Prenatal Diagnosis of 22q11.2 Deletion Syndrome Using Fluorescence In Situ Hybridization (FISH)

INTRODUCTION

Prenatal diagnosis is a critical component of modern maternal-fetal medicine, enabling the early detection of genetic anomalies and supporting informed decision-making in pregnancy management. Among the various genetic conditions detectable through prenatal testing, 22q11.2 deletion syndrome (22q11.2DS) (also known as DiGeorge syndrome or velocardiofacial syndrome) is one of the most common chromosomal microdeletion syndromes.

This condition results from the deletion of a small segment on the long arm (q arm) of chromosome 22 at position 11.2. The deletion disrupts genes essential for the normal development of multiple organ systems, leading to a wide range of clinical manifestations, including congenital heart defects, immune dysfunction, cleft palate, developmental delays, and neuropsychiatric disorders.

Given the potential severity of 22q11.2DS, prenatal diagnosis plays a pivotal role in identifying affected pregnancies early. Early detection enables healthcare providers and families to anticipate medical requirements, plan timely interventions, and provide psychosocial support. One of the most reliable methods for detecting this microdeletion prenatally is fluorescence in situ hybridization (FISH).

FISH is a molecular cytogenetic technique in which fluorescently labeled DNA probes bind specifically to targeted regions of chromosomes. For 22q11.2DS, FISH provides highly specific and sensitive detection of the microdeletion, which may be missed by conventional karyotyping due to its small size.

FISH analysis is usually performed on fetal cells obtained through chorionic villus sampling (CVS) or amniocentesis. CVS is performed between 10 and 13 weeks of gestation, while amniocentesis is typically conducted at 15–20 weeks. Once fetal cells are obtained, FISH can yield results within 24–48 h, offering a major advantage over other techniques, such as chromosomal microarray analysis (CMA), which may require longer turnaround times.

During the FISH procedure, fluorescent probes targeting the TUPLE1 region (within 22q11.2) hybridize to the corresponding chromosomal site. A missing fluorescent signal on one of the chromosome 22 pairs indicates a deletion. Although highly specific and sensitive, FISH does not provide information on the size of the deletion or detect other chromosomal abnormalities. For this reason, it is often complemented by CMA when broader genomic information is required.

Beyond its technical role, prenatal detection of 22q11.2DS is a crucial element of genetic counseling. Families receiving this diagnosis face complex decisions, and understanding the wide spectrum of clinical outcomes ranging from severe congenital heart disease to relatively mild learning difficulties is essential. Early identification through FISH allows for appropriate emotional, medical, and logistical preparation, ensuring that affected infants receive immediate care, if necessary, after birth.

Thus, the use of FISH in the prenatal diagnosis of 22q11.2DS remains a cornerstone of maternal fetal medicine. Its ability to provide rapid, reliable results makes it an indispensable diagnostic tool, facilitating early intervention planning and comprehensive genetic counseling.

MATERIAL AND METHODS

A retrospective study was conducted between January 21 and June 24. A total of 70 suspected cases of 22q11.2 abnormalities were analyzed. FISH, using a DiGeorge syndrome specific probe targeting the 22q11.2 region, was performed according to the manufacturer’s instructions (Abbott Laboratories, USA). At least, 30 interphase nuclei were examined per case, and the presence or absence of orange and green fluorescent signals was evaluated.

RESULTS

Seventy consecutive cases with suspected congenital structural heart defects were studied using conventional cytogenetic and FISH analyses. Standard cytogenetic evaluation of all 70 amniotic fluid samples revealed normal chromosomal complements, with no additional chromosomal abnormalities detected. On the contrary, FISH analysis suggest 22q11.2 microdeletions in 3 out of 70 cases (4.3%) [Table 1 and Figure 1a-c].

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Figure 1:

(a) Fluorescence in situ hybridization (FISH) image showing two signals each of orange and green indicating no 22q11.2 microdeletion. (b and c) FISH image is showing one orange signals indicating 22q11.2 microdeletion. Significance of green signals targets a \”control\” region on the distal part of chromosome 22 to confirm the probe worked and to identify both copies of chromosome.

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DISCUSSION

22q11.2DS is the most common microdeletion disorder in humans,^[1] associated with a broad spectrum of congenital anomalies and long-term health complications. The published frequency is approximately 1 in 2,148 live births,^[2] with higher prevalence observed in prenatal groups which was 1 in 992 unselected pregnancies and 1 in 497 miscarriages^[3,4]irrespective of maternal age.

Among available diagnostic tools, FISH using a probe specific for the DiGeorge chromosomal region remains widely utilized for detecting 22q11.2 deletions. Prenatal detection can be performed on interphase or metaphase chromosomes. Although conventional cytogenetics may occasionally reveal an unbalanced translocation or interstitial deletion involving 22q11.2, its sensitivity is limited. Consequently, most diagnostic laboratories rely on FISH as the primary tool for evaluating pregnancies at risk of 22q11.2DS.

In our study, microdeletions were identified in three cases, whereas the remaining 67 showed no abnormalities. Similarly, Kowalczyk et al.^[5] reported a 22q11.2 deletion in approximately 1 in 19 pregnancies undergoing diagnostic testing. Earlier studies have also reported a significant proportion of patients with conotruncal cardiac defects carry the 22q11.2 deletion.^[6,7]

FISH remains a highly reliable technique for detecting typical 22q11.2 deletions, particularly in cases with a family history, such as an affected sibling or when one parent is a known carrier. However, prenatal detection is often limited to targeted testing in high-risk cases rather than population wide screening, which remains a challenge.^[8]

CONCLUSION

A retrospective analysis of 70 pregnancies suspected of having congenital structural heart defects was conducted between January 21 and June 24. Conventional cytogenetic analysis of all cases showed normal karyotypes. In contrast, FISH detected microdeletions in 3 cases (4.3%), demonstrating its higher sensitivity compared to standard cytogenetics.

The findings underscore the importance of FISH as a rapid, specific, and reliable diagnostic tool for detecting 22q11.2 deletions prenatally. Although FISH does not provide information on deletion size or detect other chromosomal abnormalities, it plays a vital role in targeted testing of high-risk pregnancies. Early detection supports timely medical intervention, perinatal care planning, and informed genetic counseling, ultimately improving outcomes for affected families.