1.130x + 0.4051 (R² = 0.7408; p = 0.0014).

CONCLUSION

Our results showed an increase in ACA frequency in irradiated lymphocytes with an increase in radiation dose; thus, ACA may serve as a candidate cytogenetic biomarker for radiation biodosimetry.

Induced Pluripotent Stem Cell (iPSC) Lines from a Family with Resistant Epileptic Encephalopathy Caused by Compound Heterozygous Mutations in SZT2 Gene

Mutations in the SZT2 gene have been associated with developmental and epileptic encephalopathy-18, a rare severe autosomal recessive neurologic disorder, characterized by psychomotor impairment/intellectual disability, dysmorphic facial features and early onset of refractory seizures. Here we report the generation of the first induced pluripotent stem cell (iPSC) lines from a patient with treatment-resistant epilepsy, carrying compound heterozygous mutations in SZT2 (Mut1: c.498G>T and Mut2: c.6553C>T), and his healthy heterozygous parents. Peripheral blood mononuclear cells were reprogrammed by a non-integrating Sendai virus-based reprogramming system. The generated human iPSC lines exhibited expression of the main pluripotency markers, the potential to differentiate into all three germ layers and presented a normal karyotype. These lines represent a valuable resource to study neurodevelopmental alterations, and to obtain mature, pathology-relevant neuronal populations as an in vitro model to perform functional assays and test the patient’s responsiveness to novel antiepileptic treatments.

Increased frequency of acrocentric chromosome association during colcemid treatment

The effect of Colcemid on satellite association frequencies was investigated in human cell lines U-937 GTB, SC-N-MC, HL-60, and Raji by silver staining method. Cell cultures were exposed to Colcemid at a concentration of 0.02 mg/ml for 15-60 minutes. As exposure time to Colcemid was increased, both the frequency of cells with satellite associations and the number of chromosomes involved in satellite associations increased significantly. Furthermore, the mean number of silver-stained chromosomes decreased with longer exposure time. Without Colcemid, we were able to obtain an adequate number of good-quality metaphase spreads. The data obtained may be of help in furthering research concerning satellite associations and in obtaining better quality chromosome preparations.

Consanguinity and the genetic control of Down syndrome

Eighty-three infants and children with a professional diagnosis of Down syndrome were studied cytogenetically. The results showed that 81.9% of them were trisomy 21 and 18.1% were the 46/47 + G type of mosaic. All cases were distributed according to parental consanguinity. The results showed that 77.9%, 16.2% and 5.9% of the trisomy 21 cases, and 53.3%, 26.7% and 20.0% of the mosaic cases were from non-consanguineous, first-cousin and second-cousin marriages, respectively. By combining all the cases, these percentages were 73.5%, 18.1% and 8.4%, respectively. Considering the high rate of inbreeding in Iraq (inbreeding coefficient = 0.0225) and using various statistical comparisons, these results revealed the lack of evidence for genetical control of non-disjunction in man.

Correlations between relatives for acrocentric association frequency

Acrocentric association was investigated in peripheral blood lymphocytes of twins (10 female monozygotic and 11 females dizygotic pairs), newborns and both parents (30 families), and spouses (51 pairs). Seventy two hour cultures were G-banded and scored for both absolute and relative acrocentric association frequency, except in the case of the spouse pairs where only the absolute frequency was measured. Both relative and absolute parameters of acrocentric association show positive correlations between relatives with the values being highest and most consistent in monozygotic twins, intermediate in parent and offspring, and most variable in dizygotic twins. Husband and wife pairs from our family collections show a positive correlation for the absolute parameter but not for relative parameters. The environmental factors responsible have not been identified. A rough estimate of broad sense heritability (0.81) has been made for the relative parameters. It probably contains a large component due to genetic dominance. Heritability of the absolute parameters is probably lower than for the relative parameters though estimation of its value is complicated by inconsistent results. A model is proposed to account for the variation in satellite association frequency which contains two elements: (i) The genotype determines the ratio of one chromosome type to another in the population of associated chromosomes (ii) All other environmental factors influence the absolute frequency of association without altering this basic ratio.