P-719 Evaluation of genetic risk of apparently balanced chromosomal rearrangement carriers by breakpoint characterization
Authors: Yanqin X , Dehua C , Keli L , Yueqiu T , Ge L , Liang H 
Published: June 2023

Abstract

Study question
What are breakpoint characteristics of ABCR carriers? What are the genetic risks of ABCR carriers caused by chromosome rearrangements?

Summary answer
We analyzed the characteristics of ABCR's breakpoints and found that heterogeneous deletion of some AD disease-related genes may not result in clinical phenotype.

What is known already
ABCRs are common chromosomal abnormalities. People with ABCRs generally lack any visible abnormal features; however, during childbirth, the abnormal pairing and missegregation of homologous chromosomes can lead to the production of unbalanced gametes, resulting in spontaneous abortion or the birth of children with chromosomal disorders. Prenatal diagnosis or preimplantation diagnosis can effectively prevent the birth of children with unbalanced chromosome rearrangements, but no substantial data has been collected to report on the characteristics and potential genetic risks of the chromosome breakage caused by the rearrangement.

Study design, size, duration
This is a retrospective study of 2441 breakpoints in 1219 ABCR carriers who underwent Preimplantation Genetic Testing (PGT) at the Reproductive and Genetic Hospital of Citic-Xiangya from August 1, 2016 to December 31, 2021.

Participants/materials, setting, methods
All patients in this study sought PGT treatment due to primary or secondary infertility caused by ABCRs. To characterize the chromosome breakpoints, the MicroSeq technique, which involves chromosome microdissection and next generation sequencing, was employed. Additionally, gap-junction PCR and Sanger sequencing were utilized to further validate some of the breakpoints.

Main results and the role of chance
In this study, we retrospectively analyzed 2441 chromosome rearrangement breakpoint MicroSeq localization data from 1219 ABCR carriers, including 2364 breakpoint regions in 1182 reciprocal translocations, 68 breakpoint regions in 34 inversions, and 9 breakpoint regions in 3 insertional translocations. We not only identified 8q24.13, 11q11.23, and 22q11.21 as previously reported hotspots of chromosomal balanced rearrangement breaks, but also identified the 10 Mb region of 12q24.13-q24.3 as a possible rare region of balanced rearrangement breaks. the average range of breakpoints identified by MicroSeq technology was only within 3.8 Kb, which clarified 95.82% (2339/2441) of balanced chromosomal rearrangements as to whether they broke genes or not. We identified 960 genes that were interrupted, of which 53 genes with autosomal dominant mode of inheritance (AD) were interrupted. Clinical phenotypic evaluation of these patients carrying interrupted AD genes identified that some of the AD genetic interruptions may not have a clinical phenotype.

Limitations, reasons for caution
MicroSeq cannot precisely pinpoint breakpoints to the gene or single base level. Moreover, carriers of ABCRs were not comprehensively assessed for other clinical characteristics, and the alteration in function of the known Haploinsufficiency gene could not be confirmed. Additionally, the position effect of the breakpoints has not been evaluated.

Wider implications of the findings
This investigation offers a reference for evaluating the pathogenicity and genetic risks of protein-truncation mutations in some AD genes.

Trial registration number
Not applicable