XRCC4-related microcephalic primordial dwarfism: description of a clinical series of 7 cases, phenotype expansion and new diagnostic approaches.

The non-homologous end joining (NHEJ) pathway is essential to repair DNA double-strand breaks. XRCC4 acts as a stabilizer of the DNA ligase LIG4 in the NHEJ process. In humans, XRCC4 pathogenic variants are responsible for a microcephalic primordial dwarfism syndrome (MPD). Currently, 17 patients have been reported with XRCC4-related MPD and we report 7 new patients from 6 different families, including one fetus. The patients present with short stature, severe microcephaly, neurodevelopmental disorder and additional features, such as transient increase in nuchal translucency, congenital glaucoma, thumb anomalies, hepatic steatosis, seizures, essential tremor and oligodontia which have not been previously described. Hyper- and hypopigmented skin macules, dermatofibrosarcoma, mandibular osteoid osteoma and pancytopenia are also new features, reminiscent of cancer susceptibility syndromes. Functional studies were performed on two patients carrying the known pathogenic p.(Trp43Arg) variant in homozygous state, using a fast, cost-effective and non-invasive approach on PBMCs: (1) Survival analyses after ionizing radiation confirm important radiosensitivity. (2) Flow cytometry showed the lack of TCR-Va7+ T-lymphocytes, suggesting recombination defect of V(D)J coding segments. (3) This was confirmed by multiplexed RT-PCR (PROMIDISα biomarker), analyzing the diversity of V(D)J coding segments in a subset of the TCRα repertoire. We therefore extend the phenotype of XRCC4-related MPD and suggest a combination of three functional assays, based on radiosensitivity and V(D)J recombination defect, to improve the interpretation of XRCC4 variants in fast, cost-effective and non-invasive manner. These findings will improve the diagnosis, genetic counselling, follow-up and management of these patients.