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Multigenerational Triphalangeal Thumb: Suspicion for a New Dominantly-Inherited Disorder of Hand Development
Narges L Horriat, MD, Niki K Patel, BS, Shireen Dogar, DO, Brian M Kirmse, MD, Hugo Palacios Vasquez, MD and Marc E Walker, MD, MBA, University of Mississippi Medical Center, Jackson, MS

Triphalangeal thumb is a congenital hand anomaly in which the thumb consists of three rather than two phalanges. It is usually inherited as an autosomal dominant trait and thus commonly seen affecting multiple family members. However, the exact genetic transmission patterns are unknown or at best variable; many are assumed undiscovered. Here we describe a novel case of triphalangeal thumb with a strong familial component. The genetic transmission for this family's phenotype remains unidentified despite extensive work-up. We propose that our patient harbors a potentially new and unidentified genetic mutation for triphalangeal thumb.
Our proband is a 16-year-old-female who presented to our clinic with a history of bilateral preaxial polydactyly status post repair in infancy as well as bilateral triphalangeal thumb. She subsequently underwent right thumb osteotomy and shortening with interphalangeal joint arthrodesis for correction. Additionally, considering her significant family history, the patient was referred for genetic evaluation.
After construction of a multiple generation pedigree, at least three other affected individuals in four generations were identified including the proband's mother, maternal great uncle and maternal great grandmother. Notably, the maternal grandmother is unaffected. No causative genetic lesion was discovered on (1) chromosomal microarray, (2) next generation sequencing panel which included genes commonly associated with limb anomalies and triphalangeal thumb (ESCO2, HDAC8, LMBR1, NIPBL, NSDHL, RAD21, SALL1, SALL4, SHH, SMC1A, SMC3, TBX5, TP63, WNT3) or (3) whole exome sequencing which included the proband and her mother and father.
We report the unique clinical presentation, radiologic characterization, surgical management and genetic evaluation of a pediatric patient with triphalangeal thumb and affected immediate and distant family members. We propose that our patient harbors an unidentified genetic mutation and thus ongoing genetic evaluation continues in order to better characterize the clinical condition of the triphalangeal thumb and identify patterns of transmission and phenotypic expression to further aid in the understanding of this congenital anomaly. This case highlights the vast genetic frontier that remains for exploration and discovery of hitherto unidentified genetic mutations accounting for well-described phenotypic presentations of congenital hand anomalies.


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