Heterotaxy Syndrome and Primary Ciliary Dyskinesia


Background

Lateralization disorders are divided into complete (ie, situs inversus totalis) and incomplete (ie, heterotaxy); the word heterotaxy is derived from the Greek heteros, meaning “other” and taxis, meaning “arrangement.” The disorders have been recognized since at least 1933 (complete) [1and 1826 (incomplete). [2Only relatively recently have genetic alterations responsible for their occurrence in humans been identified. The discovery of kindreds in which both heterotaxy and situs inversus totalis occur strongly suggests that these are not truly separate diseases. [3At least 12% of primary ciliary dyskinesia (PCD) patients have heterotaxy. [4]

Because PCD is present in about a quarter of situs inversus totalis patients and due to the fact that situs inversus totalis is present in nearly half of individuals with PCD, this article covers both heterotaxy syndrome and PCD. Moreover, because asplenia and polysplenia can occur in the same family, [56a patient’s splenic phenotype should be viewed as merely one phenotypic aspect of an underlying laterality disorder, even though many prior reviews of heterotaxy syndrome have divided patients into two groups based on only splenic phenotype.

Pathophysiology

Embryology and developmental biology

Ivemark’s review of 65 cases of human patients with asplenia, [7in which most but not all had heart disease, firmly established the spectrum of congenital heart lesions that occurred in patients with lateralization disorders. Four years later in 1959, the recovery of a spontaneous, autosomal recessive, viable mutation in mice was reported and was named iv, for inverted viscera. [8]

Although the stomach position in the iv mutant colony remained perfectly randomized over 15 years of breeding, including only one outcross, the prevalence of discordance between thoracoabdominal venous anatomy and the situs of the rest of the body decreased from 42% to 26%. [9That is, the ratio of the heterotaxy phenotype to the situs inversus totalis phenotype decreased with progressive inbreeding. This suggests that the wild type allele of the iv locus controls overall thoracoabdominal sidedness and not individual organ sidedness. Indeed, when Icardo and Sanchez de Vega examined the hearts of iv homozygotes, only 40% were abnormal, and only 36% had abnormal splenic morphology. [1011]

The absence of outer dynein arms in the spermatozoa and airway cilia of humans with the Kartagener triad (ie, situs inversus totalis, sinusitis, and bronchiectasis) was also noted by Bj√∂rn Afzelius (see the image below).

The structure and function of cilia is shown here.The structure and function of cilia is shown here. (A) Most motile cilia are organized with 9 microtubule doublets surrounding a core pair of doublets (9+2 configuration). Outer dynein arms (green) and inner dynein arms (blue) are shown. Cilia on the cells of the ventral node in the normal mouse embryo have no core doublet (a 9+0 configuration) and were initially thought to be nonmotile; however, upon closer scrutiny, node cilia were seen to have a rotatory motion (600 rpm). [Figure A is from Hirokawa N, Tanaka Y, Okada Y. Left-right determination: involvement of molecular motor KIF3, cilia, and nodal flow. Cold Spring Harb Perspect Biol. Jul 2009;1(1):a000802 and is reprinted with permission of Cold Spring Harbor Press.] (B) lrd (left-right dynein), the protein (green) mutated by the iv mutation, is also known as DNAH11, DNAHC11, and DLP11. [Figure B is from the United States Department of Energy Genomes to Life Program.] (C) The rotatory cone of each cilium is tilted posteriorly. Hence, the cilia make a leftward swing at the fluid surface and a rightward swing at the cellular surface. Because more viscous drag is present at the cellular surface, the rightward sweep is less effective at generating fluid movement than is the leftward sweep. [Figure C is from Hirokawa N, Tanaka Y, Okada Y, Takeda S. Nodal flow and the generation of left-right asymmetry. Cell 2006; 125:33-45 and is reproduced with permission from Cell Press.] A = anterior; L = left; P = posterior; r = Right.

When the iv mouse mutation was cloned, it was found to encode a molecular motor protein, an axonemal dynein, and was named lrd, for left-right dynein (human homolog is DNAH11/DNAHC11Dynein heavy chain 11axonemal). However, its expression at embryonic day 7.5 was confined to the few hundred ciliated cells of the ventral surface of the node, a fluid-covered, pit-shaped structure at the anterior end of the primitive streak. Because these cilia, 5 microns in length and 0.3 microns in diameter, are missing the central doublet (i.e., have a 9+0 configuration of microtubule doublets, rather than the 9+2 configuration typically seen in motile cilia), they were not believed to be motile. Although the node was known to have important roles in organizing the body plan of the mouse embryo, the function of lrd remained mysterious.

The next year, the phenotype of mice missing the kinesin Kif3b was reported. Kif3b is a molecular motor which, like dynein, is responsible for transport along microtubules within cilia; kinesins transport their cargo toward the "plus end" of the microtubule, whereas dyneins are "minus end directed motors." Fifty percent of the 9.5-day embryos had L-looped hearts. Closer scrutiny of the cilia on normal ventral node cells showed that they do in fact move, despite their 9+0 arrangement of microtubules. In fact, uniquely among cilia, they rotate at 600 rpm. Ventral node cells of Kif3b nulls had either sporadic, very short cilia or absent cilia. [12Whereas iv heterozygotes had cilia that rotated at 600 rpm, iv homozygotes had immotile cilia.

Because of a posterior tilt in the orientation of the cilia, as well as a difference in the viscous drag at the fluid surface compared with the base of the pit, the fluid within the node pit moves unidirectionally to the left, as verified by the movement of submicron-sized fluorescent beads applied to the fluid as passive tracers. [13This fluid flow sets up a left-right asymmetric distribution of signaling molecules (eg, the evolutionarily conserved nodal/Pitx2 pathway—specifying the right side in protostomes and nonchordate deuterostomes but the left side in chordates [14) within the embryo during gastrulation, when the three germ layers (ie, ectoderm, mesoderm, and endoderm) are specified. Although some investigators initially proposed that perinodally situated sensory cilia might transduce the force of fluid flow via intracellular calcium signaling, mechanical forces do not in fact evoke measurable increases in intracellular calcium. [15]

In the zebrafish (Danio rerio), rotatory cilia-bearing structures homologous to the mouse node have been identified; however, they do not appear to be present in chicks or pigs, [16 which both appear to have short, nonmotile cilia [17and may utilize an alternative left-right specification strategy at the embryonic node. [18Moreover, the African clawed frog (Xenopus laevis) specifies the embryonic left-right axis long before cilia can be identified. Numerous steps in the process of embryonic development that precede the development of node cilia remain unknown. In fact, three temporal phases in which molecular and cellular decisions determine the left-right axis of the body plan are likely (ie, pregastrulation, gastrulation, and organogenesis), as is shown in the image below.

Three phases of elaboration of left-right (LR) asyThree phases of elaboration of left-right (LR) asymmetry are shown. The first step consists of differentiating the left and right sides on the cellular level. This probably takes place by means of a chiral molecule. (A) A subset of the cells (yellow) of the fairly early embryo undergo this process. (B) Localized cellular asymmetry is propagated between cells to cause LR determinants to accumulate on one side of the embryonic midline, possibly by a process involving transport through gap junctions. These determinants would then induce cascades of factors in multicellular fields of the embryo. (C) Finally, the asymmetric presence of these factors induces or suppresses asymmetrically located organs such as the spleen and regulates asymmetric morphogenesis of other organs such as the heart tube. Courtesy of Levin M, Mercola M. The compulsion of chirality: toward an understanding of left-right asymmetry. Genes Dev. Mar 15 1998;12(6):763-9.

For technical reasons, the pregastrulation time period is particularly difficult to study in mammals. Important left-right axis specification decisions may occur in this developmental time interval in mouse and human; thus, the frog, chick, and pig may not be “outliers.”

In addition, the underlying cellular biology of why improper left-right specification of the lateral plate mesoderm has such a profound effect on the patterning of the heart, particularly the venous inflow and arterial outflow, has yet to be understood.

Anatomy

Predominantly endodermal structures

The bronchial branching pattern (and lung lobation) can be normal, inversus, right isomeric, or left isomeric. Liver lobation can be normal, inversus, or symmetric. In the gallbladder and biliary tree, hypoplasia, absence, and duplication can be noted. The spleen can be normal, absent, hypoplastic, or multiple. The intestine can develop malrotation.

Predominantly mesodermal structures

The hepatic segment of inferior vena cava (IVC) can be present or absent (the so-called “interrupted IVC"). The hepatic veins can be normal (ie, join the IVC just proximal to the IVC-atrial junction) or can connect independently to the atria. The coronary sinus can be normal, absent, or completely unroofed. The superior vena cava (SVC) can be normal (unilateral) or bilateral. Pulmonary veins can be partially anomalous or totally anomalous. Appendage morphology can be normal, inversus, right isomeric, or left isomeric. The common atrioventricular canal (CAVC) is usually significantly malaligned toward the morphologic right ventricle (RV), but it can be malaligned toward the morphologic left ventricle (LV). The ventricles can be D-loop or L-loop (superoinferior ventricles is rare). In the outflow tract, subpulmonary stenosis or atresia is usually noted, but subaortic stenosis can be observed. Double-outlet RV is most common, but tetralogy of Fallot can occur. Transposition of the great arteries can occur.

Either a left aortic arch with a left upper descending aorta or a right aortic arch with a right upper descending aorta can occur. Double aortic arch is exceedingly rare. In cases of left aortic arch with left upper descending aorta, the abdominal aorta is left of the spine. In cases of right aortic arch with right upper descending aorta, the abdominal aorta is right of the spine (unlike the situation without heterotaxy, in which the abdominal aorta is left of the spine). Many cases have both the abdominal aorta and the IVC (or azygos, if the hepatic IVC is absent) on the same side of the spine (unlike without heterotaxy, in which the IVC is right of the spine, whereas the abdominal aorta is left of the spine).

Although the genetic underpinnings of left-right patterning of the embryonic brain and spinal cord have been studied extensively in some vertebrate systems, relatively little is known about this in humans so far.

Etiology

More than 60 genes have been identified as required for normal left-right axis specification, left-right patterning, or respiratory ciliary function. [19202122232425262728293031323334353637383940414243]

See the image below.

Genes required for proper left-right asymmetry areGenes required for proper left-right asymmetry are shown. Genes are presented in five columns, according to the developmental phase in which they are currently thought to function. The leftmost column has the earliest functioning genes. The second column has genes required for the development of the node (or its equivalent). The third and fourth column have genes that are required for normal node cilia function. Genes in white, green, or blue denote those in which the proof came from studies of fruit fly (Drosophila melanogaster), zebrafish (Danio rerio), or frog (Xenopus laevis), respectively. Genes in brown are those studied in mice (Mus musculus), whereas those discovered in humans (Homo sapiens) are shown in red.

 

Epidemiology

United States data

The worldwide incidence of heterotaxy syndrome is reportedly 1 case per 10,000 births. [44This closely approximates the findings of the Baltimore-Washington Infant Study, in which the incidence of cardiac malformations associated with abnormal laterality was estimated at 1.44 cases per 10,000 live births. [45]

The true prevalence of heterotaxy syndrome is unknown, because many patients, especially those with left atrial appendage isomerism or polysplenia, have sufficiently mild heart disease such that the underlying diagnosis of heterotaxy may not even be considered by the clinician.

Race-, sex-, and age-related demographics

Although no predilection based on race has been identified, two studies noted a male-to-female ratio of 2:1.

The age at presentation is largely dependen on the severity of the heart disease.

Prognosis

The vast majority of patients with heterotaxy syndrome who have cardiovascular phenotypes significant enough to warrant cardiac surgical palliation undergo staged reconstruction to create Fontan-type circulatory arrangements; all of these patients can be expected to need cardiac transplantation in the second or third decades of life. Whether patients with heterotaxy syndrome who have cardiovascular defects that can be managed without the Fontan procedure fare worse than comparable patients without heterotaxy is currently unknown.

In a retrospective, single-center study (1997-2014) of 35 children with heterotaxy syndrome, the reported survival was 83% over a median follow-up of 65 months. [46Of the 12 patients with poor outcomes (34.3%), 6 died, 1 underwent cardiac transplantation, and 5 had a New York Heart Association heart failure classification above III.

CLINICAL PRESENTATION


History

Patients with heart malformations most frequently present with cyanosis in the first few days of life due to subpulmonary stenosis or atresia. Some individuals with subaortic stenosis and aortic arch obstruction present with poor perfusion. Severe common atrioventricular valve regurgitation, contributing to low output syndrome, is another possible presentation.

A small percentage of patients with heterotaxy are first identified because of abdominal pain and vomiting related to malrotation-caused intestinal obstruction.

Physical Examination

Cyanosis (if subpulmonary stenosis or atresia is present) or poor peripheral perfusion (if severe aortic arch obstruction or common atrioventricular valve regurgitation is present) are the most common findings in patients with heterotaxy. The liver may be on the left, rather than on the right, and it may span the abdomen. Dextrocardia may be identifiable.

Patients who present because of malrotation-caused obstruction may have abdominal distension, bilious vomiting and, rarely, melena

DDx

Differential Diagnoses

Workup

Laboratory Studies and Imaging Studies

Laboratory testing

A complete blood cell (CBC) count with peripheral smear to assess for Howell-Jolly bodies (evidence of impaired splenic function) as well as an arterial blood gas (ABG) assessment are indicated in patients with heterotaxy syndrome.

Imaging studies

The following imaging studies are indicated:

  • Chest roentgenography

  • Echocardiography

  • Upper gastrointestinal (GI) series to screen for malrotation (See the image below.)

     
    Malrotation of the gut. This upper gastrointestinaMalrotation of the gut. This upper gastrointestinal (GI) barium study of the same heterotaxy patient as shown in the previous two images shows a right-sided stomach (St), opposite of the normal site. The duodenum heads to the left, the duodenal-jejunal junction is to the left of the spine (opposite to what would be expected for situs inversus totalis), and the jejunum (J) stays left-sided.
  • Magnetic resonance imaging (See images below.)

    Coronal magnetic resonance image (MRI) of the sameCoronal magnetic resonance image (MRI) of the same patient as shown in the previous image. (A) Both superior vena cava (SVC)–to–pulmonary artery (PA) anastomoses can be seen. LCCA = left common carotid artery. (B) Three-dimensional surface rendering. RIA = right innominate artery. (C) Three-dimensional reconstruction of only the systemic venous pathway.
    Axial magnetic resonance image (MRI) of a case of Axial magnetic resonance image (MRI) of a case of heterotaxy with polysplenia. (A) The abdominal aorta (abd ao) is on the left side of the spine (S), as is the left-sided azygos (L Azy). Two right-sided spleens (spl) are visible. LHV = left hepatic vein; RHV = right hepatic vein. (B) A common atrioventricular valve (black unlabelled arrows) is markedly malaligned to the right ventricle (RV). A diminutive left atrium (LA) is represented by only an appendage. The patient had an extracardiac conduit (EC) type of Fontan operation. No fenestration is noted between the EC and the neo-left atrium (neoLA). (C) Because this patient had subaortic stenosis, a proximal pulmonary artery-to-ascending aortic anastomosis was performed early in life, along with augmentation of the aortic arch. The L Azy connects to the left superior vena cava (LSVC). LU DAo = left upper descending aorta; Prox = proximal. (D) The LSVC connected originally to the coronary sinus (CS) and then to the right atrium. Despite the fact that the LSVC has been disconnected from the heart and anastomosed end-to-side to the left pulmonary artery, the CS remains large. The narrowed left ventricular outflow tract (LVOT) is seen. Ao = aorta; PA = pulmonary root; RLL PV = right lower lobe pulmonary vein. (E) Because this patient had absence of the hepatic segment of the inferior vena cava, the left-sided SVC-to-left pulmonary artery (LPA) anastomosis is referred to a left-sided Kawashima (LK). The anastomosis of the right superior vena cava to the right pulmonary artery is a right-sided bidirectional Glenn (R BDG) shunt. (F) The left lower lobe pulmonary vein (LLL PV), as part of this patient's totally anomalous pulmonary venous connection, connects to the original right atrium, which is now the neoLA.
  • Liver-spleen scanning

Other Tests

Holter monitoring is indicated, especially in cases of left atrial appendage isomerism or polysplenia, because this subset has a high prevalence of sinus node dysfunction and atrioventricular block. Electrocardiography (ECG) is also indicated.

In addition, direct imaging of cilia obtained from nasal biopsy specimens and nasal nitric oxide measurement are both more sensitive than standard transmission electron microscopy (TEM) in detecting ciliary abnormalities. [30]

More than a quarter of the genes responsible for normal left-right patterning encode components of the cilium. Because respiratory complications following surgical palliation of heterotaxy patients are associated with ciliary dysfunction, [47the suggestion has been made that presurgical evaluation of ciliary morphology and function could inform postoperative management. For example, owing to the fact that aggressive microbial diagnosis and antibiotic therapy is the standard of care for primary ciliary dyskinesia with respiratory infections, it would be sensible to extend that strategy to the heterotaxy population

TREATMENT AND MANAGEMENT


Medical Care

The cardiovascular phenotype of heterotaxy syndrome dictates inpatient care, and the selection of inpatient and outpatient medications is dependent on the cardiovascular phenotype and the success of the surgical palliation.

Continuous oral amoxicillin prophylaxis is currently recommended for those patients with an abnormal splenic phenotype. For ductal-dependent pulmonary blood flow or ductal-dependent systemic blood flow, prostaglandin E1 infusion is life saving. For common atrioventricular valve regurgitation, vasodilator therapy (eg, angiotensin-converting enzyme [ACE] inhibition) can be palliative. (See Medication.)

Arrhythmias, predominantly supraventricular, frequently complicate the preoperative and postoperative management.

Consultations

Patients with presumptive heterotaxy should have a comprehensive evaluation by a pediatric cardiologist, a geneticist, and a cardiac surgeon.

Surgical Care

Surgical care is dependent on the underlying cardiovascular phenotype, with the most severe individuals typically being staged toward Fontan-type circulatory configurations. Pacing may be indicated in patients with third-degree atrioventricular block or sinus node dysfunction.

Cardiothoracic surgeons tend to use cavopulmonary connections rather than complicated intra-atrial baffles; thus, early survival after the Fontan operation has improved. However, survival is still clearly lower than for other patients without heterotaxy. Although part of this disparity can be attributed to the propensity of atrioventricular valve regurgitation to progress in heterotaxy patients, whether this is actually primarily due to ventricular deterioration (rather than the presence of an inherently different atrioventricular valve) is unclear.

Outpatient care

Further outpatient care depends on the cardiovascular phenotype, the success of the surgical palliation, and the presence of noncardiac anomalies, such as intestinal malrotation

MEDICATION SUMMURY


Medication Summary

Patients with significant left-to-right shunts may benefit from digoxin. Those with severe common atrioventricular valve regurgitation may benefit from vasodilator therapy.

In patients with impaired splenic function, immunization with Haemophilus influenzae vaccine, pneumococcal vaccine, and meningococcal vaccine, as well as antibiotics for subacute bacterial endocarditis (SBE) prophylaxis are necessary. [4849 In a systematic review specifically assessing the risk of infection in heterotaxy syndrome (English language only), investigators noted 42 cases of bacteremia in 32 patients, with more than three quarters (79%) of these involving asplenia. [49Moreover, patients with heterotaxy syndrome had an increased risk of bacteremia leading to mortality, regardless of the anatomic splenic type. [49Thus, antibiotic prophylaxis is administered to patients before procedures that may cause bacteremia are performed. For more information, see Antibiotic Prophylactic Regimens for Endocarditis and Infectious Endocarditis.

In addition, because ciliary dysfunction is so common in heterotaxy and primary ciliary dyskinesia, [30antibiotic therapy should be considered for all patients with respiratory symptoms.

Seasonal flu vaccine and H1N1 vaccine are recommended, especially for those who have undergone Fontan operation.

Diuretic agents

Class Summary

These agents promote excretion of water and electrolytes by the kidneys. They are used to treat heart failure or hepatic, renal, or pulmonary disease when sodium and water retention has resulted in edema or ascites. They may be used as monotherapy or in combination to treat hypertension.

Furosemide (Lasix)

Used to treat edema. Increases excretion of water by interfering with chloride-binding cotransport system, which, in turn, inhibits sodium and chloride reabsorption in ascending loop of Henle and distal renal tubule. Dose must be individualized to patient. Depending on response, administer at increments of 20-40 mg, no sooner than 6-8 h after previous dose, until desired diuresis occurs. When treating infants, titrate with 1-mg/kg/dose increments until satisfactory effect achieved.

Spironolactone (Aldactone)

For management of edema resulting from excessive aldosterone excretion. Competes with aldosterone for receptor sites in distal renal tubules, increasing water excretion while retaining potassium and hydrogen ions.

Inotropic agents

Class Summary

Positive inotropes increase the force of contraction of the myocardium and are used to treat acute and chronic congestive heart failure. Some may also increase or decrease the heart rate (eg, positive or negative chronotropic agents), provide vasodilatation, or improve myocardial relaxation. These additional properties influence the choice of drug for specific circumstances. Those used predominantly for their inotropic effects include cardiac glycosides and phosphodiesterase inhibitors.

Digoxin (Lanoxin)

Used to treat congestive heart failure. Cardiac glycoside with direct inotropic effects in addition to indirect effects on the cardiovascular system. Acts directly on cardiac muscle, increasing myocardial systolic contractions. Its indirect actions result in increased carotid sinus nerve activity and enhanced sympathetic withdrawal for any given increase in mean arterial pressure.

Angiotensin-converting enzyme (ACE) inhibitors

Class Summary

ACE inhibitors are beneficial in all stages of congestive heart failure. Pharmacologic effects result in a decrease in systemic vascular resistance, reducing blood pressure, preload, and afterload. Dyspnea and exercise tolerance are improved. Unlike diuretics, studies demonstrate improvement of survival and reduced progression of mild or moderate heart failure to more severe stages. Benefits asymptomatic left ventricular dysfunction.

Enalapril (Vasotec)

Used to treat congestive heart failure. Competitive inhibitor of ACE. Reduces angiotensin II levels, decreasing aldosterone secretion.

Vaccines

Class Summary

Active immunization increases resistance to infection. Vaccines consist of microorganisms or cellular components, which act as antigens. Administration of the vaccine stimulates the production of antibodies with specific protective properties.

Pneumococcal vaccine polyvalent (Pneumovax-23, Pnu-Imune 23)

Polyvalent vaccine used for prophylaxis against infection from Streptococcus pneumoniae. Used in populations at increased risk of pneumococcal pneumonia (ie, age >55 y, chronic infection, asplenia, immunocompromise).

Antibiotics, prophylactic

Class Summary

Antibiotic prophylaxis is administered to patients before performing procedures that may cause bacteremia.

Amoxicillin (Amoxil, Trimox)

Interferes with synthesis of cell wall mucopeptides during active multiplication, resulting in bactericidal activity against susceptible bacteria. Used as prophylaxis in minor procedures.

Ampicillin (Marcillin, Omnipen)

For prophylaxis in patients undergoing dental, PO, or respiratory tract procedures.

Coadministered with gentamicin for prophylaxis in GI or genitourinary procedures.

Clindamycin (Cleocin)

Used in penicillin-allergic patients undergoing dental, PO, or respiratory tract procedures. Useful for treatment against streptococcal and most staphylococcal infections.

Gentamicin (Garamycin)

Aminoglycoside antibiotic for gram-negative coverage. Used in combination with an agent against gram-positive organisms and one that covers anaerobes.

Used in conjunction with ampicillin or vancomycin for prophylaxis in GI or genitourinary procedures.

Vancomycin (Vancocin)

Potent antibiotic directed against gram-positive organisms and active against Enterococcus species. Useful in the treatment of septicemia and skin structure infections. Indicated for patients who cannot receive or have not responded to penicillins and cephalosporins or have infections with resistant staphylococci.

Use CrCl to adjust dose in renal impairment.

Used in conjunction with gentamicin for prophylaxis in penicillin-allergic patients undergoing GI or genitourinary procedures.

Cefazolin (Ancef)

First-generation semisynthetic cephalosporin that arrests bacterial cell wall synthesis, inhibiting bacterial growth. Primarily active against skin flora, including Staphylococcus aureus.

Cephalexin (Keflex)

First-generation cephalosporin that arrests bacterial growth by inhibiting bacterial cell wall synthesis. Bactericidal activity against rapidly growing organisms. Primary activity against skin flora and used for skin infections or prophylaxis in minor procedures.

Cefadroxil (Duricef)

First-generation cephalosporin that arrests bacterial growth by inhibiting bacterial cell wall synthesis. Bactericidal activity against rapidly growing organisms. Primary activity against skin flora and used for skin infections or prophylaxis in minor procedures.

Azithromycin (Zithromax)

Inhibits bacterial growth, possibly by blocking dissociation of peptidyl tRNA from ribosomes, causing RNA-dependent protein synthesis to arrest.

Clarithromycin (Biaxin)

Inhibits bacterial growth, possibly by blocking dissociation of peptidyl tRNA from ribosomes, causing RNA-dependent protein synthesis to arrest.