Re-evaluation of whole exome sequencing, including intronic region, in combination with genetic intolerance score for detecting foetal structural anomalies in X-linked disorders
Authors: Taniguchi Kosuke , Hasegawa Fuyuki , Okazaki Yuka , Hori Asuka , Ogata-Kawata Hiroko , Aoto Saki , Migita Osuke , Kawai Tomoko , Nakabayashi Kazuhiko , Okamura Kohji , Fukui Kana , Wada Seiji , Ozawa Katsusuke , Ito Yushi , Sago Haruhiko , Hata Kenichiro 
Published: February 2023

Background

Whole-exome sequencing (WES) is a strong diagnostic tool for foetal structural anomalies, but the causative gene for more than half the anomalies have not been identified. Therefore, improving the diagnostic yield based on WES data is essential.

Methods

First, 138 foetuses with structural anomalies were assessed using conventional WES and copy number variation (CNV) analyses. For undiagnosed cases, we employed a three-step approach for diagnosis. We re-evaluated 1) candidate variants using a loss-of-function observed/expected upper bound fraction (LOEUF) score, 2) all variants of disease-causing genes for clinically diagnosed cases using spliceAI, and 3) the rare variants in all low LOEUF scored genes (< 0.35) using spliceAI.

Results

We identified molecular diagnoses in 53 of 138 cases (38.4%) using conventional WES and CNV. For undiagnosed cases, for the first step, we diagnosed two X-linked recessive diseases. For the second step, we diagnosed Meckel-Gruber syndrome by detecting likely pathogenic intron variant inTMEM67. In the third step, we identified ade novohemizygous pathogenic variant in one severe hydrops fetalis male, which caused aberrant splicing inCASK. We found a novel phenotype, hydrops fetalis, in CASK-related X-linked dominant disorder. Moreover, we revealed that the LOEUF score of X-linked disease-causing genes was significantly lower than that of autosomal genes among all OMIM-registered genes.

Conclusion

We showed that the evaluation of variants, including introns of WES data, in combination with the LOEUF score, could improve the WES diagnostic yield and be useful for evaluation of variants, especially on chromosome X.

What is already known on this topic?

Molecular genetic diagnosis of foetal structural anomalies using WES is being increasingly implemented. However, more than half of the cases cannot be diagnosed. There is signigicant potential to increase the diagnostic yield by re-analysing WES data.

What this study adds

In the present study, we focused on loss-of-function observed/expected upper bound fraction (LOEUF) scores to quantify genetic intolerance, and additional intron analysis for undiagnosed cases using conventional WES data. These approaches enabled the appropriate evaluation of candidate variants and detected overlooked candidate variants on intron. We diagnosed two X-linked recessive disorder cases (Hardikar syndrome and Ritscher-Schinzel syndrome), by re-evaluating candidate variants using the LOEUF score. We also diagnosed one Meckel-Gruber syndrome case caused by an intronic pathogenic variant, that had been overlooked by the conventional method. Moreover, evaluating all variants, including introns with low LOEUF score genes (2,971 genes) that could cause haploinsufficiency helped us find a pathogenic intronic variant onCASKin one hydrops fetalis case, which revealed that CASK-related X-linked dominant disorder could cause hydrops fetalis as severe phenotypes. Finally, the LOEUF score of X-linked genes was significantly lower than that of autosomal genes among all OMIM-registered genes, which meant that gene evaluation using the LOEUF score was helpful, for genetic diagnosis, especially for genes on chromosome X.

How this study might affect research, practice, or policy

The evaluation of variants, including introns, in combination with the LOEUF score is expected to contribute to the improvement of the diagnostic yield in WES. These approaches are easy and convenient to implement. The LOEUF score might be useful for evaluation of variants, especially in chromosome X.