Familial Bainbridge‐Ropers syndrome: Report of familial ASXL3 inheritance and a milder phenotype

Abstract De novo truncating and splicing pathogenic variants in the Additional Sex Combs‐Like 3 (ASXL3) gene are known to cause neurodevelopmental delay, intellectual disability, behavioral difficulties, hypotonia, feeding problems and characteristic facial features. We previously reported 45 patients with ASXL3‐related disorder including three individuals with a familial variant. Here we report the detailed clinical and molecular characteristics of these three families with inherited ASXL3‐related disorder. First, a father and son with c.2791_2792del p.Gln931fs pathogenic variant. The second, a mother, daughter and son with c.4534C > T, p.Gln1512Ter pathogenic variant. The third, a mother and her daughter with c.4441dup, p.Leu1481fs maternally inherited pathogenic variant. This report demonstrates intrafamilial phenotypic heterogeneity and confirms heritability of ASXL3‐related disorder.

splicing pathogenic variants in the additional sex combs-like 3 (ASXL3) gene resulting in loss-of-function as the predominant disease mechanism. It is characterized by neurodevelopmental delay, moderate to severe learning difficulties, behavioral problems, hypotonia and feeding problems. Characteristic craniofacial features include prominent forehead, down-slanting palpebral fissures, prominent nasal bridge, broad nasal tip, low columella, high-arched palate, and crowded teeth.
Here, we report detailed clinical and molecular characteristics of three families with ASXL3-related disorder which represents the first detailed description of familial ASXL3-related disorder. Almost all patients with ASXL3-related disorder have de novo pathogenic variants in ASXL3 (Balasubramanian et al., 2017;Bainbridge et al., 2013;Dinwiddie et al., 2013;Schirwani et al., 2021). No clear genotypephenotype correlation exists and there is significant phenotypic heterogeneity between individuals. Intrafamilial phenotypic variation has not been previously explored and there has not been a prior detailed report of familial cases.

| Family 1
The proband was referred for genetic evaluation at 4 years of age because of developmental delay and possible episodes of apnoea.
This patient was born at term via normal delivery weighing 3.85 kg (75th centile). At age 4 years, his height was 98.5 cm (20th centile), weight was 13.6 kg (5th centile) and head circumference of 47.5 cm (1st centile). He had initial feeding difficulties with poor sucking reflex in the neonatal period. His fontanelles closed prematurely.
Developmentally, motor milestones were achieved appropriately but speech and language development were severely delayed. At His father was born at term, with a birth weight of 3.6 kg. Motor and speech and language milestones were met appropriately. Growth parameters for the proband's 37-year-old father were: height 182 cm (80th centile) and weight 90 kg (90th centile).
He was diagnosed with ADHD at 11 years and had gone to a special school during childhood. He had difficulties with reading and information processing.

| Family 2
This family of mother, daughter and son, were referred for genetic evalu-  (Wright et al., 2015).
Putative de novo mutations were identified from exome data using DeNovoGear software, interpreted for their clinical relevance based on ACMG/AMP and ACGS guidelines and were validated using targeted Sanger sequencing (Richards et al., 2015;Ellard et al., 2020).
The mother was born at 42 weeks gestation by emergency caesarean section due to umbilical prolapse, weighing 3.9 kg (77th centile).
Final growth parameters were also all within normal range; height 168 cm (85th centile), weight 91 kg (78th centile). There was no admission to the Neonatal Unit, and there were no feeding difficulties during the neonatal period. Motor milestones were slightly delayed and she first walked at 18 months. Global neurodevelopmental delay became apparent in nursery and she was later diagnosed with mild intellectual disability and attended a special school. A special programme following on from school presented a job in a supermarket, where she worked for 11 years. In terms of behavioral issues, there were severe temper tantrums in childhood and oppositional behavior during teenage years, but no evidence of autism or ADHD. She was diagnosed with epilepsy, having atonic and absence seizures, aged 7 years and was on antiepileptics until the age of 16 years old. Epilepsy recurred after the birth of her first child and she was started on levetiracetam at that time.
Her daughter was born by elective caesarean section due to maternal epilepsy, weighing 3.3 kg (56th centile). Now at age 15 years, her weight is 52 kg (25th centile), height is 155 cm (9th centile) and she is relatively microcephalic with head circumference of 52 cm (2nd centile) ( Figure 2a). During the neonatal period, she was slow to develop effective feeding and had mild failure to thrive. Motor milestones were delayed; she sat at 12 months and walked at 2 years.
Speech and language development were also significantly delayed, with her first word at 2.5 years of age. She attended mainstream school with additional support provided for mild intellectual disability and persistent speech and language difficulties. In terms of behavior, she had poor attention, but no stereotypies of autism or ADHD. At 5 years of age, she was diagnosed with epilepsy after having seizures that were characteristically similar to her mother's. Her epilepsy has been well controlled on lamotrigine and oxcarbazepine since. Imaging (MRI) of her head was normal.
Her son was born by emergency caesarean section for breech presentation, weighing 3.9 kg (77th centile). At aged 16 months, growth parameters were: height 75 cm (4th centile), weight 8.6 kg (1st centile) and head circumference 45 cm (1st centile). He was severely hypotonic and had feeding difficulties from birth, with recurrent hypoglycemic episodes, which required percutaneous endoscopic gastrostomy feeding.
Developmental milestones were significantly delayed; he first sat independently at 20 months of age and could not walk or talk by 7 years of age. He attended a special school due to severe intellectual disability and significant behavioral difficulties including stereotypies, head rolling and biting. He was extremely hypermobile and repeatedly put his legs behind his ears (Figure 2b-d). Unlike his mother and sister, he had not been diagnosed with epilepsy, but had had one seizure in the past. He had an MRI head as part of investigations into hypotonia, which was normal.
Genetic testing also identified two copy number variants (CNV); 2q32. The common dysmorphic feature present in all three individuals was a tall forehead (Figure 2a,b). Despite absence of other dysmorphisms in the mother and daughter, the son additionally had hypermobility, hypotonia, nevus flammeus, long slender fingers and scoliosis ( Figure 2b-d). There was evidence of microcephaly in the daughter and son, but normal head circumference in their mother. There is no head circumference measurement for their father for comparison.

| Family 3
The proband was referred for genetic evaluation because of intellectual disability, intractable epilepsy and autism. Trio whole exome sequencing identified a heterozygous likely pathogenic frameshift variant that was maternally inherited (c.4441dupC, p.Leu1481fsX12).
Previous testing had revealed a MECP2 variant of unknown significance, which was re-classified to benign when it was found to be paternally inherited from an asymptomatic male.
This patient was born via caesarean section at 41 weeks due to postmaturity with no complications or need for neonatal admission.
There were no feeding difficulties during the neonatal period. Developmental milestones were delayed and she first sat at 15 months, first walked at 2 years, and had developed five words by the age of 2-3 years, after which, speech regressed and she became nonverbal. MRI head showed slight prominence of the right temporal horn and slight volume loss of the right hippocampal region without abnormal signaling, but was otherwise normal.   (Sarkar et al., 1991). They can be asymptomatic but have the potential to cause significant morbidity.
The father, who carries the same pathogenic variant, will also be screened for aneurysms with an initial echocardiogram and subsequent evaluation based on this. There have been no prior reports of connective tissue features linked to ASXL3 in other patients, however, with ASXL3-related syndrome, we would normally recommend an initial echocardiogram on diagnosis. Further surveillance would be dependent on any abnormal initial findings, or whether there were clinical findings as seen in Family 1. As we collate more data on ASXL3-related disorder, we will understand better the natural history of this disorder to allow meaningful conclusions on need for additional screening and ongoing surveillance for features of a connective tissue disorder.

| CONCLUSION
We describe the clinical and molecular characteristics of three families with known familial ASXL3-related disorder. We confirm heritability of this condition but show intrafamilial phenotypic heterogeneity. This report has immediate clinical ramifications with respect to recurrence risk counseling for parents, parental testing and reproductive health advice for patients with ASXL3-related disorder.

DATA AVAILABILITY STATEMENT
Data sharing is not applicable to this article as no new data were created or analyzed in this study.