Page 6 - CBAC Newsletter 2014

a new spin on CardiaC ConduCtion and

rhythm From developmental Biology and

human genetiCs

1
By patriCk y. Jay and staCey rentsChler 2

1 departments oF pediatriCs and genetiCs
2 departments oF mediCine and developmental Biology
washington university sChool oF mediCine, st. louis, mo

Historically, key observations and technical advances have spurred scientific progress in the cardiac conduction sys-
tem. Tawara performed painstaking histological reconstructions to delineate the anatomy of the central and peripheral
conduction systems in 1906. Einthoven described the electrocardiogram in 1903. Subsequent advances in the 20th
century, such as patch clamping and cloning methods, enabled discoveries that bring us to modern cardiac electro-
physiology, a field that is mainly focused on the initiation and propagation of the action potential. In this review, we
provide an overview of key discoveries from developmental biology and human genetics. They herald new avenues of
investigation and could suggest targets for the next generation of anti-arrhythmic therapy.

Cardiac developmental genes regulate the programming of the conduction system.

Most cells and tissues in the body can be studied as isolated cells, in cell culture or in simple models like the fly, worm,
or fish. Such methods and model organisms facilitate the molecular genetic dissection of pathways and phenotypes.
The cardiac conduction system, however, is unlike most tissues. Due to its intricately complex nature, functional stud-
ies must be performed in situ in human or mammalian hearts, which impedes mechanistic studies. Furthermore, the
function of components of the conduction system is usually experimentally inferred. For example, conduction from the
atria to the ventricles is measured using electrodes, or voltage- and calcium-sensitive fluorescent dyes, while conduc-
tion through the AV node, His bundle and Purkinje system are not observed directly. Until relatively recently, the diffi-
culties in studying this complex cellular network hampered mechanistic exploration of its developmental programming,
until several key discoveries piqued the interest of the developmental biology community.

Developmental biologists seek to understand how cells and tissues arise and grow in an organism. To do so, they
utilize molecular markers to trace the origin and fate of a cell. The markers can be antibodies or RNA probes for genes
specifically expressed in the cell of interest. The markers can also be genetic. An old, yet elegant genetic method was
the use of a retroviral vector that inserts into the genome of a progenitor cell. The cell in turn passes its transgenic
reporter to its progeny. More recently, Cre-lox systems have largely replaced the use of retroviruses. Cre is a bacterio-
phage enzyme that recombines genomic DNA that is flanked or “flox’ed” by two 32-base pair sequences, known as loxP.
The intervening sequence is excised. In a typical lineage analysis experiment, a transgenic mouse is engineered to
express Cre in a specific cell-type and at a specific time. Induction of Cre activity removes the flox’ed sequence which
had prevented the expression of a transgenic reporter gene. Thus, all the daughter cells of a Cre-positive progenitor
inherit a genetic label. As no practical markers of the conduction system were available until just 15 years ago, the
retroviral strategy was used to settle a longstanding controversy: Does the conduction system arise from a myocyte
or neural lineage?

1 | CBAC Center Heartbeat