Despite their clinical significance, characterization of balanced chromosomal abnormalities (BCAs) has largely been restricted to cytogenetic resolution. We explored the landscape of BCAs at nucleotide resolution in 273 subjects with a spectrum of congenital anomalies. Whole-genome sequencing revised 93% of karyotypes and revealed complexity that was cryptic to karyotyping in 21% of BCAs, highlighting the limitations of conventional cytogenetic approaches. At least 33.9% of BCAs resulted in gene disruption that likely contributed to the developmental phenotype, 5.2% were associated with pathogenic genomic imbalances, and 7.3% disrupted topologically associated domains (TADs) encompassing known syndromic loci. Remarkably, BCA breakpoints in eight subjects altered a single TAD encompassing MEF2C, a known driver of 5q14.3 microdeletion syndrome, resulting in decreased MEF2C expression. This study proposes that sequence-level resolution dramatically improves prediction of clinical outcomes for balanced rearrangements, and provides insight into novel pathogenic mechanisms such as altered regulation due to changes in chromosome topology.

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Human butyrylcholinesterase (BChE; EC 3.1.1.8) is codified by the BCHE gene (3q26.1-q26.2) in which 65 variants have been identified. BChE is a scavenger of organophosphorus and carbamate compounds and hydrolyzes succinylcholine, mivacurium and cocaine. The present study describes 12 naturally occurring BCHE mutations including five new mutations (K12R, G15G, V294M, G333C and R470W) identified in 366 blood donors from Southern Brazil. Exons 2 and 4 of the BCHE gene were examined by PCR-SSCA and samples with unexpected electrophoretic patterns were sequenced. The respective nucleotide substitution that characterizes each of the four new nonsynonymous mutations was introduced into BCHE cDNA by site directed mutagenesis and transfected into human embryonic kidney 293T cells and/or Chinese hamster ovary cells. The catalyzed hydrolysis of butyrylthiocholine (BTC) by BChE was measured by the Ellman method. Enzyme kinetic parameters obtained after the expression of the respective recombinant BChE evaluated the effects of the four nonsynonymous mutations. Thirty-four out of 366 individuals carried a BChE mutation in exon 2. The K variant mutation, A539T in exon 4, was present in one out of three persons. Gene expression showed that only one of the newly identified mutations (G333C) altered BChE activity, leading to a decrease of about 80% in relation to the wild-type enzyme.