Trisomy 21 Variants Among 3,124 Mexican Patients with Down Syndrome: A Three-Decade Experience of a Referral...

Down syndrome (DS) is the most common genetic cause of intellectual disability, results from trisomy 21 (T21). T21 can be regular or due to translocation, isochromosome, or mosaicism, among other chromosomal alterations. In Mexico, frequencies of these chromosomal variants reported internationally are used; therefore, national data are necessary.

To determine the frequencies of cytogenetic findings in patients with DS referred to a research center in western Mexico over the past 31 years (1993-2024).

Physical and digital records from the cytogenetics laboratory were reviewed to extract data from patients who were clinically diagnosed with DS, including date of birth, age at diagnosis, parental age at birth, Mexican state of origin, and karyotype. Descriptive statistics and a non-parametric test were used.

3,238 patients were identified, of whom 3,124 were confirmed to have T21. Of those 94.17% had regular T21; 2.72% had T21 by Robertsonian translocation; 1.54% were mosaic; 0.93% had T21 by isochromosome; and 0.64% had T21 with other chromosomal alterations. Of the patients without T21, 105 had a normal karyotype without alterations, and nine showed other chromosomal alterations, mainly sex aneuploidies. The patients' age at diagnosis was 1 year (ranging from newborn to 28 years), with maternal and paternal ages both averaging 34 years old (13-49 years and 15-69 years, respectively).

These findings align with international reports but differ slightly from local data, possibly due to sample size. This is the largest study of T21 chromosomal variant frequencies in the Mexican population.