A Case Study of Monozygotic Twins Apparently Homozygous for a Novel Variant of UDP-Galactose 4′-epimerase (GALE)

Abstract

Epimerase deficiency galactosemia is an autosomal recessive disorder that results from partial impairment of UDP-galactose 4′-epimerase (GALE), the third enzyme in the Leloir pathway of galactose metabolism. Clinical severity of epimerase deficiency ranges from potentially lethal to apparently benign, likely reflecting the extent of GALE enzyme impairment, among other factors. We report here a case study of monozygotic twins identified by newborn screening with elevated total galactose and normal galactose-1P uridylyltransferase (GALT). Follow-up testing revealed partial impairment of GALE in hemolysates but near-normal activity in lymphoblasts; molecular testing identified a missense substitution, R220W, apparently in the homozygous state. The twins were treated with dietary galactose restriction for the first 18 months of life. During this time, independent testing revealed concurrent diagnoses of Williams Syndrome in both twins, and cytomegalovirus (CMV) infection in one. Expression studies of R220W-hGALE in a null-background strain of Saccharomyces cerevisiae demonstrated a very limited impairment of V _max for UDP-galactose (UDP-Gal) and K _m for UDP-N-acetylgalactosamine (UDP-GalNAc), but a galactose challenge in vivo failed to uncover any evidence of impaired Leloir function. Similarly, both twins demonstrated normal hemolysate galactose-1-phosphate (Gal-1P) levels following normalization of their diets at 18 months of age. While these studies cannot rule out a negative consequence from some cryptic GALE impairment in a specific tissue or developmental stage, they suggest that the substitution, R220W, is mild to neutral, so that any GALE impairment in these twins is likely to be peripheral and therefore unlikely to be the cause of the negative outcomes observed.

Competing interests: None declared

Appendices

One Sentence Summary

Detailed biochemical studies of a rare variant of human GALE, R220W, suggest that it is unlikely to account for the negative outcomes observed in the twins who carry it.

Contributions of the Individual Authors

Y Liu performed the majority of experiments presented, assembled the figures and tables, and wrote the first draft of the manuscript. K Bentler identified this family, assembled all of the clinical and some of the laboratory data, and wrote the manuscript section dealing with clinical presentation. B Coffee oversaw and interpreted the GALE genotyping and wrote the section of the manuscript relevant to those data. JS Chhay generated the hGALE-R220W yeast expression plasmid and also prepared and initially characterized the EBV-transformed lymphoblasts from both subjects. K Sarafoglou provided clinical consultation on endocrinology issues and assisted with writing of the manuscript section dealing with clinical presentation and some tables. JL Fridovich-Keil oversaw the project, wrote some sections of the manuscript, and edited and finalized all sections of the manuscript for submission. All authors assisted with editing the final manuscript.

Guarantor

Judith L. Fridovich-Keil is the guarantor for this work.

Competing Interest Statement

None of the authors has any competing interests to disclose.

Funding

This work was funded, in part, by grant NIH R01 DK059904 (to JLFK). “The author(s) confirm(s) independence from the sponsors; the content of the article has not been influenced by the sponsors.”

Ethics Approval

This project was conducted with approval from the Emory University Institutional Review Board (Protocol # 618–99, PI: JL Fridovich-Keil) and study volunteers were consented prior to the study in accordance with that protocol.

No vertebrate animals were used in this study.